1.\" $NetBSD: sh.1,v 1.106 2011/10/05 13:15:30 christos Exp $ 2.\" Copyright (c) 1991, 1993 3.\" The Regents of the University of California. All rights reserved. 4.\" 5.\" This code is derived from software contributed to Berkeley by 6.\" Kenneth Almquist. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 3. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)sh.1 8.6 (Berkeley) 5/4/95 33.\" 34.Dd October 4, 2011 35.Dt SH 1 36.Os 37.Sh NAME 38.Nm sh 39.Nd command interpreter (shell) 40.Sh SYNOPSIS 41.Nm 42.Bk -words 43.Op Fl aCefnuvxIimqVEb 44.Op Cm +aCefnuvxIimqVEb 45.Ek 46.Bk -words 47.Op Fl o Ar option_name 48.Op Cm +o Ar option_name 49.Ek 50.Bk -words 51.Op Ar command_file Oo Ar argument ... Oc 52.Ek 53.Nm 54.Fl c 55.Bk -words 56.Op Fl aCefnuvxIimqVEb 57.Op Cm +aCefnuvxIimqVEb 58.Ek 59.Bk -words 60.Op Fl o Ar option_name 61.Op Cm +o Ar option_name 62.Ek 63.Bk -words 64.Ar command_string 65.Op Ar command_name Oo Ar argument ... Oc 66.Ek 67.Nm 68.Fl s 69.Bk -words 70.Op Fl aCefnuvxIimqVEb 71.Op Cm +aCefnuvxIimqVEb 72.Ek 73.Bk -words 74.Op Fl o Ar option_name 75.Op Cm +o Ar option_name 76.Ek 77.Bk -words 78.Op Ar argument ... 79.Ek 80.Sh DESCRIPTION 81.Nm 82is the standard command interpreter for the system. 83The current version of 84.Nm 85is in the process of being changed to conform with the 86.Tn POSIX 871003.2 and 1003.2a specifications for the shell. 88This version has many 89features which make it appear similar in some respects to the Korn shell, 90but it is not a Korn shell clone (see 91.Xr ksh 1 ) . 92Only features designated by 93.Tn POSIX , 94plus a few Berkeley extensions, are being incorporated into this shell. 95.\" We expect 96.\" .Tn POSIX 97.\" conformance by the time 4.4 BSD is released. 98This man page is not intended 99to be a tutorial or a complete specification of the shell. 100.Ss Overview 101The shell is a command that reads lines from either a file or the 102terminal, interprets them, and generally executes other commands. 103It is the program that is running when a user logs into the system 104(although a user can select a different shell with the 105.Xr chsh 1 106command). 107The shell implements a language that has flow control 108constructs, a macro facility that provides a variety of features in 109addition to data storage, along with built in history and line editing 110capabilities. 111It incorporates many features to aid interactive use and 112has the advantage that the interpretative language is common to both 113interactive and non-interactive use (shell scripts). 114That is, commands 115can be typed directly to the running shell or can be put into a file and 116the file can be executed directly by the shell. 117.Ss Invocation 118If no arguments are present and if the standard input of the shell 119is connected to a terminal (or if the 120.Fl i 121flag is set), 122and the 123.Fl c 124option is not present, the shell is considered an interactive shell. 125An interactive shell generally prompts before each command and handles 126programming and command errors differently (as described below). 127When first starting, 128the shell inspects argument 0, and if it begins with a dash 129.Sq - , 130the shell is also considered 131a login shell. 132This is normally done automatically by the system 133when the user first logs in. 134A login shell first reads commands 135from the files 136.Pa /etc/profile 137and 138.Pa .profile 139if they exist. 140If the environment variable 141.Ev ENV 142is set on entry to a shell, or is set in the 143.Pa .profile 144of a login shell, the shell next reads 145commands from the file named in 146.Ev ENV . 147Therefore, a user should place commands that are to be executed only at 148login time in the 149.Pa .profile 150file, and commands that are executed for every shell inside the 151.Ev ENV 152file. 153To set the 154.Ev ENV 155variable to some file, place the following line in your 156.Pa .profile 157of your home directory 158.Pp 159.Dl ENV=$HOME/.shinit; export ENV 160.Pp 161substituting for 162.Dq .shinit 163any filename you wish. 164Since the 165.Ev ENV 166file is read for every invocation of the shell, including shell scripts 167and non-interactive shells, the following paradigm is useful for 168restricting commands in the 169.Ev ENV 170file to interactive invocations. 171Place commands within the 172.Dq case 173and 174.Dq esac 175below (these commands are described later): 176.Pp 177.Bl -item -compact -offset indent 178.It 179.Li case $- in *i*) 180.Bl -item -compact -offset indent 181.It 182.Li # commands for interactive use only 183.It 184.Li ... 185.El 186.It 187.Li esac 188.El 189.Pp 190If command line arguments besides the options have been specified, then 191the shell treats the first argument as the name of a file from which to 192read commands (a shell script), and the remaining arguments are set as the 193positional parameters of the shell ($1, $2, etc). 194Otherwise, the shell 195reads commands from its standard input. 196.Ss Argument List Processing 197All of the single letter options have a corresponding name that can be 198used as an argument to the 199.Fl o 200option. 201The set 202.Fl o 203name is provided next to the single letter option in 204the description below. 205Specifying a dash 206.Dq - 207turns the option on, while using a plus 208.Dq + 209disables the option. 210The following options can be set from the command line or 211with the 212.Ic set 213built-in (described later). 214.Bl -tag -width aaaallexportfoo -offset indent 215.It Fl a Em allexport 216Export all variables assigned to. 217.It Fl c 218Read commands from the 219.Ar command_string 220operand instead of from the standard input. 221Special parameter 0 will be set from the 222.Ar command_name 223operand and the positional parameters ($1, $2, etc.) 224set from the remaining argument operands. 225.It Fl C Em noclobber 226Don't overwrite existing files with 227.Dq \*[Gt] . 228.It Fl e Em errexit 229If not interactive, exit immediately if any untested command fails. 230The exit status of a command is considered to be 231explicitly tested if the command is used to control an 232.Ic if , 233.Ic elif , 234.Ic while , 235or 236.Ic until ; 237or if the command is the left hand operand of an 238.Dq \*[Am]\*[Am] 239or 240.Dq || 241operator. 242.It Fl f Em noglob 243Disable pathname expansion. 244.It Fl n Em noexec 245If not interactive, read commands but do not execute them. 246This is useful for checking the syntax of shell scripts. 247.It Fl u Em nounset 248Write a message to standard error when attempting to expand a variable 249that is not set, and if the shell is not interactive, exit immediately. 250.It Fl v Em verbose 251The shell writes its input to standard error as it is read. 252Useful for debugging. 253.It Fl x Em xtrace 254Write each command to standard error (preceded by a 255.Sq +\ ) 256before it is executed. 257Useful for debugging. 258.It Fl q Em quietprofile 259If the 260.Fl v 261or 262.Fl x 263options have been set, do not apply them when reading 264initialization files, these being 265.Pa /etc/profile , 266.Pa .profile , 267and the file specified by the 268.Ev ENV 269environment variable. 270.It Fl I Em ignoreeof 271Ignore EOFs from input when interactive. 272.It Fl i Em interactive 273Force the shell to behave interactively. 274.It Fl m Em monitor 275Turn on job control (set automatically when interactive). 276.It Fl s Em stdin 277Read commands from standard input (set automatically if no file arguments 278are present). 279This option has no effect when set after the shell has 280already started running (i.e. with 281.Ic set ) . 282.It Fl V Em vi 283Enable the built-in 284.Xr vi 1 285command line editor (disables 286.Fl E 287if it has been set). 288(See the 289.Sx Command Line Editing 290section below.) 291.It Fl E Em emacs 292Enable the built-in emacs style 293command line editor (disables 294.Fl V 295if it has been set). 296(See the 297.Sx Command Line Editing 298section below.) 299.It Fl b Em notify 300Enable asynchronous notification of background job completion. 301(Not implemented.) 302.It "\ \ " Em cdprint 303Make an interactive shell always print the new directory name when 304changed by the 305.Ic cd 306command. 307.It "\ \ " Em tabcomplete 308Enables filename completion in the command line editor. 309Typing a tab character will extend the current input word to match a 310filename. 311If more than one filename matches it is only extended to be the common prefix. 312Typing a second tab character will list all the matching names. 313One of the editing modes, either 314.Fl E 315or 316.Fl V , 317must be enabled for this to work. 318.El 319.Ss Lexical Structure 320The shell reads input in terms of lines from a file and breaks it up into 321words at whitespace (blanks and tabs), and at certain sequences of 322characters that are special to the shell called 323.Dq operators . 324There are two types of operators: control operators and redirection 325operators (their meaning is discussed later). 326Following is a list of operators: 327.Bl -ohang -offset indent 328.It "Control operators:" 329.Dl \*[Am] \*[Am]\*[Am] \&( \&) \&; ;; | || \*[Lt]newline\*[Gt] 330.It "Redirection operators:" 331.Dl \*[Lt] \*[Gt] \*[Gt]| \*[Lt]\*[Lt] \*[Gt]\*[Gt] \*[Lt]\*[Am] \*[Gt]\*[Am] \*[Lt]\*[Lt]- \*[Lt]\*[Gt] 332.El 333.Ss Quoting 334Quoting is used to remove the special meaning of certain characters or 335words to the shell, such as operators, whitespace, or keywords. 336There are three types of quoting: matched single quotes, 337matched double quotes, and backslash. 338.Ss Backslash 339A backslash preserves the literal meaning of the following 340character, with the exception of 341.Aq newline . 342A backslash preceding a 343.Aq newline 344is treated as a line continuation. 345.Ss Single Quotes 346Enclosing characters in single quotes preserves the literal meaning of all 347the characters (except single quotes, making it impossible to put 348single-quotes in a single-quoted string). 349.Ss Double Quotes 350Enclosing characters within double quotes preserves the literal 351meaning of all characters except dollar sign 352.Pq $ , 353backquote 354.Pq ` , 355and backslash 356.Pq \e . 357The backslash inside double quotes is historically weird, and serves to 358quote only the following characters: 359.Dl $ ` \*q \e \*[Lt]newline\*[Gt] . 360Otherwise it remains literal. 361.Ss Reserved Words 362Reserved words are words that have special meaning to the 363shell and are recognized at the beginning of a line and 364after a control operator. 365The following are reserved words: 366.Bl -column while while while while while -offset indent 367.It ! Ta elif Ta fi Ta while Ta case 368.It else Ta for Ta then Ta { Ta } 369.It do Ta done Ta until Ta if Ta esac 370.El 371.Pp 372Their meaning is discussed later. 373.Ss Aliases 374An alias is a name and corresponding value set using the 375.Ic alias 376built-in command. 377Whenever a reserved word may occur (see above), 378and after checking for reserved words, the shell 379checks the word to see if it matches an alias. 380If it does, it replaces it in the input stream with its value. 381For example, if there is an alias called 382.Dq lf 383with the value 384.Dq "ls -F" , 385then the input: 386.Pp 387.Dl lf foobar Aq return 388.Pp 389would become 390.Pp 391.Dl ls -F foobar Aq return 392.Pp 393Aliases provide a convenient way for naive users to create shorthands for 394commands without having to learn how to create functions with arguments. 395They can also be used to create lexically obscure code. 396This use is discouraged. 397.Ss Commands 398The shell interprets the words it reads according to a language, the 399specification of which is outside the scope of this man page (refer to the 400BNF in the 401.Tn POSIX 4021003.2 document). 403Essentially though, a line is read and if the first 404word of the line (or after a control operator) is not a reserved word, 405then the shell has recognized a simple command. 406Otherwise, a complex 407command or some other special construct may have been recognized. 408.Ss Simple Commands 409If a simple command has been recognized, the shell performs 410the following actions: 411.Bl -enum -offset indent 412.It 413Leading words of the form 414.Dq name=value 415are stripped off and assigned to the environment of the simple command. 416Redirection operators and their arguments (as described below) are 417stripped off and saved for processing. 418.It 419The remaining words are expanded as described in 420the section called 421.Dq Expansions , 422and the first remaining word is considered the command name and the 423command is located. 424The remaining words are considered the arguments of the command. 425If no command name resulted, then the 426.Dq name=value 427variable assignments recognized in item 1 affect the current shell. 428.It 429Redirections are performed as described in the next section. 430.El 431.Ss Redirections 432Redirections are used to change where a command reads its input or sends 433its output. 434In general, redirections open, close, or duplicate an 435existing reference to a file. 436The overall format used for redirection is: 437.Pp 438.Dl [n] Va redir-op Ar file 439.Pp 440where 441.Va redir-op 442is one of the redirection operators mentioned previously. 443Following is a list of the possible redirections. 444The 445.Bq n 446is an optional number, as in 447.Sq 3 448(not 449.Sq Bq 3 ) , 450that refers to a file descriptor. 451.Bl -tag -width aaabsfiles -offset indent 452.It [n] Ns \*[Gt] file 453Redirect standard output (or n) to file. 454.It [n] Ns \*[Gt]| file 455Same, but override the 456.Fl C 457option. 458.It [n] Ns \*[Gt]\*[Gt] file 459Append standard output (or n) to file. 460.It [n] Ns \*[Lt] file 461Redirect standard input (or n) from file. 462.It [n1] Ns \*[Lt]\*[Am] Ns n2 463Duplicate standard input (or n1) from file descriptor n2. 464.It [n] Ns \*[Lt]\*[Am]- 465Close standard input (or n). 466.It [n1] Ns \*[Gt]\*[Am] Ns n2 467Duplicate standard output (or n1) to n2. 468.It [n] Ns \*[Gt]\*[Am]- 469Close standard output (or n). 470.It [n] Ns \*[Lt]\*[Gt] file 471Open file for reading and writing on standard input (or n). 472.El 473.Pp 474The following redirection is often called a 475.Dq here-document . 476.Bl -item -offset indent 477.It 478.Li [n]\*[Lt]\*[Lt] delimiter 479.Dl here-doc-text ... 480.Li delimiter 481.El 482.Pp 483All the text on successive lines up to the delimiter is saved away and 484made available to the command on standard input, or file descriptor n if 485it is specified. 486If the delimiter as specified on the initial line is 487quoted, then the here-doc-text is treated literally, otherwise the text is 488subjected to parameter expansion, command substitution, and arithmetic 489expansion (as described in the section on 490.Dq Expansions ) . 491If the operator is 492.Dq \*[Lt]\*[Lt]- 493instead of 494.Dq \*[Lt]\*[Lt] , 495then leading tabs in the here-doc-text are stripped. 496.Ss Search and Execution 497There are three types of commands: shell functions, built-in commands, and 498normal programs -- and the command is searched for (by name) in that order. 499They each are executed in a different way. 500.Pp 501When a shell function is executed, all of the shell positional parameters 502(except $0, which remains unchanged) are set to the arguments of the shell 503function. 504The variables which are explicitly placed in the environment of 505the command (by placing assignments to them before the function name) are 506made local to the function and are set to the values given. 507Then the command given in the function definition is executed. 508The positional parameters are restored to their original values 509when the command completes. 510This all occurs within the current shell. 511.Pp 512Shell built-ins are executed internally to the shell, without spawning a 513new process. 514.Pp 515Otherwise, if the command name doesn't match a function or built-in, the 516command is searched for as a normal program in the file system (as 517described in the next section). 518When a normal program is executed, the shell runs the program, 519passing the arguments and the environment to the program. 520If the program is not a normal executable file (i.e., if it does 521not begin with the "magic number" whose 522.Tn ASCII 523representation is "#!", so 524.Xr execve 2 525returns 526.Er ENOEXEC 527then) the shell will interpret the program in a subshell. 528The child shell will reinitialize itself in this case, 529so that the effect will be as if a 530new shell had been invoked to handle the ad-hoc shell script, except that 531the location of hashed commands located in the parent shell will be 532remembered by the child. 533.Pp 534Note that previous versions of this document and the source code itself 535misleadingly and sporadically refer to a shell script without a magic 536number as a "shell procedure". 537.Ss Path Search 538When locating a command, the shell first looks to see if it has a shell 539function by that name. 540Then it looks for a built-in command by that name. 541If a built-in command is not found, one of two things happen: 542.Bl -enum 543.It 544Command names containing a slash are simply executed without performing 545any searches. 546.It 547The shell searches each entry in 548.Ev PATH 549in turn for the command. 550The value of the 551.Ev PATH 552variable should be a series of entries separated by colons. 553Each entry consists of a directory name. 554The current directory may be indicated 555implicitly by an empty directory name, or explicitly by a single period. 556.El 557.Ss Command Exit Status 558Each command has an exit status that can influence the behavior 559of other shell commands. 560The paradigm is that a command exits 561with zero for normal or success, and non-zero for failure, 562error, or a false indication. 563The man page for each command 564should indicate the various exit codes and what they mean. 565Additionally, the built-in commands return exit codes, as does 566an executed shell function. 567.Pp 568If a command consists entirely of variable assignments then the 569exit status of the command is that of the last command substitution 570if any, otherwise 0. 571.Ss Complex Commands 572Complex commands are combinations of simple commands with control 573operators or reserved words, together creating a larger complex command. 574More generally, a command is one of the following: 575.Bl -bullet 576.It 577simple command 578.It 579pipeline 580.It 581list or compound-list 582.It 583compound command 584.It 585function definition 586.El 587.Pp 588Unless otherwise stated, the exit status of a command is that of the last 589simple command executed by the command. 590.Ss Pipelines 591A pipeline is a sequence of one or more commands separated 592by the control operator |. 593The standard output of all but 594the last command is connected to the standard input 595of the next command. 596The standard output of the last 597command is inherited from the shell, as usual. 598.Pp 599The format for a pipeline is: 600.Pp 601.Dl [!] command1 [ | command2 ...] 602.Pp 603The standard output of command1 is connected to the standard input of 604command2. 605The standard input, standard output, or both of a command is 606considered to be assigned by the pipeline before any redirection specified 607by redirection operators that are part of the command. 608.Pp 609If the pipeline is not in the background (discussed later), the shell 610waits for all commands to complete. 611.Pp 612If the reserved word ! does not precede the pipeline, the exit status is 613the exit status of the last command specified in the pipeline. 614Otherwise, the exit status is the logical NOT of the exit status of the 615last command. 616That is, if the last command returns zero, the exit status 617is 1; if the last command returns greater than zero, the exit status is 618zero. 619.Pp 620Because pipeline assignment of standard input or standard output or both 621takes place before redirection, it can be modified by redirection. 622For example: 623.Pp 624.Dl $ command1 2\*[Gt]\*[Am]1 | command2 625.Pp 626sends both the standard output and standard error of command1 627to the standard input of command2. 628.Pp 629A ; or 630.Aq newline 631terminator causes the preceding AND-OR-list (described 632next) to be executed sequentially; a \*[Am] causes asynchronous execution of 633the preceding AND-OR-list. 634.Pp 635Note that unlike some other shells, each process in the pipeline is a 636child of the invoking shell (unless it is a shell built-in, in which case 637it executes in the current shell -- but any effect it has on the 638environment is wiped). 639.Ss Background Commands -- \*[Am] 640If a command is terminated by the control operator ampersand (\*[Am]), the 641shell executes the command asynchronously -- that is, the shell does not 642wait for the command to finish before executing the next command. 643.Pp 644The format for running a command in background is: 645.Pp 646.Dl command1 \*[Am] [command2 \*[Am] ...] 647.Pp 648If the shell is not interactive, the standard input of an asynchronous 649command is set to 650.Pa /dev/null . 651.Ss Lists -- Generally Speaking 652A list is a sequence of zero or more commands separated by newlines, 653semicolons, or ampersands, and optionally terminated by one of these three 654characters. 655The commands in a list are executed in the order they are written. 656If command is followed by an ampersand, the shell starts the 657command and immediately proceed onto the next command; otherwise it waits 658for the command to terminate before proceeding to the next one. 659.Ss Short-Circuit List Operators 660.Dq \*[Am]\*[Am] 661and 662.Dq || 663are AND-OR list operators. 664.Dq \*[Am]\*[Am] 665executes the first command, and then executes the second command if and only 666if the exit status of the first command is zero. 667.Dq || 668is similar, but executes the second command if and only if the exit status 669of the first command is nonzero. 670.Dq \*[Am]\*[Am] 671and 672.Dq || 673both have the same priority. 674Note that these operators are left-associative, so 675.Dq true || echo bar \*[Am]\*[Am] echo baz 676writes 677.Dq baz 678and nothing else. 679This is not the way it works in C. 680Also, if you forget the left-hand side (for example when continuing lines but 681forgetting to use a backslash) it defaults to a true statement. 682This behavior is not useful and should not be relied upon. 683.Ss Flow-Control Constructs -- if, while, for, case 684The syntax of the if command is 685.Bd -literal -offset indent 686if list 687then list 688[ elif list 689then list ] ... 690[ else list ] 691fi 692.Ed 693.Pp 694The syntax of the while command is 695.Bd -literal -offset indent 696while list 697do list 698done 699.Ed 700.Pp 701The two lists are executed repeatedly while the exit status of the 702first list is zero. 703The until command is similar, but has the word 704until in place of while, which causes it to 705repeat until the exit status of the first list is zero. 706.Pp 707The syntax of the for command is 708.Bd -literal -offset indent 709for variable in word ... 710do list 711done 712.Ed 713.Pp 714The words are expanded, and then the list is executed repeatedly with the 715variable set to each word in turn. 716do and done may be replaced with 717.Dq { 718and 719.Dq } . 720.Pp 721The syntax of the break and continue command is 722.Bd -literal -offset indent 723break [ num ] 724continue [ num ] 725.Ed 726.Pp 727Break terminates the num innermost for or while loops. 728Continue continues with the next iteration of the innermost loop. 729These are implemented as built-in commands. 730.Pp 731The syntax of the case command is 732.Bd -literal -offset indent 733case word in 734pattern) list ;; 735\&... 736esac 737.Ed 738.Pp 739The pattern can actually be one or more patterns (see 740.Sx Shell Patterns 741described later), separated by 742.Dq \*(Ba 743characters. 744.Ss Grouping Commands Together 745Commands may be grouped by writing either 746.Pp 747.Dl (list) 748.Pp 749or 750.Pp 751.Dl { list; } 752.Pp 753The first of these executes the commands in a subshell. 754Built-in commands grouped into a (list) will not affect the current shell. 755The second form does not fork another shell so is slightly more efficient. 756Grouping commands together this way allows you to redirect 757their output as though they were one program: 758.Pp 759.Bd -literal -offset indent 760{ echo -n \*q hello \*q ; echo \*q world" ; } \*[Gt] greeting 761.Ed 762.Pp 763Note that 764.Dq } 765must follow a control operator (here, 766.Dq \&; ) 767so that it is recognized as a reserved word and not as another command argument. 768.Ss Functions 769The syntax of a function definition is 770.Pp 771.Dl name ( ) command 772.Pp 773A function definition is an executable statement; when executed it 774installs a function named name and returns an exit status of zero. 775The command is normally a list enclosed between 776.Dq { 777and 778.Dq } . 779.Pp 780Variables may be declared to be local to a function by using a local 781command. 782This should appear as the first statement of a function, and the syntax is 783.Pp 784.Dl local [ variable | - ] ... 785.Pp 786.Dq Local 787is implemented as a built-in command. 788.Pp 789When a variable is made local, it inherits the initial value and exported 790and read-only flags from the variable with the same name in the surrounding 791scope, if there is one. 792Otherwise, the variable is initially unset. 793The shell uses dynamic scoping, so that if you make the variable x local to 794function f, which then calls function g, references to the variable x made 795inside g will refer to the variable x declared inside f, not to the global 796variable named x. 797.Pp 798The only special parameter that can be made local is 799.Dq - . 800Making 801.Dq - 802local causes any shell options that are changed via the set command inside the 803function to be restored to their original values when the function 804returns. 805.Pp 806The syntax of the return command is 807.Pp 808.Dl return [ exitstatus ] 809.Pp 810It terminates the currently executing function. 811Return is implemented as a built-in command. 812.Ss Variables and Parameters 813The shell maintains a set of parameters. 814A parameter denoted by a name is called a variable. 815When starting up, the shell turns all the environment 816variables into shell variables. 817New variables can be set using the form 818.Pp 819.Dl name=value 820.Pp 821Variables set by the user must have a name consisting solely of 822alphabetics, numerics, and underscores - the first of which must not be 823numeric. 824A parameter can also be denoted by a number or a special 825character as explained below. 826.Ss Positional Parameters 827A positional parameter is a parameter denoted by a number (n \*[Gt] 0). 828The shell sets these initially to the values of its command line arguments 829that follow the name of the shell script. 830The 831.Ic set 832built-in can also be used to set or reset them. 833.Ss Special Parameters 834A special parameter is a parameter denoted by one of the following special 835characters. 836The value of the parameter is listed next to its character. 837.Bl -tag -width thinhyphena 838.It * 839Expands to the positional parameters, starting from one. 840When the 841expansion occurs within a double-quoted string it expands to a single 842field with the value of each parameter separated by the first character of 843the 844.Ev IFS 845variable, or by a 846.Aq space 847if 848.Ev IFS 849is unset. 850.It @ 851Expands to the positional parameters, starting from one. 852When the expansion occurs within double-quotes, each positional 853parameter expands as a separate argument. 854If there are no positional parameters, the 855expansion of @ generates zero arguments, even when @ is 856double-quoted. 857What this basically means, for example, is 858if $1 is 859.Dq abc 860and $2 is 861.Dq def ghi , 862then 863.Qq $@ 864expands to 865the two arguments: 866.Pp 867.Sm off 868.Dl \*q abc \*q \ \*q def\ ghi \*q 869.Sm on 870.It # 871Expands to the number of positional parameters. 872.It \&? 873Expands to the exit status of the most recent pipeline. 874.It - (Hyphen.) 875Expands to the current option flags (the single-letter 876option names concatenated into a string) as specified on 877invocation, by the set built-in command, or implicitly 878by the shell. 879.It $ 880Expands to the process ID of the invoked shell. 881A subshell retains the same value of $ as its parent. 882.It \&! 883Expands to the process ID of the most recent background 884command executed from the current shell. 885For a pipeline, the process ID is that of the last command in the pipeline. 886.It 0 (Zero.) 887Expands to the name of the shell or shell script. 888.El 889.Ss Word Expansions 890This clause describes the various expansions that are performed on words. 891Not all expansions are performed on every word, as explained later. 892.Pp 893Tilde expansions, parameter expansions, command substitutions, arithmetic 894expansions, and quote removals that occur within a single word expand to a 895single field. 896It is only field splitting or pathname expansion that can 897create multiple fields from a single word. 898The single exception to this 899rule is the expansion of the special parameter @ within double-quotes, as 900was described above. 901.Pp 902The order of word expansion is: 903.Bl -enum 904.It 905Tilde Expansion, Parameter Expansion, Command Substitution, 906Arithmetic Expansion (these all occur at the same time). 907.It 908Field Splitting is performed on fields 909generated by step (1) unless the 910.Ev IFS 911variable is null. 912.It 913Pathname Expansion (unless set 914.Fl f 915is in effect). 916.It 917Quote Removal. 918.El 919.Pp 920The $ character is used to introduce parameter expansion, command 921substitution, or arithmetic evaluation. 922.Ss Tilde Expansion (substituting a user's home directory) 923A word beginning with an unquoted tilde character (~) is 924subjected to tilde expansion. 925All the characters up to 926a slash (/) or the end of the word are treated as a username 927and are replaced with the user's home directory. 928If the username is missing (as in 929.Pa ~/foobar ) , 930the tilde is replaced with the value of the 931.Va HOME 932variable (the current user's home directory). 933.Ss Parameter Expansion 934The format for parameter expansion is as follows: 935.Pp 936.Dl ${expression} 937.Pp 938where expression consists of all characters until the matching 939.Dq } . 940Any 941.Dq } 942escaped by a backslash or within a quoted string, and characters in 943embedded arithmetic expansions, command substitutions, and variable 944expansions, are not examined in determining the matching 945.Dq } . 946.Pp 947The simplest form for parameter expansion is: 948.Pp 949.Dl ${parameter} 950.Pp 951The value, if any, of parameter is substituted. 952.Pp 953The parameter name or symbol can be enclosed in braces, which are 954optional except for positional parameters with more than one digit or 955when parameter is followed by a character that could be interpreted as 956part of the name. 957If a parameter expansion occurs inside double-quotes: 958.Bl -enum 959.It 960Pathname expansion is not performed on the results of the expansion. 961.It 962Field splitting is not performed on the results of the 963expansion, with the exception of the special rules for @. 964.El 965.Pp 966In addition, a parameter expansion can be modified by using one of the 967following formats. 968If the 969.Dq Dv \&: 970is omitted in the following modifiers, then the expansion is applied only 971to unset parameters, not null ones. 972.Bl -tag -width aaparameterwordaaaaa 973.It ${parameter:-word} 974Use Default Values. 975If parameter is unset or null, the expansion of word 976is substituted; otherwise, the value of parameter is substituted. 977.It ${parameter:=word} 978Assign Default Values. 979If parameter is unset or null, the expansion of 980word is assigned to parameter. 981In all cases, the final value of parameter is substituted. 982Only variables, not positional parameters or special 983parameters, can be assigned in this way. 984.It ${parameter:?[word]} 985Indicate Error if Null or Unset. 986If parameter is unset or null, the 987expansion of word (or a message indicating it is unset if word is omitted) 988is written to standard error and the shell exits with a nonzero exit status. 989Otherwise, the value of parameter is substituted. 990An interactive shell need not exit. 991.It ${parameter:+word} 992Use Alternative Value. 993If parameter is unset or null, null is 994substituted; otherwise, the expansion of word is substituted. 995.El 996.Pp 997.Bl -tag -width aaparameterwordaaaaa 998.It ${#parameter} 999String Length. 1000The length in characters of the value of parameter. 1001.El 1002.Pp 1003The following four varieties of parameter expansion provide for substring 1004processing. 1005In each case, pattern matching notation (see 1006.Sx Shell Patterns ) , 1007rather than regular expression notation, is used to evaluate the patterns. 1008If parameter is * or @, the result of the expansion is unspecified. 1009Enclosing the full parameter expansion string in double-quotes does not 1010cause the following four varieties of pattern characters to be quoted, 1011whereas quoting characters within the braces has this effect. 1012.Bl -tag -width aaparameterwordaaaaa 1013.It ${parameter%word} 1014Remove Smallest Suffix Pattern. 1015The word is expanded to produce a pattern. 1016The parameter expansion then results in parameter, with the 1017smallest portion of the suffix matched by the pattern deleted. 1018.It ${parameter%%word} 1019Remove Largest Suffix Pattern. 1020The word is expanded to produce a pattern. 1021The parameter expansion then results in parameter, with the largest 1022portion of the suffix matched by the pattern deleted. 1023.It ${parameter#word} 1024Remove Smallest Prefix Pattern. 1025The word is expanded to produce a pattern. 1026The parameter expansion then results in parameter, with the 1027smallest portion of the prefix matched by the pattern deleted. 1028.It ${parameter##word} 1029Remove Largest Prefix Pattern. 1030The word is expanded to produce a pattern. 1031The parameter expansion then results in parameter, with the largest 1032portion of the prefix matched by the pattern deleted. 1033.El 1034.Ss Command Substitution 1035Command substitution allows the output of a command to be substituted in 1036place of the command name itself. 1037Command substitution occurs when the command is enclosed as follows: 1038.Pp 1039.Dl $(command) 1040.Pp 1041or 1042.Po 1043.Dq backquoted 1044version 1045.Pc : 1046.Pp 1047.Dl `command` 1048.Pp 1049The shell expands the command substitution by executing command in a 1050subshell environment and replacing the command substitution with the 1051standard output of the command, removing sequences of one or more 1052.Ao newline Ac Ns s 1053at the end of the substitution. 1054(Embedded 1055.Ao newline Ac Ns s 1056before 1057the end of the output are not removed; however, during field splitting, 1058they may be translated into 1059.Ao space Ac Ns s , 1060depending on the value of 1061.Ev IFS 1062and quoting that is in effect.) 1063.Ss Arithmetic Expansion 1064Arithmetic expansion provides a mechanism for evaluating an arithmetic 1065expression and substituting its value. 1066The format for arithmetic expansion is as follows: 1067.Pp 1068.Dl $((expression)) 1069.Pp 1070The expression is treated as if it were in double-quotes, except 1071that a double-quote inside the expression is not treated specially. 1072The shell expands all tokens in the expression for parameter expansion, 1073command substitution, and quote removal. 1074.Pp 1075Next, the shell treats this as an arithmetic expression and 1076substitutes the value of the expression. 1077.Pp 1078Arithmetic expressions use a syntax similar to that 1079of the C language, and are evaluated using the 1080.Ql intmax_t 1081data type (this is an extension to 1082.Tn POSIX , 1083which requires only 1084.Ql long 1085arithmetic). 1086Shell variables may be referenced by name inside an arithmetic 1087expression, without needing a 1088.Dq \&$ 1089sign. 1090.Ss White Space Splitting (Field Splitting) 1091After parameter expansion, command substitution, and 1092arithmetic expansion the shell scans the results of 1093expansions and substitutions that did not occur in double-quotes for 1094field splitting and multiple fields can result. 1095.Pp 1096The shell treats each character of the 1097.Ev IFS 1098as a delimiter and use the delimiters to split the results of parameter 1099expansion and command substitution into fields. 1100.Pp 1101Non-whitespace characters in 1102.Ev IFS 1103are treated strictly as parameter terminators. 1104So adjacent non-whitespace 1105.Ev IFS 1106characters will produce empty parameters. 1107.Pp 1108If 1109.Ev IFS 1110is unset it is assumed to contain space, tab, and newline. 1111.Ss Pathname Expansion (File Name Generation) 1112Unless the 1113.Fl f 1114flag is set, file name generation is performed after word splitting is 1115complete. 1116Each word is viewed as a series of patterns, separated by slashes. 1117The process of expansion replaces the word with the names of all 1118existing files whose names can be formed by replacing each pattern with a 1119string that matches the specified pattern. 1120There are two restrictions on 1121this: first, a pattern cannot match a string containing a slash, and 1122second, a pattern cannot match a string starting with a period unless the 1123first character of the pattern is a period. 1124The next section describes the 1125patterns used for both Pathname Expansion and the 1126.Ic case 1127command. 1128.Ss Shell Patterns 1129A pattern consists of normal characters, which match themselves, 1130and meta-characters. 1131The meta-characters are 1132.Dq \&! , 1133.Dq * , 1134.Dq \&? , 1135and 1136.Dq \&[ . 1137These characters lose their special meanings if they are quoted. 1138When command or variable substitution is performed 1139and the dollar sign or back quotes are not double quoted, 1140the value of the variable or the output of 1141the command is scanned for these characters and they are turned into 1142meta-characters. 1143.Pp 1144An asterisk 1145.Pq Dq * 1146matches any string of characters. 1147A question mark matches any single character. 1148A left bracket 1149.Pq Dq \&[ 1150introduces a character class. 1151The end of the character class is indicated by a 1152.Pq Dq \&] ; 1153if the 1154.Dq \&] 1155is missing then the 1156.Dq \&[ 1157matches a 1158.Dq \&[ 1159rather than introducing a character class. 1160A character class matches any of the characters between the square brackets. 1161A range of characters may be specified using a minus sign. 1162The character class may be complemented 1163by making an exclamation point the first character of the character class. 1164.Pp 1165To include a 1166.Dq \&] 1167in a character class, make it the first character listed (after the 1168.Dq \&! , 1169if any). 1170To include a minus sign, make it the first or last character listed. 1171.Ss Built-ins 1172This section lists the built-in commands which are built-in because they 1173need to perform some operation that can't be performed by a separate 1174process. 1175In addition to these, there are several other commands that may 1176be built in for efficiency (e.g. 1177.Xr printf 1 , 1178.Xr echo 1 , 1179.Xr test 1 , 1180etc). 1181.Bl -tag -width 5n 1182.It : 1183A null command that returns a 0 (true) exit value. 1184.It \&. file 1185The commands in the specified file are read and executed by the shell. 1186.It alias Op Ar name Ns Op Ar "=string ..." 1187If 1188.Ar name=string 1189is specified, the shell defines the alias 1190.Ar name 1191with value 1192.Ar string . 1193If just 1194.Ar name 1195is specified, the value of the alias 1196.Ar name 1197is printed. 1198With no arguments, the 1199.Ic alias 1200built-in prints the 1201names and values of all defined aliases (see 1202.Ic unalias ) . 1203.It bg [ Ar job ] ... 1204Continue the specified jobs (or the current job if no 1205jobs are given) in the background. 1206.It command Oo Fl p Oc Oo Fl v Oc Oo Fl V Oc Ar command Oo Ar arg ... Oc 1207Execute the specified command but ignore shell functions when searching 1208for it. 1209(This is useful when you 1210have a shell function with the same name as a built-in command.) 1211.Bl -tag -width 5n 1212.It Fl p 1213search for command using a 1214.Ev PATH 1215that guarantees to find all the standard utilities. 1216.It Fl V 1217Do not execute the command but 1218search for the command and print the resolution of the 1219command search. 1220This is the same as the 1221.Ic type 1222built-in. 1223.It Fl v 1224Do not execute the command but 1225search for the command and print the absolute pathname 1226of utilities, the name for built-ins or the expansion of aliases. 1227.El 1228.It cd Oo Fl P Oc Op Ar directory Op Ar replace 1229Switch to the specified directory (default 1230.Ev $HOME ) . 1231If 1232.Ar replace 1233is specified, then the new directory name is generated by replacing 1234the first occurrence of 1235.Ar directory 1236in the current directory name with 1237.Ar replace . 1238If 1239.Ar directory 1240is 1241.Sq - , 1242then the current working directory is changed to the previous current 1243working directory as set in 1244.Ev OLDPWD . 1245Otherwise if an entry for 1246.Ev CDPATH 1247appears in the environment of the 1248.Ic cd 1249command or the shell variable 1250.Ev CDPATH 1251is set and the directory name does not begin with a slash, 1252or its first (or only) component isn't dot or dot dot, 1253then the directories listed in 1254.Ev CDPATH 1255will be searched for the specified directory. 1256The format of 1257.Ev CDPATH 1258is the same as that of 1259.Ev PATH . 1260.Pp 1261The 1262.Fl P 1263option instructs the shell to update 1264.Ev PWD 1265with the specified physical directory path and change to that directory. 1266This is the default. 1267.Pp 1268When the directory changes, the variable 1269.Ev OLDPWD 1270is set to the working directory before the change. 1271.Pp 1272Some shells also support a 1273.Fl L 1274option, which instructs the shell to update 1275.Ev PWD 1276with the logical path and to change the current directory 1277accordingly. 1278This is not supported. 1279.Pp 1280In an interactive shell, the 1281.Ic cd 1282command will print out the name of the 1283directory that it actually switched to if this is different from the name 1284that the user gave. 1285These may be different either because the 1286.Ev CDPATH 1287mechanism was used or because a symbolic link was crossed. 1288.It eval Ar string ... 1289Concatenate all the arguments with spaces. 1290Then re-parse and execute the command. 1291.It exec Op Ar command arg ... 1292Unless command is omitted, the shell process is replaced with the 1293specified program (which must be a real program, not a shell built-in or 1294function). 1295Any redirections on the 1296.Ic exec 1297command are marked as permanent, so that they are not undone when the 1298.Ic exec 1299command finishes. 1300.It exit Op Ar exitstatus 1301Terminate the shell process. 1302If 1303.Ar exitstatus 1304is given it is used as the exit status of the shell; otherwise the 1305exit status of the preceding command is used. 1306.It export Ar name ... 1307.It export Fl p 1308The specified names are exported so that they will appear in the 1309environment of subsequent commands. 1310The only way to un-export a variable is to unset it. 1311The shell allows the value of a variable to be set at the 1312same time it is exported by writing 1313.Pp 1314.Dl export name=value 1315.Pp 1316With no arguments the export command lists the names of all exported variables. 1317With the 1318.Fl p 1319option specified the output will be formatted suitably for non-interactive use. 1320.It fc Oo Fl e Ar editor Oc Oo Ar first Oo Ar last Oc Oc 1321.It fc Fl l Oo Fl nr Oc Oo Ar first Oo Ar last Oc Oc 1322.It fc Fl s Oo Ar old=new Oc Oo Ar first Oc 1323The 1324.Ic fc 1325built-in lists, or edits and re-executes, commands previously entered 1326to an interactive shell. 1327.Bl -tag -width 5n 1328.It Fl e No editor 1329Use the editor named by editor to edit the commands. 1330The editor string is a command name, subject to search via the 1331.Ev PATH 1332variable. 1333The value in the 1334.Ev FCEDIT 1335variable is used as a default when 1336.Fl e 1337is not specified. 1338If 1339.Ev FCEDIT 1340is null or unset, the value of the 1341.Ev EDITOR 1342variable is used. 1343If 1344.Ev EDITOR 1345is null or unset, 1346.Xr ed 1 1347is used as the editor. 1348.It Fl l No (ell) 1349List the commands rather than invoking an editor on them. 1350The commands are written in the sequence indicated by 1351the first and last operands, as affected by 1352.Fl r , 1353with each command preceded by the command number. 1354.It Fl n 1355Suppress command numbers when listing with -l. 1356.It Fl r 1357Reverse the order of the commands listed (with 1358.Fl l ) 1359or edited (with neither 1360.Fl l 1361nor 1362.Fl s ) . 1363.It Fl s 1364Re-execute the command without invoking an editor. 1365.It first 1366.It last 1367Select the commands to list or edit. 1368The number of previous commands that 1369can be accessed are determined by the value of the 1370.Ev HISTSIZE 1371variable. 1372The value of first or last or both are one of the following: 1373.Bl -tag -width 5n 1374.It [+]number 1375A positive number representing a command number; command numbers can be 1376displayed with the 1377.Fl l 1378option. 1379.It Fl number 1380A negative decimal number representing the command that was executed 1381number of commands previously. 1382For example, \-1 is the immediately previous command. 1383.El 1384.It string 1385A string indicating the most recently entered command that begins with 1386that string. 1387If the old=new operand is not also specified with 1388.Fl s , 1389the string form of the first operand cannot contain an embedded equal sign. 1390.El 1391.Pp 1392The following environment variables affect the execution of fc: 1393.Bl -tag -width HISTSIZE 1394.It Ev FCEDIT 1395Name of the editor to use. 1396.It Ev HISTSIZE 1397The number of previous commands that are accessible. 1398.El 1399.It fg Op Ar job 1400Move the specified job or the current job to the foreground. 1401.It getopts Ar optstring var 1402The 1403.Tn POSIX 1404.Ic getopts 1405command, not to be confused with the 1406.Em Bell Labs 1407-derived 1408.Xr getopt 1 . 1409.Pp 1410The first argument should be a series of letters, each of which may be 1411optionally followed by a colon to indicate that the option requires an 1412argument. 1413The variable specified is set to the parsed option. 1414.Pp 1415The 1416.Ic getopts 1417command deprecates the older 1418.Xr getopt 1 1419utility due to its handling of arguments containing whitespace. 1420.Pp 1421The 1422.Ic getopts 1423built-in may be used to obtain options and their arguments 1424from a list of parameters. 1425When invoked, 1426.Ic getopts 1427places the value of the next option from the option string in the list in 1428the shell variable specified by 1429.Va var 1430and its index in the shell variable 1431.Ev OPTIND . 1432When the shell is invoked, 1433.Ev OPTIND 1434is initialized to 1. 1435For each option that requires an argument, the 1436.Ic getopts 1437built-in will place it in the shell variable 1438.Ev OPTARG . 1439If an option is not allowed for in the 1440.Va optstring , 1441then 1442.Ev OPTARG 1443will be unset. 1444.Pp 1445.Va optstring 1446is a string of recognized option letters (see 1447.Xr getopt 3 ) . 1448If a letter is followed by a colon, the option is expected to have an 1449argument which may or may not be separated from it by whitespace. 1450If an option character is not found where expected, 1451.Ic getopts 1452will set the variable 1453.Va var 1454to a 1455.Dq \&? ; 1456.Ic getopts 1457will then unset 1458.Ev OPTARG 1459and write output to standard error. 1460By specifying a colon as the first character of 1461.Va optstring 1462all errors will be ignored. 1463.Pp 1464A nonzero value is returned when the last option is reached. 1465If there are no remaining arguments, 1466.Ic getopts 1467will set 1468.Va var 1469to the special option, 1470.Dq -- , 1471otherwise, it will set 1472.Va var 1473to 1474.Dq \&? . 1475.Pp 1476The following code fragment shows how one might process the arguments 1477for a command that can take the options 1478.Op a 1479and 1480.Op b , 1481and the option 1482.Op c , 1483which requires an argument. 1484.Pp 1485.Bd -literal -offset indent 1486while getopts abc: f 1487do 1488 case $f in 1489 a | b) flag=$f;; 1490 c) carg=$OPTARG;; 1491 \e?) echo $USAGE; exit 1;; 1492 esac 1493done 1494shift $(expr $OPTIND - 1) 1495.Ed 1496.Pp 1497This code will accept any of the following as equivalent: 1498.Pp 1499.Bd -literal -offset indent 1500cmd \-acarg file file 1501cmd \-a \-c arg file file 1502cmd \-carg -a file file 1503cmd \-a \-carg \-\- file file 1504.Ed 1505.It hash Fl rv Ar command ... 1506The shell maintains a hash table which remembers the 1507locations of commands. 1508With no arguments whatsoever, 1509the 1510.Ic hash 1511command prints out the contents of this table. 1512Entries which have not been looked at since the last 1513.Ic cd 1514command are marked with an asterisk; it is possible for these entries 1515to be invalid. 1516.Pp 1517With arguments, the 1518.Ic hash 1519command removes the specified commands from the hash table (unless 1520they are functions) and then locates them. 1521With the 1522.Fl v 1523option, hash prints the locations of the commands as it finds them. 1524The 1525.Fl r 1526option causes the hash command to delete all the entries in the hash table 1527except for functions. 1528.It inputrc Ar file 1529Read the 1530.Va file 1531to set keybindings as defined by 1532.Xr editrc 5 . 1533.It jobid Op Ar job 1534Print the process id's of the processes in the job. 1535If the 1536.Ar job 1537argument is omitted, the current job is used. 1538.It jobs 1539This command lists out all the background processes 1540which are children of the current shell process. 1541.It pwd Op Fl \&LP 1542Print the current directory. 1543If 1544.Fl L 1545is specified the cached value (initially set from 1546.Ev PWD ) 1547is checked to see if it refers to the current directory; if it does 1548the value is printed. 1549Otherwise the current directory name is found using 1550.Xr getcwd 3 . 1551The environment variable 1552.Ev PWD 1553is set to the printed value. 1554.Pp 1555The default is 1556.Ic pwd 1557.Fl L , 1558but note that the built-in 1559.Ic cd 1560command doesn't currently support the 1561.Fl L 1562option and will cache (almost) the absolute path. 1563If 1564.Ic cd 1565is changed, 1566.Ic pwd 1567may be changed to default to 1568.Ic pwd 1569.Fl P . 1570.Pp 1571If the current directory is renamed and replaced by a symlink to the 1572same directory, or the initial 1573.Ev PWD 1574value followed a symbolic link, then the cached value may not 1575be the absolute path. 1576.Pp 1577The built-in command may differ from the program of the same name because 1578the program will use 1579.Ev PWD 1580and the built-in uses a separately cached value. 1581.It read Oo Fl p Ar prompt Oc Oo Fl r Oc Ar variable Oo Ar ... Oc 1582The prompt is printed if the 1583.Fl p 1584option is specified and the standard input is a terminal. 1585Then a line is read from the standard input. 1586The trailing newline is deleted from the 1587line and the line is split as described in the section on word splitting 1588above, and the pieces are assigned to the variables in order. 1589If there are more pieces than variables, the remaining pieces 1590(along with the characters in 1591.Ev IFS 1592that separated them) are assigned to the last variable. 1593If there are more variables than pieces, 1594the remaining variables are assigned the null string. 1595The 1596.Ic read 1597built-in will indicate success unless EOF is encountered on input, in 1598which case failure is returned. 1599.Pp 1600By default, unless the 1601.Fl r 1602option is specified, the backslash 1603.Dq \e 1604acts as an escape character, causing the following character to be treated 1605literally. 1606If a backslash is followed by a newline, the backslash and the 1607newline will be deleted. 1608.It readonly Ar name ... 1609.It readonly Fl p 1610The specified names are marked as read only, so that they cannot be 1611subsequently modified or unset. 1612The shell allows the value of a variable 1613to be set at the same time it is marked read only by writing 1614.Pp 1615.Dl readonly name=value 1616.Pp 1617With no arguments the readonly command lists the names of all read only 1618variables. 1619With the 1620.Fl p 1621option specified the output will be formatted suitably for non-interactive use. 1622.Pp 1623.It set Oo { Fl options | Cm +options | Cm \-- } Oc Ar arg ... 1624The 1625.Ic set 1626command performs three different functions. 1627.Pp 1628With no arguments, it lists the values of all shell variables. 1629.Pp 1630If options are given, it sets the specified option 1631flags, or clears them as described in the section called 1632.Sx Argument List Processing . 1633.Pp 1634The third use of the set command is to set the values of the shell's 1635positional parameters to the specified arguments. 1636To change the positional 1637parameters without changing any options, use 1638.Dq -- 1639as the first argument to set. 1640If no arguments are present, the set command 1641will clear all the positional parameters (equivalent to executing 1642.Dq shift $# . ) 1643.It setvar Ar variable Ar value 1644Assigns value to variable. 1645(In general it is better to write 1646variable=value rather than using 1647.Ic setvar . 1648.Ic setvar 1649is intended to be used in 1650functions that assign values to variables whose names are passed as 1651parameters.) 1652.It shift Op Ar n 1653Shift the positional parameters n times. 1654A 1655.Ic shift 1656sets the value of 1657.Va $1 1658to the value of 1659.Va $2 , 1660the value of 1661.Va $2 1662to the value of 1663.Va $3 , 1664and so on, decreasing 1665the value of 1666.Va $# 1667by one. 1668If there are zero positional parameters, 1669.Ic shift 1670does nothing. 1671.It trap Oo Fl l Oc 1672.It trap Oo Ar action Oc Ar signal ... 1673Cause the shell to parse and execute action when any of the specified 1674signals are received. 1675The signals are specified by signal number or as the name of the signal. 1676If 1677.Ar signal 1678is 1679.Li 0 1680or its equivalent, EXIT, 1681the action is executed when the shell exits. 1682.Ar action 1683may be null, which cause the specified signals to be ignored. 1684With 1685.Ar action 1686omitted or set to 1687.Sq - 1688the specified signals are set to their default action. 1689When the shell forks off a subshell, it resets trapped (but not ignored) 1690signals to the default action. 1691On non-interactive shells, the 1692.Ic trap 1693command has no effect on signals that were 1694ignored on entry to the shell. 1695On interactive shells, the 1696.Ic trap 1697command will catch or reset signals ignored on entry. 1698Issuing 1699.Ic trap 1700with option 1701.Ar -l 1702will print a list of valid signal names. 1703.Ic trap 1704without any arguments cause it to write a list of signals and their 1705associated action to the standard output in a format that is suitable 1706as an input to the shell that achieves the same trapping results. 1707.Pp 1708Examples: 1709.Pp 1710.Dl trap 1711.Pp 1712List trapped signals and their corresponding action 1713.Pp 1714.Dl trap -l 1715.Pp 1716Print a list of valid signals 1717.Pp 1718.Dl trap '' INT QUIT tstp 30 1719.Pp 1720Ignore signals INT QUIT TSTP USR1 1721.Pp 1722.Dl trap date INT 1723.Pp 1724Print date upon receiving signal INT 1725.It type Op Ar name ... 1726Interpret each name as a command and print the resolution of the command 1727search. 1728Possible resolutions are: 1729shell keyword, alias, shell built-in, 1730command, tracked alias and not found. 1731For aliases the alias expansion is 1732printed; for commands and tracked aliases the complete pathname of the 1733command is printed. 1734.It ulimit Oo Fl H \*(Ba Fl S Oc Oo Fl a \*(Ba Fl tfdscmlpnv Oo Ar value Oc Oc 1735Inquire about or set the hard or soft limits on processes or set new 1736limits. 1737The choice between hard limit (which no process is allowed to 1738violate, and which may not be raised once it has been lowered) and soft 1739limit (which causes processes to be signaled but not necessarily killed, 1740and which may be raised) is made with these flags: 1741.Bl -tag -width Fl 1742.It Fl H 1743set or inquire about hard limits 1744.It Fl S 1745set or inquire about soft limits. 1746If neither 1747.Fl H 1748nor 1749.Fl S 1750is specified, the soft limit is displayed or both limits are set. 1751If both are specified, the last one wins. 1752.El 1753.Pp 1754The limit to be interrogated or set, then, is chosen by specifying 1755any one of these flags: 1756.Bl -tag -width Fl 1757.It Fl a 1758show all the current limits 1759.It Fl b 1760show or set the limit on the socket buffer size of a process (in bytes) 1761.It Fl t 1762show or set the limit on CPU time (in seconds) 1763.It Fl f 1764show or set the limit on the largest file that can be created 1765(in 512-byte blocks) 1766.It Fl d 1767show or set the limit on the data segment size of a process (in kilobytes) 1768.It Fl s 1769show or set the limit on the stack size of a process (in kilobytes) 1770.It Fl c 1771show or set the limit on the largest core dump size that can be produced 1772(in 512-byte blocks) 1773.It Fl m 1774show or set the limit on the total physical memory that can be 1775in use by a process (in kilobytes) 1776.It Fl l 1777show or set the limit on how much memory a process can lock with 1778.Xr mlock 2 1779(in kilobytes) 1780.It Fl p 1781show or set the limit on the number of processes this user can 1782have at one time 1783.It Fl n 1784show or set the limit on the number of files a process can have open at once 1785.It Fl v 1786show or set the limit on how large a process address space can be 1787.El 1788.Pp 1789If none of these is specified, it is the limit on file size that is shown 1790or set. 1791If value is specified, the limit is set to that number; otherwise 1792the current limit is displayed. 1793.Pp 1794Limits of an arbitrary process can be displayed or set using the 1795.Xr sysctl 8 1796utility. 1797.Pp 1798.It umask Op Ar mask 1799Set the value of umask (see 1800.Xr umask 2 ) 1801to the specified octal value. 1802If the argument is omitted, the umask value is printed. 1803.It unalias Oo Fl a Oc Oo Ar name Oc 1804If 1805.Ar name 1806is specified, the shell removes that alias. 1807If 1808.Fl a 1809is specified, all aliases are removed. 1810.It unset Ar name ... 1811The specified variables and functions are unset and unexported. 1812If a given name corresponds to both a variable and a function, both 1813the variable and the function are unset. 1814.It wait Op Ar job 1815Wait for the specified job to complete and return the exit status of the 1816last process in the job. 1817If the argument is omitted, wait for all jobs to 1818complete and then return an exit status of zero. 1819.El 1820.Ss Command Line Editing 1821When 1822.Nm 1823is being used interactively from a terminal, the current command 1824and the command history (see 1825.Ic fc 1826in 1827.Sx Built-ins ) 1828can be edited using emacs-mode or vi-mode command-line editing. 1829The command 1830.Ql set -o emacs 1831enables emacs-mode editing. 1832The command 1833.Ql set -o vi 1834enables vi-mode editing and places the current shell process into 1835.Ar vi 1836insert mode. 1837(See the 1838.Sx Argument List Processing 1839section above.) 1840.Pp 1841The 1842.Ar vi 1843mode uses commands similar to a subset of those described in the 1844.Xr vi 1 1845man page. 1846With vi-mode 1847enabled, 1848.Nm sh 1849can be switched between insert mode and command mode. 1850It's similar to 1851.Xr vi 1 : 1852pressing the 1853.Aq ESC 1854key will throw you into command VI command mode. 1855Pressing the 1856.Aq return 1857key while in command mode will pass the line to the shell. 1858.Pp 1859The 1860.Ar emacs 1861mode uses commands similar to a subset available in 1862the 1863.Xr emacs 1 1864editor. 1865With emacs-mode enabled, special keys can be used to modify the text 1866in the buffer using the control key. 1867.Pp 1868.Nm 1869uses the 1870.Xr editline 3 1871library. 1872.Sh ENVIRONMENT 1873.Bl -tag -width MAILCHECK 1874.It Ev HOME 1875Set automatically by 1876.Xr login 1 1877from the user's login directory in the password file 1878.Pq Xr passwd 5 . 1879This environment variable also functions as the default argument for the 1880.Ic cd 1881built-in. 1882.It Ev PATH 1883The default search path for executables. 1884See the above section 1885.Sx Path Search . 1886.It Ev CDPATH 1887The search path used with the 1888.Ic cd 1889built-in. 1890.It Ev LANG 1891The string used to specify localization information that allows users 1892to work with different culture-specific and language conventions. 1893See 1894.Xr nls 7 . 1895.It Ev MAIL 1896The name of a mail file, that will be checked for the arrival of new mail. 1897Overridden by 1898.Ev MAILPATH . 1899.It Ev MAILCHECK 1900The frequency in seconds that the shell checks for the arrival of mail 1901in the files specified by the 1902.Ev MAILPATH 1903or the 1904.Ev MAIL 1905file. 1906If set to 0, the check will occur at each prompt. 1907.It Ev MAILPATH 1908A colon 1909.Dq \&: 1910separated list of file names, for the shell to check for incoming mail. 1911This environment setting overrides the 1912.Ev MAIL 1913setting. 1914There is a maximum of 10 mailboxes that can be monitored at once. 1915.It Ev PS1 1916The primary prompt string, which defaults to 1917.Dq $ \ , 1918unless you are the superuser, in which case it defaults to 1919.Dq # \ . 1920.It Ev PS2 1921The secondary prompt string, which defaults to 1922.Dq \*[Gt] \ . 1923.It Ev PS4 1924Output before each line when execution trace (set -x) is enabled, 1925defaults to 1926.Dq + \ . 1927.It Ev IFS 1928Input Field Separators. 1929This is normally set to 1930.Aq space , 1931.Aq tab , 1932and 1933.Aq newline . 1934See the 1935.Sx White Space Splitting 1936section for more details. 1937.It Ev TERM 1938The default terminal setting for the shell. 1939This is inherited by 1940children of the shell, and is used in the history editing modes. 1941.It Ev HISTSIZE 1942The number of lines in the history buffer for the shell. 1943.El 1944.Sh FILES 1945.Bl -item -width HOMEprofilexxxx 1946.It 1947.Pa $HOME/.profile 1948.It 1949.Pa /etc/profile 1950.El 1951.Sh EXIT STATUS 1952Errors that are detected by the shell, such as a syntax error, will cause the 1953shell to exit with a non-zero exit status. 1954If the shell is not an 1955interactive shell, the execution of the shell file will be aborted. 1956Otherwise 1957the shell will return the exit status of the last command executed, or 1958if the exit built-in is used with a numeric argument, it will return the 1959argument. 1960.Sh SEE ALSO 1961.Xr csh 1 , 1962.Xr echo 1 , 1963.Xr getopt 1 , 1964.Xr ksh 1 , 1965.Xr login 1 , 1966.Xr printf 1 , 1967.Xr test 1 , 1968.Xr editline 3 , 1969.Xr getopt 3 , 1970.\" .Xr profile 4 , 1971.Xr editrc 5 , 1972.Xr passwd 5 , 1973.Xr environ 7 , 1974.Xr nls 7 , 1975.Xr sysctl 8 1976.Sh HISTORY 1977A 1978.Nm 1979command appeared in 1980.At v1 . 1981It was, however, unmaintainable so we wrote this one. 1982.Sh BUGS 1983Setuid shell scripts should be avoided at all costs, as they are a 1984significant security risk. 1985.Pp 1986PS1, PS2, and PS4 should be subject to parameter expansion before 1987being displayed. 1988.Pp 1989The characters generated by filename completion should probably be quoted 1990to ensure that the filename is still valid after the input line has been 1991processed. 1992