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Section: User Commands (1)
BSD mandoc


sh - command interpreter (shell) 


sh[-/+aCefnuvxIimqsVEbc][-o longname]-words[target ...] 


Sh is the standard command interpreter for the system. The current versionofshis in the process of being changed to conform with thePOSIX1003.2 and 1003.2a specifications for the shell. This version has manyfeatures which make it appear similar in some respects to the Korn shell,but it is not a Korn shell clone (seeksh(1)).Only features designated byPOSIX plus a few Berkeley extensions, are being incorporated into this shell.We expectPOSIXconformance by the time 4.4 BSD is released. This man page is not intendedto be a tutorial or a complete specification of the shell. 


The shell is a command that reads lines from either a file or theterminal, interprets them, and generally executes other commands. It isthe program that is running when a user logs into the system (although auser can select a different shell with thechsh(1)command). The shell implements a language that has flow controlconstructs, a macro facility that provides a variety of features inaddition to data storage. It incorporates many features to aid interactiveuse and has the advantage that the interpretative language is common to bothinteractive and non-interactive use (shell scripts). That is, commandscan be typed directly to the running shell or can be put into a file andthe file can be executed directly by the shell. 


If no args are present and if the standard input of the shellis connected to a terminal (or if the-iflag is set),and the -coption is not present, the shell is considered an interactive shell. Aninteractive shell generally prompts before each command and handlesprogramming and command errors differently (as described below). Whenfirst starting, the shell inspects argument 0, and if it begins with adash`-' the shell is also considereda login shell. This is normally done automatically by the systemwhen the user first logs in. A login shell first reads commandsfrom the files/etc/profileand.profileif they exist.If the environment variableENVis set on entry to a shell, or is set in the.profileof a login shell, the shell next readscommands from the file named inENV Therefore, a user should place commands that are to be executed only atlogin time in the.profilefile, and commands that are executed for every shell inside theENVfile. To set theENVvariable to some file, place the following line in your.profileof your home directory

ENV=$HOME/.shinit; export ENV

substituting for``.shinit''any filename you wish. Since theENVfile is read for every invocation of the shell, including shell scriptsand non-interactive shells, the following paradigm is useful forrestricting commands in theENVfile to interactive invocations. Place commands within the``case''and``esac''below (these commands are described later):

case $- in *i*)

# commands for interactive use only


If command line arguments besides the options have been specified, thenthe shell treats the first argument as the name of a file from which toread commands (a shell script), and the remaining arguments are set as thepositional parameters of the shell ($1, $2, etc). Otherwise, the shellreads commands from its standard input. 

Argument List Processing

All of the single letter options have a corresponding name that can beused as an argument to the-ooption. The set-oname is provided next to the single letter option inthe description below. Specifying a dash``-''turns the option on, while using a plus``+''disables the option.The following options can be set from the command line orwith theset(1)builtin (described later).

-a allexport
Export all variables assigned to. (UNIMPLEMENTED for 4.4alpha)
Read commands from the command line.No commands will be read from the standard input.
-C noclobber
Don't overwrite existing files with``>'' (UNIMPLEMENTED for 4.4alpha)
-e errexit
If not interactive, exit immediately if any untested command fails.The exit status of a command is considered to beexplicitly tested if the command is used to controlanif elif while oruntil or if the command is the left hand operand of an``&&''or``||''operator.
-f noglob
Disable pathname expansion.
-n noexec
If not interactive, read commands but do not execute them. This is usefulfor checking the syntax of shell scripts.
-u nounset
Write a message to standard error when attempting to expand a variablethat is not set, and if the shell is not interactive, exit immediately.(UNIMPLEMENTED for 4.4alpha)
-v verbose
The shell writes its input to standard error as it is read. Useful fordebugging.
-x xtrace
Write each command to standard error (precededby a`+ ' before it is executed. Useful for debugging.
-q quietprofile
If the-vor-xoptions have been set, do not apply them when readinginitialization files, these being/etc/profile .profile and the file specified by theENVenvironment variable.
-I ignoreeof
Ignore EOF's from input when interactive.
-i interactive
Force the shell to behave interactively.
-m monitor
Turn on job control (set automatically when interactive).
-s stdin
Read commands from standard input (set automatically if no file argumentsare present). This option has no effect when set after the shell hasalready started running (i.e. withset(1)).
-V vi
Enable the built-invi(1)command line editor (disables-Eif it has been set).
-E emacs
Enable the built-inemacs(1)command line editor (disables-Vif it has been set).
-b notify
Enable asynchronous notification of background job completion.(UNIMPLEMENTED for 4.4alpha)


Lexical Structure

The shell reads input in terms of lines from a file and breaks it up intowords at whitespace (blanks and tabs), and at certain sequences ofcharacters that are special to the shell called``operators'' There are two types of operators: control operators and redirectionoperators (their meaning is discussed later). Following is a list of operators:

"Control operators:"
& && ( ) ; ;; | || <newline>
"Redirection operator:"
< > >| << >> <& >& <<- <>



Quoting is used to remove the special meaning of certain characters orwords to the shell, such as operators, whitespace, or keywords. There arethree types of quoting: matched single quotes, matched double quotes, andbackslash. 


A backslash preserves the literal meaning of the followingcharacter, with the exception ofAq newline .A backslash preceding aAq newlineis treated as a line continuation. 

Single Quotes

Enclosing characters in single quotes preserves the literal meaning of allthe characters (except single quotes, making it impossible to putsingle-quotes in a single-quoted string). 

Double Quotes

Enclosing characters within double quotes preserves the literalmeaning of all characters except dollarsign($) backquote(`) and backslash(\) The backslash inside double quotes is historically weird, and serves toquote only the following characters:
$ ` \ <newline>
Otherwise it remains literal. 

Reserved Words

Reserved words are words that have special meaning to theshell and are recognized at the beginning of a line andafter a control operator. The following are reserved words:

! Ta elif Ta fi Ta while Ta case
else Ta for Ta then Ta { Ta }
do Ta done Ta until Ta if Ta esac

Their meaning is discussed later. 


An alias is a name and corresponding value set using thealias(1)builtin command. Whenever a reserved word may occur (see above),and after checking for reserved words, the shellchecks the word to see if it matches an alias. If it does,it replaces it in the input stream with its value. For example,if there is an alias called``lf''with the value``ls -F'' then the input:

lf foobar <return>

would become

ls -F foobar <return>

Aliases provide a convenient way for naive users to create shorthands forcommands without having to learn how to create functions with arguments.They can also be used to create lexically obscure code. This use isdiscouraged. 


The shell interprets the words it reads according to a language, thespecification of which is outside the scope of this man page (refer to theBNF in thePOSIX1003.2 document). Essentially though, a line is read and if the firstword of the line (or after a control operator) is not a reserved word,then the shell has recognized a simple command. Otherwise, a complexcommand or some other special construct may have been recognized. 

Simple Commands

If a simple command has been recognized, the shell performsthe following actions:

  1. Leading words of the form``name=value''are stripped off and assigned to the environment of the simple command.Redirection operators and their arguments (as described below) arestripped off and saved for processing.
  2. The remaining words are expanded as described inthe section called``Expansions'' and the first remaining word is considered the command name and thecommand is located. The remaining words are considered the arguments ofthe command. If no command name resulted, then the``name=value''variable assignments recognized in item 1 affect the current shell.
  3. Redirections are performed as described in the next section.



Redirections are used to change where a command reads its input or sendsits output. In general, redirections open, close, or duplicate anexisting reference to a file. The overall format used for redirection is:

[n] redir-op file

whereredir-opis one of the redirection operators mentioned previously. Following is alist of the possible redirections. TheBq nis an optional number, as in`3'(not`Bq 3' that refers to a file descriptor.

[n] > file
Redirect standard output (or n) to file.
[n] >| file
Same, but override the-Coption.
[n] >> file
Append standard output (or n) to file.
[n] < file
Redirect standard input (or n) from file.
[n1] <& n2
Duplicate standard input (or n1) from file descriptor n2.
[n] <&-
Close standard input (or n).
[n1] >& n2
Duplicate standard output (or n1) from n2.
[n] >&-
Close standard output (or n).
[n] <> file
Open file for reading and writing on standard input (or n).

The following redirection is often called a``here-document''

[n]<< delimiter

All the text on successive lines up to the delimiter is saved away andmade available to the command on standard input, or file descriptor n ifit is specified. If the delimiter as specified on the initial line isquoted, then the here-doc-text is treated literally, otherwise the text issubjected to parameter expansion, command substitution, and arithmeticexpansion (as described in the section on``Expansions )'' If the operator is``<<-''instead of``<<'' then leading tabs in the here-doc-text are stripped. 

Search and Execution

There are three types of commands: shell functions, builtin commands, andnormal programs -- and the command is searched for (by name) in thatorder. They each are executed in a different way.

When a shell function is executed, all of the shell positional parameters(except $0, which remains unchanged) are set to the arguments of the shellfunction. The variables which are explicitly placed in the environment ofthe command (by placing assignments to them before the function name) aremade local to the function and are set to the values given. Then thecommand given in the function definition is executed. The positionalparameters are restored to their original values when the commandcompletes. This all occurs within the current shell.

Shell builtins are executed internally to the shell, without spawning anew process.

Otherwise, if the command name doesn't match a function or builtin, thecommand is searched for as a normal program in the filesystem (asdescribed in the next section). When a normal program is executed, theshell runs the program, passing the arguments and the environment to theprogram. If the program is not a normal executable file (i.e., if it doesnot begin with the "magic number" whoseASCIIrepresentation is "#!", soexecve(2)returnsEr ENOEXECthen) the shell will interpret the program in a subshell. The child shellwill reinitialize itself in this case, so that the effect will be as if anew shell had been invoked to handle the ad-hoc shell script, except thatthe location of hashed commands located in the parent shell will beremembered by the child.

Note that previous versions of this document and the source code itselfmisleadingly and sporadically refer to a shell script without a magicnumber as a "shell procedure". 

Path Search

When locating a command, the shell first looks to see if it has a shellfunction by that name. Then it looks for a builtin command by that name.If a builtin command is not found, one of two things happen:

  1. Command names containing a slash are simply executed without performingany searches.
  2. The shell searches each entry inPATHin turn for the command. The value of thePATHvariable should be a series of entries separated by colons. Each entryconsists of a directory name. The current directory may be indicatedimplicitly by an empty directory name, or explicitly by a single period.


Command Exit Status

Each command has an exit status that can influence the behaviorof other shell commands. The paradigm is that a command exitswith zero for normal or success, and non-zero for failure,error, or a false indication. The man page for each commandshould indicate the various exit codes and what they mean.Additionally, the builtin commands return exit codes, as doesan executed shell function. 

Complex Commands

Complex commands are combinations of simple commands with controloperators or reserved words, together creating a larger complex command.More generally, a command is one of the following:

  • simple command
  • pipeline
  • list or compound-list
  • compound command
  • function definition

Unless otherwise stated, the exit status of a command is that of the lastsimple command executed by the command. 


A pipeline is a sequence of one or more commands separatedby the control operator |. The standard output of all butthe last command is connected to the standard inputof the next command. The standard output of the lastcommand is inherited from the shell, as usual.

The format for a pipeline is:

[!] command1 [ | command2 ...]

The standard output of command1 is connected to the standard input ofcommand2. The standard input, standard output, or both of a command isconsidered to be assigned by the pipeline before any redirection specifiedby redirection operators that are part of the command.

If the pipeline is not in the background (discussed later), the shellwaits for all commands to complete.

If the reserved word ! does not precede the pipeline, the exit status isthe exit status of the last command specified in the pipeline.Otherwise, the exit status is the logical NOT of the exit status of thelast command. That is, if the last command returns zero, the exit statusis 1; if the last command returns greater than zero, the exit status iszero.

Because pipeline assignment of standard input or standard output or bothtakes place before redirection, it can be modified by redirection. Forexample:

$ command1 2>&1 | command2

sends both the standard output and standard error of command1to the standard input of command2.

A ; or <newline> terminator causes the preceding AND-OR-list (describednext) to be executed sequentially; a & causes asynchronous execution ofthe preceding AND-OR-list.

Note that unlike some other shells, each process in the pipeline is achild of the invoking shell (unless it is a shell builtin, in which caseit executes in the current shell -- but any effect it has on theenvironment is wiped). 

Background Commands --

If a command is terminated by the control operator ampersand (&), theshell executes the command asynchronously -- that is, the shell does notwait for the command to finish before executing the next command.

The format for running a command in background is:

command1 & [command2 & ...]

If the shell is not interactive, the standard input of an asynchronouscommand is set to/dev/null  

Lists -- Generally Speaking

A list is a sequence of zero or more commands separated by newlines,semicolons, or ampersands, and optionally terminated by one of these threecharacters. The commands in a list are executed in the order they arewritten. If command is followed by an ampersand, the shell starts thecommand and immediately proceed onto the next command; otherwise it waitsfor the command to terminate before proceeding to the next one. 

Short-Circuit List Operators

``&&''and``||''are AND-OR list operators.``&&''executes the first command, and then executes the second command iff theexit status of the first command is zero.``||''is similar, but executes the second command iff the exit status of the firstcommand is nonzero.``&&''and``||''both have the same priority. 

Flow-Control Constructs -- if, while, for, case

The syntax of the if command is
if listthen list[ elif listthen    list ] ...[ else list ]fi

The syntax of the while command is

while listdo   listdone

The two lists are executed repeatedly while the exit status of thefirst list is zero. The until command is similar, but has the worduntil in place of while, which causes it torepeat until the exit status of the first list is zero.

The syntax of the for command is

for variable in   listdone

The words are expanded, and then the list is executed repeatedly with thevariable set to each word in turn. do and done may be replaced with``{''and``}''

The syntax of the break and continue command is

break [ num ]continue [ num ]

Break terminates the num innermost for or while loops.Continue continues with the next iteration of the innermost loop.These are implemented as builtin commands.

The syntax of the case command is

case word inpattern) list ;;...esac

The pattern can actually be one or more patterns (see ShellPatterns described later), separated by``''characters. 

Grouping Commands Together

Commands may be grouped by writing either



{ list;

The first of these executes the commands in a subshell. Builtin commandsgrouped into a (list) will not affect the current shell. The second formdoes not fork another shell so is slightly more efficient. Groupingcommands together this way allows you to redirect their output as thoughthey were one program:

{ printf  hello  ; printf  world\n" ; } > greeting



The syntax of a function definition is

name ( ) command

A function definition is an executable statement; when executed itinstalls a function named name and returns an exit status of zero. Thecommand is normally a list enclosed between``{''and``}''

Variables may be declared to be local to a function by using a localcommand. This should appear as the first statement of a function, and thesyntax is

local [ variable | - ] ...

Local is implemented as a builtin command.

When a variable is made local, it inherits the initial value and exportedand readonly flags from the variable with the same name in the surroundingscope, if there is one. Otherwise, the variable is initially unset. Theshell uses dynamic scoping, so that if you make the variable x local tofunction f, which then calls function g, references to the variable x madeinside g will refer to the variable x declared inside f, not to the globalvariable named x.

The only special parameter than can be made local is``-'' Making``-''local any shell options that are changed via the set command inside thefunction to be restored to their original values when the functionreturns.

The syntax of the return command is

return [ exitstatus

It terminates the currently executing function. Return isimplemented as a builtin command. 

Variables and Parameters

The shell maintains a set of parameters. A parameter denoted by a name iscalled a variable. When starting up, the shell turns all the environmentvariables into shell variables. New variables can be set using the form


Variables set by the user must have a name consisting solely ofalphabetics, numerics, and underscores - the first of which must not benumeric. A parameter can also be denoted by a number or a specialcharacter as explained below. 

Positional Parameters

A positional parameter is a parameter denoted by a number (n > 0). Theshell sets these initially to the values of its command line argumentsthat follow the name of the shell script. Theset(1)builtin can also be used to set or reset them. 

Special Parameters

A special parameter is a parameter denoted by one of the following specialcharacters. The value of the parameter is listed next to its character.

Expands to the positional parameters, starting from one. When theexpansion occurs within a double-quoted string it expands to a singlefield with the value of each parameter separated by the first character oftheIFSvariable, or by a <space> ifIFSis unset.
Expands to the positional parameters, starting from one. Whenthe expansion occurs within double-quotes, each positionalparameter expands as a separate argument.If there are no positional parameters, theexpansion of @ generates zero arguments, even when @ isdouble-quoted. What this basically means, for example, isif $1 is``abc''and $2 is``def ghi'' thenQq $@expands tothe two arguments:

abc   def ghi
Expands to the number of positional parameters.
Expands to the exit status of the most recent pipeline.
- (Hyphen.)
Expands to the current option flags (the single-letteroption names concatenated into a string) as specified oninvocation, by the set builtin command, or implicitlyby the shell.
Expands to the process ID of the invoked shell. A subshellretains the same value of $ as its parent.
Expands to the process ID of the most recent backgroundcommand executed from the current shell. For apipeline, the process ID is that of the last command in thepipeline.
0 (Zero.)
Expands to the name of the shell or shell script.


Word Expansions

This clause describes the various expansions that are performed on words.Not all expansions are performed on every word, as explained later.

Tilde expansions, parameter expansions, command substitutions, arithmeticexpansions, and quote removals that occur within a single word expand to asingle field. It is only field splitting or pathname expansion that cancreate multiple fields from a single word. The single exception to thisrule is the expansion of the special parameter @ within double-quotes, aswas described above.

The order of word expansion is:

  1. Tilde Expansion, Parameter Expansion, Command Substitution,Arithmetic Expansion (these all occur at the same time).
  2. Field Splitting is performed on fieldsgenerated by step (1) unless theIFSvariable is null.
  3. Pathname Expansion (unless set-fis in effect).
  4. Quote Removal.

The $ character is used to introduce parameter expansion, commandsubstitution, or arithmetic evaluation. 

Tilde Expansion (substituting a user's home directory)

A word beginning with an unquoted tilde character (~) issubjected to tilde expansion. All the characters up toa slash (/) or the end of the word are treated as a usernameand are replaced with the user's home directory. If theusername is missing (as in~/foobar ) the tilde is replaced with the value of theHOMEvariable (the current user's home directory). 

Parameter Expansion

The format for parameter expansion is as follows:


where expression consists of all characters until the matching``}'' Any``}''escaped by a backslash or within a quoted string, and characters inembedded arithmetic expansions, command substitutions, and variableexpansions, are not examined in determining the matching``}''

The simplest form for parameter expansion is:


The value, if any, of parameter is substituted.

The parameter name or symbol can be enclosed in braces, which areoptional except for positional parameters with more than one digit orwhen parameter is followed by a character that could be interpreted aspart of the name.If a parameter expansion occurs insidedouble-quotes:

  1. Pathname expansion is not performed on the results of theexpansion.
  2. Field splitting is not performed on the results of theexpansion, with the exception of @.

In addition, a parameter expansion can be modified by using one of thefollowing formats.

Use Default Values. If parameter is unset or null, the expansion of wordis substituted; otherwise, the value of parameter is substituted.
Assign Default Values. If parameter is unset or null, the expansion ofword is assigned to parameter. In all cases, the final value of parameteris substituted. Only variables, not positional parameters or specialparameters, can be assigned in this way.
Indicate Error if Null or Unset. If parameter is unset or null, theexpansion of word (or a message indicating it is unset if word is omitted)is written to standard error and the shell exits with a nonzero exitstatus. Otherwise, the value of parameter is substituted. An interactiveshell need not exit.
Use Alternative Value. If parameter is unset or null, null issubstituted; otherwise, the expansion of word is substituted.

In the parameter expansions shown previously, use of the colon in theformat results in a test for a parameter that is unset or null; omissionof the colon results in a test for a parameter that is only unset.

String Length. The length in characters ofthe value of parameter.

The following four varieties of parameter expansion provide for substringprocessing. In each case, pattern matching notation (see Shell Patterns),rather than regular expression notation, is used to evaluate the patterns.If parameter is * or @, the result of the expansion is unspecified.Enclosing the full parameter expansion string in double-quotes does notcause the following four varieties of pattern characters to be quoted,whereas quoting characters within the braces has this effect.

Remove Smallest Suffix Pattern. The word is expanded to produce apattern. The parameter expansion then results in parameter, with thesmallest portion of the suffix matched by the pattern deleted.
Remove Largest Suffix Pattern. The word is expanded to produce a pattern.The parameter expansion then results in parameter, with the largestportion of the suffix matched by the pattern deleted.
Remove Smallest Prefix Pattern. The word is expanded to produce apattern. The parameter expansion then results in parameter, with thesmallest portion of the prefix matched by the pattern deleted.
Remove Largest Prefix Pattern. The word is expanded to produce a pattern.The parameter expansion then results in parameter, with the largestportion of the prefix matched by the pattern deleted.


Command Substitution

Command substitution allows the output of a command to be substituted inplace of the command name itself. Command substitution occurs whenthe command is enclosed as follows:


orPo ``backquoted''versionPc :


The shell expands the command substitution by executing command in asubshell environment and replacing the command substitution with thestandard output of the command, removing sequences of one or more<newline>s at the end of the substitution. (Embedded <newline>s beforethe end of the output are not removed; however, during field splitting,they may be translated into <space>s, depending on the value ofIFSand quoting that is in effect.) 

Arithmetic Expansion

Arithmetic expansion provides a mechanism for evaluating an arithmeticexpression and substituting its value. The format for arithmeticexpansion is as follows:


The expression is treated as if it were in double-quotes, exceptthat a double-quote inside the expression is not treated specially. Theshell expands all tokens in the expression for parameter expansion,command substitution, and quote removal.

Next, the shell treats this as an arithmetic expression andsubstitutes the value of the expression. 

White Space Splitting (Field Splitting)

After parameter expansion, command substitution, andarithmetic expansion the shell scans the results ofexpansions and substitutions that did not occur in double-quotes forfield splitting and multiple fields can result.

The shell treats each character of theIFSas a delimiter and use the delimiters to split the results of parameterexpansion and command substitution into fields. 

Pathname Expansion (File Name Generation)

Unless the-fflag is set, file name generation is performed after word splitting iscomplete. Each word is viewed as a series of patterns, separated byslashes. The process of expansion replaces the word with the names of allexisting files whose names can be formed by replacing each pattern with astring that matches the specified pattern. There are two restrictions onthis: first, a pattern cannot match a string containing a slash, andsecond, a pattern cannot match a string starting with a period unless thefirst character of the pattern is a period. The next section describes thepatterns used for both Pathname Expansion and thecase(1)command. 

Shell Patterns

A pattern consists of normal characters, which match themselves,and meta-characters. The meta-characters are``!'' ``*'' ``?'' and``['' These characters lose their special meanings if they are quoted. Whencommand or variable substitution is performed and the dollar sign or backquotes are not double quoted, the value of the variable or the output ofthe command is scanned for these characters and they are turned intometa-characters.

An asterisk(``*'')matches any string of characters. A question mark matches any singlecharacter. A left bracket(``['')introduces a character class. The end ofthe character class is indicated by a(``]'') if the``]''is missing then the``[''matches a``[''rather than introducing a character class. A character class matches anyof the characters between the square brackets. A range of characters maybe specified using a minus sign. The character class may be complementedby making an exclamation point the first character of the character class.

To include a``]''in a character class, make it the first character listed (after the``!'' if any). To include a minus sign, make it the first or last character listed 


This section lists the builtin commands which are builtin because theyneed to perform some operation that can't be performed by a separateprocess. In addition to these, there are several other commands that maybe builtin for efficiency (e.g.printf(1),echo(1),test(1),etc).

A null command that returns a 0 (true) exit value.
. file
The commands in the specified file are read and executed by the shell.
alias [name [=string ...]]
Ifname=stringis specified, the shell defines the aliasnamewith valuestring If justnameis specified, the value of the aliasnameis printed. With no arguments, thealiasbuiltin prints thenames and values of all defined aliases (seeunalias )
bg [ job ] ...
Continue the specified jobs (or the current job if nojobs are given) in the background.
command command arg...
Execute the specified builtin command. (This is useful when youhave a shell function with the same name as a builtin command.)
cd [directory]
Switch to the specified directory (default$HOME ) If an entry forCDPATHappears in the environment of thecdcommand or the shell variableCDPATHis set and the directory name does not begin with a slash, then thedirectories listed inCDPATHwill be searched for the specified directory. The format ofCDPATHis the same as that ofPATH In an interactive shell, thecdcommand will print out the name of thedirectory that it actually switched to if this is different from the namethat the user gave. These may be different either because theCDPATHmechanism was used or because a symbolic link was crossed.
eval string...
Concatenate all the arguments with spaces. Then re-parse and executethe command.
exec [command arg...]
Unless command is omitted, the shell process is replaced with thespecified program (which must be a real program, not a shell builtin orfunction). Any redirections on theexeccommand are marked as permanent, so that they are not undone when theexeccommand finishes.
exit [exitstatus]
Terminate the shell process. Ifexitstatusis given it is used as the exit status of the shell; otherwise theexit status of the preceding command is used.
export name...
export -p
The specified names are exported so that they will appear in theenvironment of subsequent commands. The only way to un-export a variableis to unset it. The shell allows the value of a variable to be set at thesame time it is exported by writing

export name=value

With no arguments the export command lists the names of all exported variables.With the-poption specified the output will be formatted suitably for non-interactive use.

fg [job]
Move the specified job or the current job to the foreground.
getopts optstring var
ThePOSIXgetoptscommand, not to be confused with theBell Labs-derivedgetopt(1).

The first argument should be a series of letters, each of which may beoptionally followed by a colon to indicate that the option requires anargument. The variable specified is set to the parsed option.

Thegetoptscommand deprecates the oldergetopt(1)utility due to its handling of arguments containing whitespace.

Thegetoptsbuiltin may be used to obtain options and their argumentsfrom a list of parameters. When invoked,getoptsplaces the value of the next option from the option string in the list inthe shell variable specified byvarand it's index in the shell variableOPTIND When the shell is invoked,OPTINDis initialized to 1. For each option that requires an argument, thegetoptsbuiltin will place it in the shell variableOPTARG If an option is not allowed for in theoptstring thenOPTARGwill be unset.

optstringis a string of recognized option letters (seegetopt(3)).If a letter is followed by a colon, the option is expected to have anargument which may or may not be separated from it by white space. If anoption character is not found where expected,getoptswill set the variablevarto a``?'' getoptswill then unsetOPTARGand write output to standard error. By specifying a colon as thefirst character ofoptstringall errors will be ignored.

A nonzero value is returned when the last option is reached.If there are no remaining arguments,getoptswill setvarto the special option,``--'' otherwise, it will setvarto``?''

The following code fragment shows how one might process the argumentsfor a command that can take the options[a]and[b] and the option[c] which requires an argument.

while getopts abc: fdo        case $f in        a | b)  flag=$f;;        c)      carg=$OPTARG;;        \?)     echo $USAGE; exit 1;;        esacdoneshift `expr $OPTIND - 1`

This code will accept any of the following as equivalent:

cmd -acarg file filecmd -a -c arg file filecmd -carg -a file filecmd -a -carg -- file file
hash -rv command...
The shell maintains a hash table which remembers thelocations of commands. With no arguments whatsoever,thehashcommand prints out the contents of this table. Entries which have notbeen looked at since the lastcdcommand are marked with an asterisk; it is possible for these entriesto be invalid.

With arguments, thehashcommand removes the specified commands from the hash table (unlessthey are functions) and then locates them. With the-voption, hash prints the locations of the commands as it finds them. The-roption causes the hash command to delete all the entries in the hash tableexcept for functions.

This command lists out all the background processeswhich are children of the current shell process.
Print the current directory. The builtin command maydiffer from the program of the same name because thebuiltin command remembers what the current directoryis rather than recomputing it each time. This makesit faster. However, if the current directory isrenamed, the builtin version ofpwdwill continue to print the old name for the directory.
read [-p prompt][-r]variable...
The prompt is printed if the-poption is specified and the standard input is a terminal. Then a line isread from the standard input. The trailing newline is deleted from theline and the line is split as described in the section on word splittingabove, and the pieces are assigned to the variables in order.At least one variable must be specified.If there aremore pieces than variables, the remaining pieces (along with thecharacters inIFSthat separated them) are assigned to the last variable. If there are morevariables than pieces, the remaining variables are assigned the nullstring. Thereadbuiltin will indicate success unless EOF is encountered on input, inwhich case failure is returned.

By default, unless the-roption is specified, the backslash``\''acts as an escape character, causing the following character to be treatedliterally. If a backslash is followed by a newline, the backslash and thenewline will be deleted.

readonly name...
readonly -p
The specified names are marked as read only, so that they cannot besubsequently modified or unset. The shell allows the value of a variableto be set at the same time it is marked read only by writing

readonly name=value

With no arguments the readonly command lists the names of all read onlyvariables.With the-poption specified the output will be formatted suitably for non-interactive use.

set[{-options | +options | -- arg...]
Thesetcommand performs three different functions.

With no arguments, it lists the values of all shellvariables.

If options are given, it sets the specified optionflags, or clears them as described in the sectioncalledSx Argument List Processing .

The third use of the set command is to set the values of the shell'spositional parameters to the specified args. To change the positionalparameters without changing any options, use``--''as the first argument to set. If no args are present, the set commandwill clear all the positional parameters (equivalent to executing``shift $# .''

setvar variable value
Assigns value to variable. (In general it is better to writevariable=value rather than usingsetvar setvar is intended to be used infunctions that assign values to variables whose names are passed asparameters.)
shift [n]
Shift the positional parameters n times. Ashiftsets the value of$1to the value of$2 the value of$2to the value of$3 and so on, decreasingthe value of$#by one. If n is greater than the number of positional parameters,shiftwill issue an error message, and exit with return status 2.
Print the accumulated user and system times for the shell and for processesrun from the shell. The return status is 0.
Cause the shell to parse and execute action when any of the specifiedsignals are received. The signals are specified by signal number. Ifsignalis0 the action is executed when the shell exits.actionmay be null or``-'' the former causes the specified signal to beignored and the latter causes the default action to be taken. When theshell forks off a subshell, it resets trapped (but not ignored) signals tothe default action. Thetrapcommand has no effect on signals that wereignored on entry to the shell.
type [name ...]
Interpret each name as a command and print the resolution of the commandsearch. Possible resolutions are:shell keyword, alias, shell builtin,command, tracked alias and not found. For aliases the alias expansion isprinted; for commands and tracked aliases the complete pathname of thecommand is printed.
ulimit [-H -S][-a -tfdscmlpn [value]]
Inquire about or set the hard or soft limits on processes or set newlimits. The choice between hard limit (which no process is allowed toviolate, and which may not be raised once it has been lowered) and softlimit (which causes processes to be signaled but not necessarily killed,and which may be raised) is made with these flags:

set or inquire about hard limits
set or inquire about soft limits. If neither-Hnor-Sis specified, the soft limit is displayed or both limits are set. If bothare specified, the last one wins.

The limit to be interrogated or set, then, is chosen by specifyingany one of these flags:

show all the current limits
show or set the limit on CPU time (in seconds)
show or set the limit on the largest file that can be created(in 512-byte blocks)
show or set the limit on the data segment size of a process (in kilobytes)
show or set the limit on the stack size of a process (in kilobytes)
show or set the limit on the largest core dump size that can be produced(in 512-byte blocks)
show or set the limit on the total physical memory that can bein use by a process (in kilobytes)
show or set the limit on how much memory a process can lock withmlock(2)(in kilobytes)
show or set the limit on the number of processes this user canhave at one time
show or set the limit on the number files a process can have open at once

If none of these is specified, it is the limit on file size that is shownor set. If value is specified, the limit is set to that number; otherwisethe current limit is displayed.

Limits of an arbitrary process can be displayed or set using thesysctl(8)utility.

umask [mask]
Set the value of umask (seeumask(2))to the specified octal value. If the argument is omitted, the umask valueis printed.
unalias [-a][name]
Ifnameis specified, the shell removes that alias. If-ais specified, all aliases are removed.
unset name...
The specified variables and functions are unset and unexported. If a givenname corresponds to both a variable and a function, both the variable andthe function are unset.
wait [job]
Wait for the specified job to complete and return the exit status of thelast process in the job. If the argument is omitted, wait for all jobs tocomplete and the return an exit status of zero.


Set automaticly bylogin(1)from the user's login directory in the password file(passwd(4)) This environment variable also functions as the default argument for thecd builtin.
The default search path for executables. See the above sectionSx Path Search .
The search path used with the cd builtin.
The name of a mail file, that will be checked for the arrival of new mail.Overridden byMAILPATH
The frequency in seconds that the shell checks for the arrival of mailin the files specified by theMAILPATHor theMAILfile. If set to 0, the check will occur at each prompt.
A colon``:''separated list of file names, for the shell to check for incoming mail.This environment setting overrides theMAILsetting. There is a maximum of 10 mailboxes that can be monitored at once.
The primary prompt string, which defaults to``$  '' unless you are the superuser, in which case it defaults to``#  ''
The secondary prompt string, which defaults to``>  ''
Input Field Separators. This is normally set to <space> <tab> and<newline>. See theSx White Space Splittingsection for more details.
The default terminal setting for the shell. This is inherited bychildren of the shell, and is used in the history editing modes.
The number of lines in the history buffer for the shell.








Ashcommand appeared inAT&T Systemv1 .It was, however, unmaintainable so we wrote this one. 


Errors that are detected by the shell, such as a syntax error, will cause theshell to exit with a non-zero exit status. If the shell is not aninteractive shell, the execution of the shell file will be aborted. Otherwisethe shell will return the exit status of the last command executed, orif the exit builtin is used with a numeric argument, it will return theargument. 


Setuid shell scripts should be avoided at all costs, as they are asignificant security risk.



Argument List Processing
Lexical Structure
Single Quotes
Double Quotes
Reserved Words
Simple Commands
Search and Execution
Path Search
Command Exit Status
Complex Commands
Background Commands -- &
Lists -- Generally Speaking
Short-Circuit List Operators
Flow-Control Constructs -- if, while, for, case
Grouping Commands Together
Variables and Parameters
Positional Parameters
Special Parameters
Word Expansions
Tilde Expansion (substituting a user's home directory)
Parameter Expansion
Command Substitution
Arithmetic Expansion
White Space Splitting (Field Splitting)
Pathname Expansion (File Name Generation)
Shell Patterns

This document was created byman2html,using the manual pages.