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Section: User Commands (1)
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sh - command interpreter (shell) 


-words[-aCefnuvxIimqVEb [+aCefnuvxIimqVEb -words]][-o option_name][+o option_name]-words[command_file [argument ... ]]
-c-words[-aCefnuvxIimqVEb [+aCefnuvxIimqVEb -words]][-o option_name][+o option_name]-wordscommand_string[command_name [argument ... ]]
-s-words[-aCefnuvxIimqVEb [+aCefnuvxIimqVEb -words]][-o option_name][+o option_name]-words[argument ...] 


is the standard command interpreter for the system.The current version ofis 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.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 is the program that is running when a user logs into the system(although a user 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, along with built in history and line editingcapabilities.It incorporates many features to aid interactive use andhas 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 arguments 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.An interactive shell generally prompts before each command and handlesprogramming and command errors differently (as described below).When first starting,the shell inspects argument 0, and if it begins with a dash`-' 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 thesetbuilt-in (described later).

-a allexport
Export all variables assigned to.
Read commands from thecommand_stringoperand instead of from the standard input.Special parameter 0 will be set from thecommand_nameoperand and the positional parameters ($1, $2, etc.)set from the remaining argument operands.
-C noclobber
Don't overwrite existing files with``Gt]''
-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 control anif elif while oruntil or if the command is the left hand operand of an``Am]Am]''or``||''operator.
-f noglob
Disable pathname expansion.
-n noexec
If not interactive, read commands but do not execute them.This is useful for 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.
-v verbose
The shell writes its input to standard error as it is read.Useful for debugging.
-x xtrace
Write each command to standard error (preceded by 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 EOFs 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 )
-V vi
Enable the built-invi(1)command line editor (disables-Eif it has been set).(See theSx Command Line Editingsection below.)
-E emacs
Enable the built-in emacs stylecommand line editor (disables-Vif it has been set).(See theSx Command Line Editingsection below.)
-b notify
Enable asynchronous notification of background job completion.(UNIMPLEMENTED for 4.4alpha)
"  " cdprint
Make an interactive shell always print the new directory name whenchanged by thecdcommand.
"  " tabcomplete
Enables filename completion in the command line editor.Typing a tab character will extend the current input word to match afilename.If more than one filename matches it is only extended to be the common prefix.Typing a second tab character will list all the matching names.One of the editing modes, either-Eor-V must be enabled for this to work.


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:"
Am] Am]Am] ( ) ; ;; | || Lt]newlineGt]
"Redirection operators:"
Lt] Gt] Gt]| Lt]Lt] Gt]Gt] Lt]Am] Gt]Am] Lt]Lt]- Lt]Gt]



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


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 dollar sign($) backquote(`) and backslash(\) The backslash inside double quotes is historically weird, and serves toquote only the following characters:
$ ` \ Lt]newlineGt]
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 thealiasbuilt-in 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 Aq return

would become

ls -F foobar Aq 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 is discouraged. 


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 of the 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 a list of the possible redirections.TheBq nis an optional number, as in`3'(not`Bq 3 )' that refers to a file descriptor.

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

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

[n]Lt]Lt] delimiter
here-doc-text ...

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``Lt]Lt]-''instead of``Lt]Lt]'' then leading tabs in the here-doc-text are stripped. 

Search and Execution

There are three types of commands: shell functions, built-in commands, andnormal programs -- and the command is searched for (by name) in that order.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 the command given in the function definition is executed.The positional parameters are restored to their original valueswhen the command completes.This all occurs within the current shell.

Shell built-ins are executed internally to the shell, without spawning anew process.

Otherwise, if the command name doesn't match a function or built-in, thecommand is searched for as a normal program in the file system (asdescribed in the next section).When a normal program is executed, the shell runs the program,passing the arguments and the environment to the program.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 shell will 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 built-in command by that name.If a built-in 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 entry consists 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 built-in commands return exit codes, as doesan executed shell function.

If a command consists entirely of variable assignments then theexit status of the command is that of the last command substitutionif any, otherwise 0. 

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.For example:

$ command1 2Gt]Am]1 | command2

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

A ; orAq newlineterminator causes the preceding AND-OR-list (describednext) to be executed sequentially; a Am] 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 built-in, in which caseit executes in the current shell -- but any effect it has on theenvironment is wiped). 

Background Commands -- Am]

If a command is terminated by the control operator ampersand (Am]), 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 Am] [command2 Am] ...]

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 are written.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

``Am]Am]''and``||''are AND-OR list operators.``Am]Am]''executes the first command, and then executes the second command if and onlyif the exit status of the first command is zero.``||''is similar, but executes the second command if and only if the exit statusof the first command is nonzero.``Am]Am]''and``||''both have the same priority.Note that these operators are left-associative, so``true || echo bar Am]Am] echo baz''writes``baz''and nothing else.This is not the way it works in C.Also, if you forget the left-hand side (for example when continuing lines butforgetting to use a backslash) it defaults to a true statement.This behavior is not useful and should not be relied upon. 

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 word   listdone

The words are expanded, and then the list is executed repeatedly with thevariable set to each word in 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 built-in commands.

The syntax of the case command is

case word inpattern) list ;;...esac

The pattern can actually be one or more patterns (seeSx Shell Patternsdescribed 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.Built-in commands grouped into a (list) will not affect the current shell.The second form does not fork another shell so is slightly more efficient.Grouping commands together this way allows you to redirecttheir output as though they were one program:

{ echo -n  hello  ; echo  world" ; } Gt] greeting

Note that``}''must follow a control operator (here,``;'' so that it is recognized as a reserved word and not as another command argument. 


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.The command 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 the syntax is

local [ variable | - ] ...

``Local''is implemented as a built-in command.

When a variable is made local, it inherits the initial value and exportedand read-only flags from the variable with the same name in the surroundingscope, if there is one.Otherwise, the variable is initially unset.The shell 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 that can be made local is``-'' Making``-''local causes 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 is implemented as a built-in command. 

Variables and Parameters

The shell maintains a set of parameters.A parameter denoted by a name is called 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 Gt] 0).The shell sets these initially to the values of its command line argumentsthat follow the name of the shell script.Thesetbuilt-in 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 aAq spaceifIFSis unset.
Expands to the positional parameters, starting from one.When the 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 built-in command, or implicitlyby the shell.
Expands to the process ID of the invoked shell.A subshell retains the same value of $ as its parent.
Expands to the process ID of the most recent backgroundcommand executed from the current shell.For a pipeline, the process ID is that of the last command in the pipeline.
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 the username 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 inside double-quotes:

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

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 parameter is 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 exit status.Otherwise, the value of parameter is substituted.An interactive shell 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 of the value of parameter.

The following four varieties of parameter expansion provide for substringprocessing.In each case, pattern matching notation (seeSx 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 a pattern.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 a pattern.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 when the 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 moreAo newline Ac Ns sat the end of the substitution.(EmbeddedAo newline Ac Ns sbeforethe end of the output are not removed; however, during field splitting,they may be translated intoAo space Ac Ns 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 arithmetic expansion 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.The shell 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.

Arithmetic expressions use a syntax similar to thatof the C language, and are evaluated using the`intmax_t'data type (this is an extension toPOSIX which requires only`long'arithmetic).Shell variables may be referenced by name inside an arithmeticexpression, without needing a``$''sign. 

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.

Non-whitespace characters inIFSare treated strictly as parameter terminators.So adjacent non-whitespaceIFScharacters will produce empty parameters.

IfIFSis unset it is assumed to contain space, tab, and newline. 

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 by slashes.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 thecasecommand. 

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.When command or variable substitution is performedand the dollar sign or back quotes 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 single character.A left bracket(``['')introduces a character class.The end of the character class is indicated by a(``]'') if the``]''is missing then the``[''matches a``[''rather than introducing a character class.A character class matches any of the characters between the square brackets.A range of characters may be 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 built-in commands which are built-in 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 built in 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, thealiasbuilt-in 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 [-p [-v [-V command [arg ... ]]]]
Execute the specified command but ignore shell functions when searchingfor it.(This is useful when youhave a shell function with the same name as a built-in command.)

search for command using aPATHthat guarantees to find all the standard utilities.
Do not execute the command butsearch for the command and print the resolution of thecommand search.This is the same as thetypebuilt-in.
Do not execute the command butsearch for the command and print the absolute pathnameof utilities, the name for built-ins or the expansion of aliases.

cd [-P [directory [replace]]]
Switch to the specified directory (default$HOME ) Ifreplaceis specified, then the new directory name is generated by replacingthe first occurrence ofdirectoryin the current directory name withreplace Otherwise if an entry forCDPATHappears in the environment of thecdcommand or the shell variableCDPATHis set and the directory name does not begin with a slash,or its first (or only) component isn't dot or dot dot,then the directories listed inCDPATHwill be searched for the specified directory.The format ofCDPATHis the same as that ofPATH

The-Poption instructs the shell to updatePWDwith the specified directory and change to that directory.This is the default.

Some shells also support a-Loption, which instructs the shell to updatePWDwith incorrect information and to change the current directoryaccordingly.This is not supported.

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 execute the 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 built-in 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 variable is 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.

fc [-e editor [first [last ]]]
fc -l [-nr [first [last ]]]
fc -s [old=new [first ]]
Thefcbuilt-in lists, or edits and re-executes, commands previously enteredto an interactive shell.

-e editor
Use the editor named by editor to edit the commands.The editor string is a command name, subject to search via thePATHvariable.The value in theFCEDITvariable is used as a default when-eis not specified.IfFCEDITis null or unset, the value of theEDITORvariable is used.IfEDITORis null or unset,ed(1)is used as the editor.
-l (ell)
List the commands rather than invoking an editor on them.The commands are written in the sequence indicated bythe first and last operands, as affected by-r with each command preceded by the command number.
Suppress command numbers when listing with -l.
Reverse the order of the commands listed (with-l or edited (with neither-lnor-s )
Re-execute the command without invoking an editor.
Select the commands to list or edit.The number of previous commands thatcan be accessed are determined by the value of theHISTSIZEvariable.The value of first or last or both are one of the following:

A positive number representing a command number; command numbers can bedisplayed with the-loption.
A negative decimal number representing the command that was executednumber of commands previously.For example, -1 is the immediately previous command.

A string indicating the most recently entered command that begins withthat string.If the old=new operand is not also specified with-s the string form of the first operand cannot contain an embedded equal sign.

The following environment variables affect the execution of fc:

Name of the editor to use.
The number of previous commands that are accessible.

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.

Thegetoptsbuilt-in 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 its index in the shell variableOPTIND When the shell is invoked,OPTINDis initialized to 1.For each option that requires an argument, thegetoptsbuilt-in 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 whitespace.If an option character is not found where expected,getoptswill set the variablevarto a``?'' getoptswill then unsetOPTARGand write output to standard error.By specifying a colon as the first 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 not been 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.

inputrc file
Read thefileto set keybindings as defined byeditrc(5).
jobid [job]
Print the process id's of the processes in the job.If thejobargument is omitted, the current job is used.
This command lists out all the background processeswhich are children of the current shell process.
pwd [-LP]
Print the current directory.If-Lis specified the cached value (initially set fromPWD is checked to see if it refers to the current directory; if it doesthe value is printed.Otherwise the current directory name is found usinggetcwd(3).The environment variablePWDis set to the printed value.

The default ispwd-L but note that the built-incdcommand doesn't currently support the-Loption and will cache (almost) the absolute path.Ifcdis changed,pwdmay be changed to default topwd-P

If the current directory is renamed and replaced by a symlink to thesame directory, or the initialPWDvalue followed a symbolic link, then the cached value may notbe the absolute path.

The built-in command may differ from the program of the same name becausethe program will usePWDand the built-in uses a separately cached value.

read [-p prompt [-r variable [... ]]]
The prompt is printed if the-poption is specified and the standard input is a terminal.Then a line is read 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.If there are more pieces than variables, the remaining pieces(along with the characters inIFSthat separated them) are assigned to the last variable.If there are more variables than pieces,the remaining variables are assigned the null string.Thereadbuilt-in 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 shell variables.

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

The third use of the set command is to set the values of the shell'spositional parameters to the specified arguments.To change the positionalparameters without changing any options, use``--''as the first argument to set.If no arguments 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 setvaris 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 there are zero positional parameters,shiftdoes nothing.
trap [-l ]
trap [action signal ...]
Cause the shell to parse and execute action when any of the specifiedsignals are received.The signals are specified by signal number or as the name of the signal.Ifsignalis0or its equivalent, EXIT,the action is executed when the shell exits.actionmay be null, which cause the specified signals to be ignored.Withactionomitted or set to `-' the specified signals are set to their default action.When the shell forks off a subshell, it resets trapped (but not ignored)signals to the default action.On non-interactive shells, thetrapcommand has no effect on signals that wereignored on entry to the shell.On interactive shells, thetrapcommand will catch or reset signals ignored on entry.Issuingtrapwith option-lwill print a list of valid signal names.trapwithout any arguments cause it to write a list of signals and theirassociated action to the standard output in a format that is suitableas an input to the shell that achieves the same trapping results.



List trapped signals and their corresponding action

trap -l

Print a list of valid signals

trap '' INT QUIT tstp 30

Ignore signals INT QUIT TSTP USR1

trap date INT

Print date upon receiving signal INT

type [name ...]
Interpret each name as a command and print the resolution of the commandsearch.Possible resolutions are:shell keyword, alias, shell built-in,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 -tfdscmlpnv [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 both are 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 the socket buffer size of a process (in bytes)
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 of files a process can have open at once
show or set the limit on how large a process address space can be

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 value is 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 given name corresponds to both a variable and a function, boththe variable and the 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 then return an exit status of zero.


Command Line Editing

Whenis being used interactively from a terminal, the current commandand the command history (seefcinSx Built-ins )can be edited using emacs-mode or vi-mode command-line editing.The command`set'-o emacsenables emacs-mode editing.The command`set'-o vienables vi-mode editing and places the current shell process intoviinsert mode.(See theSx Argument List Processingsection above.)

Thevimode uses commands similar to a subset of those described in thevi(1)man page.With vi-modeenabled,shcan be switched between insert mode and command mode.It's similar tovi(1):pressing theAq ESCkey will throw you into command VI command mode.Pressing theAq returnkey while in command mode will pass the line to the shell.

Theemacsmode uses commands similar to a subset available intheemacs(1)editor.With emacs-mode enabled, special keys can be used to modify the textin the buffer using the control key.

uses theeditline(3)library. 


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 built-in is used with a numeric argument, it will return theargument. 


Set automatically bylogin(1)from the user's login directory in the password file(passwd(5)) This environment variable also functions as the default argument for thecdbuilt-in.
The default search path for executables.See the above sectionSx Path Search .
The search path used with thecdbuilt-in.
The string used to specify localization information that allows usersto work with different culture-specific and language conventions.Seenls(7).
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``Gt]  ''
Output before each line when execution trace (set -x) is enabled,defaults to``+  ''
Input Field Separators.This is normally set toAq space ,Aq tab ,andAq 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.








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


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

PS1, PS2, and PS4 should be subject to parameter expansion beforebeing displayed.

The characters generated by filename completion should probably be quotedto ensure that the filename is still valid after the input line has beenprocessed.



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 -- Am]
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