Syntax

The shell reads, parses, and executes command line by line. If there is more than one command on a line, all the commands are parsed before executed. If a command is continued to next lines, the shell reads more enough lines to complete the command. On a syntax error, the shell neither reads nor executes any more commands.

Tokens and keywords

A command is composed of one or more tokens. In the shell syntax, a token is a word that is part of a command. Normally, tokens are separated by whitespaces, that is, the space or tab character. Whitespaces inside a command substitution or a parameter expansion, however, do not separate tokens.

The following symbols have special meanings in the shell syntax and in most cases separate tokens:

; & | < > ( ) [newline]

The following symbols do not separate tokens, but have syntactic meanings:

$ ` \ " ' * ? [ # ~ = %

The following tokens are treated as keywords depending on the context in which they appear:

! { } [[ case do done elif else esac fi
for function if in then until while

A token is treated as a keyword when:

it is the first token of a command,
it follows another keyword (except case, for, and in), or
it is a non-first token of a command and is supposed to be a keyword to compose a composite command.

If a token begins with #, then the # and any following characters up to the end of the line are treated as a comment, which is completely ignored in syntax parsing.

Quotations

If you want whitespaces, separator characters, or keywords described above to be treated as a normal characters, you must quote the characters using appropriate quotation marks. Quotation marks are not treated as normal characters unless they are themselves quoted. You can use the following three quotation marks:

A backslash (\) quotes a character that immediately follows.
The only exception about a backslash is the case where a backslash is followed by a newline. In this case, the two characters are treated as a line continuation rather than a newline being quoted. The two characters are removed from the input and the two lines surrounding the line continuation are concatenated into a single line.
A pair of single-quotation marks (') quote any characters between them except another single-quotation. Note that newlines can be quoted using single-quotations.
Double-quotation marks (") are like single-quotations, but they have a few exceptions: Parameter expansion, command substitution, and arithmetic expansion are interpreted as usual even between double-quotations. A backslash between double-quotations is treated as a quotation mark only when it is followed by $, `, ", \, or a newline; other backslashes are treated as normal characters.

Aliases

Tokens that compose a command are subject to alias substitution. A token that matches the name of an alias that has already been defined is substituted with the value of the alias before the command is parsed.

Tokens that contain quotations are not alias-substituted since an alias name cannot contain quotation marks. Keywords and command separator characters are not alias-substituted either.

There are two kinds of aliases: normal aliases and global aliases. A normal alias can only substitute the first token of a command while a global alias can substitute any part of a command. Global aliases are yash extension that is not defined in POSIX.

If a token is alias-substituted with the value of a normal alias that ends with a whitespace, the next token is exceptionally subject to alias substitution for normal aliases.

The results of alias substitution are again subject to alias substitution for other aliases (but not for the aliases that have been already applied).

You can define aliases using the alias built-in and remove using the unalias built-in.

Simple commands

A command that does not start with a keyword token is a simple command. Simple commands are executed as defined in Execution of simple commands.

If the first and any number of following tokens of a simple command have the form name=value, they are interpreted as variable assignments. A variable name must consist of one or more alphabets, digits and/or underlines (_) and must not start with a digit. The first token that is not a variable assignment is considered as a command name and all the following tokens (whether or not they have the form name=value) as command arguments.

A variable assignment of the form var=(tokens) is interpreted as assignment to an array. You can write any number of tokens between a pair of parentheses. Tokens can be separated by not only spaces and tabs but also newlines.

Pipelines

A pipeline is a sequence of one or more simple commands, compound commands, and/or function definitions that are separated by |.

A pipeline that has more than one subcommand is executed by executing each subcommand of the pipeline in a subshell simultaneously. The standard output of each subcommand except the last one is redirected to the standard input of the next subcommand. The standard input of the first subcommand and the standard output of the last subcommand are not redirected.

The exit status of the pipeline is that of the last subcommand unless the pipe-fail option is enabled, in which case the exit status of the pipeline is that of the last subcommand that exits with a non-zero exit status. If all the subcommands exit with an exit status of zero, the exit status of the pipeline is also zero.

A pipeline can be prefixed by !, in which case the exit status of the pipeline is reversed: the exit status of the pipeline is 1 if that of the last subcommand is 0, and 0 otherwise.

Korn shell treats a word of the form !(…) as an extended pathname expansion pattern that is not defined in POSIX. In the POSIXly-correct mode, the tokens ! and ( must be separated by one or more white spaces.

Note

When the execution of a pipeline finishes, at least the execution of the last subcommand has finished since the exit status of the last subcommand defines that of the whole pipeline. The execution of other subcommands, however, may not have finished then. On the other hand, the execution of the pipeline may not finish soon after that of the last subcommand finished because the shell may choose to wait for the execution of other subcommands to finish.

Note	The POSIX standard allows executing any of subcommands in the current shell rather than subshells, though yash does not do so.

And/or lists

An and/or list is a sequence of one or more pipelines separated by && or ||.

An and/or list is executed by executing some of the pipelines conditionally. The first pipeline is always executed. The other pipelines are either executed or not executed according to the exit status of the previous pipelines.

If two pipelines are separated by && and the exit status of the first pipeline is zero, the second pipeline is executed.
If two pipelines are separated by || and the exit status of the first pipeline is not zero, the second pipeline is executed.
In other cases, the execution of the and/or list ends: the second and any remaining pipelines are not executed.

The exit status of an and/or list is that of the last pipeline that was executed.

Normally, an and/or list must be terminated by a semicolon, ampersand, or newline. See Command separators and asynchronous commands.

Command separators and asynchronous commands

The whole input to the shell must be composed of any number of and/or lists separated by a semicolon or ampersand. A terminating semicolon can be omitted if it is followed by ;;, ), or a newline. Otherwise, an and/or list must be terminated by a semicolon or ampersand.

If an and/or list is terminated by a semicolon, it is executed synchronously: the shell waits for the and/or list to finish before executing the next and/or list. If an and/or list is terminated by an ampersand, it is executed asynchronously: after the execution of the and/or list is started, the next and/or list is executed immediately. An asynchronous and/or list is always executed in a subshell and its exit status is zero.

If the shell is not doing job control, the standard input of an asynchronous and/or list is automatically redirected to /dev/null. Signal handlers of the and/or list for the SIGINT and SIGQUIT signals are set to “ignore” the signal so that the execution of the and/or list cannot be stopped by those signals.

When the execution of an asynchronous and/or list is started, the shell remembers its process ID. You can obtain the ID by referencing the ! special parameter. You can obtain the current and exit status of the asynchronous list as well by using the jobs and wait built-ins.

Compound commands

Compound commands provide you with programmatic control of shell command execution.

Grouping

A grouping is a list of commands that is treated as a simple command.

Normal grouping syntax: { command…; }
Subshell grouping syntax: (command…)

The { and } tokens are keywords, which must be separated from other tokens. The ( and ) tokens, however, are special separators that need not to be separated.

In the normal grouping syntax, the commands in a grouping are executed in the current shell. In the subshell grouping syntax, the commands are executed in a new subshell.

In the POSIXly-correct mode, a grouping must contain at least one command. If the shell is not in the POSIXly-correct mode, a grouping may contain no commands.

The exit status of a grouping is that of the last command in the grouping. If the grouping contains no commands, its exit status is that of the last executed command before the grouping.

If command

The if command performs a conditional branch.

Basic if command syntax: if condition…; then body…; fi
Syntax with the else clause: if condition…; then body…; else body…; fi
Syntax with the elif clause: if condition…; then body…; elif condition…; then body…; fi
Syntax with the elif clause: if condition…; then body…; elif condition…; then body…; else body…; fi

For all the syntaxes, the execution of an if command starts with the execution of the condition commands that follows the if token. If the exit status of the condition commands is zero, the condition is considered as “true”. In this case, the body commands that follows the then token are executed and the execution of the if command finishes. If the exit status of the condition commands is non-zero, the condition is considered as “false”. In this case, the condition commands for the next elif clause are executed and the exit status is tested in the same manner as above. If there is no elif clause, the body commands that follow the else token are executed and the execution of the if command finishes. If there is no else clause either, the execution of the if command just ends.

An if command may have more than one elif-then clause.

The exit status of an if command is that of the body commands that were executed. The exit status is zero if no body commands were executed, that is, all the conditions were false and there was no else clause.

While and until loops

The while loop and until loop are simple loops with condition.

While loop syntax: while condition…; do body…; done
Until loop syntax: until condition…; do body…; done

If the shell is not in the POSIXly-correct mode, you can omit the condition and/or body commands of a while/until loop.

The execution of a while loop is started by executing the condition commands. If the exit status of the condition commands is zero, the shell executes the body commands and returns to the execution of the condition commands. The condition and body commands are repeatedly executed until the exit status of the condition commands is non-zero.

Note	The `body` commands are not executed at all if the first execution of the `condition` commands yields a non-zero exit status.

An until loop is executed in the same manner as a while loop except that the condition to repeat the loop is reversed: the body commands are executed when the exit status of the condition commands is non-zero.

The exit status of a while/until loop is that of the last executed body command. The exit status is zero if the body commands are empty or were not executed at all.

For loop

The for loop repeats commands with a variable assigned one of given values in each round.

For loop syntax: for varname in word…; do command…; done
for varname do command…; done

The word list after the in token may be empty, but the semicolon (or newline) before the do token is required even in that case. The words are not treated as keywords, but you need to quote separator characters (such as & and |) to include them as part of a word. The command list may be empty if not in the POSIXly-correct mode.

The varname must be a portable (ASCII-only) name in the POSIXly-correct mode.

The execution of a for loop is started by expanding the words in the same manner as in the execution of a simple command. If the in and word tokens are omitted, the shell assumes the word tokens to be "$@". Next, the following steps are taken for each word expanded (in the order the words were expanded):

Assign the word to the variable whose name is varname.
Execute the commands.

By default, if a for loop is executed within a function, varname is created as a local variable, even if it already exists globally. Turning off the for-local shell option or enabling the POSIXly-correct mode mode will disable this behavior.

If the expansion of the words yields no words, no variable is created and the commands are not executed at all.

The exit status of a for loop is that of the last executed command. The exit status is zero if the commands are not empty and not executed at all. If the commands are empty, the exit status is that of the last executed command before the for loop.

If the variable is read-only, the execution of the for loop is interrupted and the exit status will be non-zero.

Case command

The case command performs a pattern matching to select commands to execute.

Case command syntax: case word in caseitem… esac
Case item syntax: (patterns) command…;;

The word between the case and in tokens must be exactly one word. The word is not treated as a keyword, but you need to quote separator characters (such as & and |) to include them as part of the word. Between the in and esac tokens you can put any number of case items (may be none). You can omit the first ( token of a case item and the last ;; token before the esac token. If the last command of a case item is terminated by a semicolon, you can omit the semicolon as well. The commands in a case item may be empty.

The patterns in a case item are one or more tokens each separated by a | token.

The execution of a case command starts with subjecting the word to the four expansions. Next, the following steps are taken for each case item (in the order of appearance):

For each word in the patterns, expand the word in the same manner as the word and test if the expanded pattern matches the expanded word. (If a pattern is found that matches the word, the remaining patterns are not expanded nor tested, so some of the patterns may not be expanded. Yash expands and tests the patterns in the order of appearance, but it may not be the case for other shells.)
If one of the patterns was found to match the word in the previous step, the commands in this case item are executed and the execution of the whole case item ends. Otherwise, proceed to the next case item.

The exit status of a case command is that of the commands executed. The exit status is zero if no commands were executed, that is, there were no case items, no matching pattern was found, or no commands were associated with the matching pattern.

In the POSIXly-correct mode, the first pattern in a case item cannot be esac (even if you do not omit the ( token).

Double-bracket command

The double-bracket command is a syntactic construct that works similarly to the test built-in. It expands and evaluates the words between the brackets.

Double-bracket command syntax: [[ expression ]]

The expression can be a single primary or combination of primaries and operators. The expression syntax is parsed when the command is parsed, not executed. Operators (either primary or non-primary) must not be quoted, or it will be parsed as a normal word.

When the command is executed, operand words are subjected to the four expansions, but not brace expansion, field splitting, or pathname expansion.

In the double-bracket command, the following primaries from the test built-in can be used:

Unary primaries: -b, -c, -d, -e, -f, -G, -g, -h, -k, -L, -N, -n, -O, -o, -p, -r, -S, -s, -t, -u, -w, -x, -z
Binary primaries: -ef, -eq, -ge, -gt, -le, -lt, -ne, -nt, -ot, -veq, -vge, -vgt, -vle, -vlt, -vne, ===, !==, =~, <, >

Additionally, some binary primaries can be used to compare strings, which works slightly differently from those for the test built-in: The = primary treats the right-hand-side operand word as a pattern and tests if it matches the left-hand-side operand word. The == primary is the same as =. The != primary is negation of the = primary (reverse result).

The operand word of a primary must be quoted if it is ]] or can be confused with another primary operator.

Note	More primaries may be added in future versions of the shell. You should quote any words that start with a hyphen.

Note	The `<=` and `>=` binary primaries cannot be used in the double-bracket command because it cannot be parsed correctly in the shell grammar.

The following operands (listed in the descending order of precedence) can be used to combine primaries:

( expression ): A pair of parentheses change operator precedence.
! expression: An exclamation mark negates (reverses) the result.
expression && expression: A double ampersand represents logical conjugation (the “and” operation). The entire expression is true if and only if the operand expressions are both true. The left-hand-side expression is first expanded and tested. The right-hand-side is expanded only if the left-hand-side is true.
expression || expression: A double vertical line represents logical conjugation (the “or” operation). The entire expression is false if and only if the operand expressions are both false. The left-hand-side expression is first expanded and tested. The right-hand-side is expanded only if the left-hand-side is false.

Note	Unlike the test built-in, neither `-a` nor `-o` can be used as a binary operator in the double-bracket command.

The exit status of the double-bracket command is 0 if expression is true, 1 if false, and 2 if it cannot be evaluated because of expansion error or any other reasons.

Note	The double-bracket command is also supported in bash, ksh, mksh, and zsh, but not defined in the POSIX standard. The behavior slightly differs between the shells. The test built-in should be preferred over the double-bracket command for maximum portability.

Function definition

The function definition command defines a function.

Function definition syntax: funcname ( ) compound_command
function funcname compound_command
function funcname ( ) compound_command

In the first syntax without the function keyword, funcname cannot contain any special characters such as semicolons and quotation marks. In the second and third syntax, which cannot be used in the POSIXly-correct mode, funcname is subjected to the four expansions when executed. In the POSIXly-correct mode, funcname is limited to a portable (ASCII-only) name.

When a function definition command is executed, a function whose name is funcname is defined with its body being compound_command.

A function definition command cannot be directly redirected. Any redirections that follow a function definition are associated with compound_command rather than the whole function definition command. In func() { cat; } >/dev/null, for example, it is not func() { cat; } but { cat; } that is redirected.

The exit status of a function definition is zero if the function was defined without errors, and non-zero otherwise.