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Commands

A simple command is a string of normal or I/O words terminated by a newline or one of the other special words, and not starting with a reserved word. Each I/O word is combined with the following word (which must be a normal word) to form an I/O directive. Then all leading assignment words or, if the `k' flag is set, all assignment words are collected to form a local environment. The remaining words form the body of the command. A number of substitutions (described below) are performed on these words before the command, determined by the first word in the body, is executed.

A pipeline is a number of commands, simple or structured (see Structured Commands below), separated by the pipe (|) word. These commands are run simultaneously with the standard output of each command but the last connected to the standard input of the next command by the use of a pipe (see pipe(2)).

A task is a number of pipelines separated by either && or ||. In order to describe the meaning of these separators, it is more convenient to describe a task as being either a pipeline or a task followed by one of && or || and then a pipeline.

A pipeline following a && is executed only if the preceding task succeeded. A pipeline following a || is executed only if the preceding task failed (see Execution for the meaning of success and failure).

Finally a command list is a number of tasks each terminated by a semicolon (;), an ampersand (&), or a newline. The & causes the task to be run in the background so that it does not respond to keyboard signals (such as INTERRUPT or QUIT) and that the shell does not wait for it to complete before proceeding. A semicolon (;) or a newline causes the task to run in the foreground and the shell waits for the completion of the task before the next command is executed.

A special type of command list is a job in which only the last task is terminated by a newline. When commands are being read from a file or the terminal, one job is read and executed at a time.

Structured Commands

The following structured commands are particularly useful for writing shell scripts --- programs which are interpreted by the shell. They can all be followed by I/O redirection directives that will affect every subcommand within the whole. Where list appears below, the list must be terminated with a semicolon (;), ampersand (&) or newline except where it is followed by a special word as is the case in the case and ( statements. Also where a semicolon is shown, a newline may be used instead.

if list then list [elif list then list ...] [else list] fi

Execute the first list. If it succeeds, execute the list after the then otherwise perform the elif or else parts if they exist. ifnot and elifnot may be used in place of if and elif to invert the sense of the test.

for name [in word ...] ; do list done

The variable name is successively assigned each word (or each positional parameter if the in clause is omitted) and the list is executed. If present, the word list is subjected to that same sequence of substitutions as words in a simple command.

while list do list done

Repeatedly execute the first list and then, if it succeeds, the second list until the first list fails.

until list do list done

Same as while except that looping continues until first list succeeds.

case word in pattern[ | pattern]) list ;; ... esac

Word is matched against each of the patterns in order using file name pattern matching. The statement list following the first pattern which matches is executed. If no pattern matches, the statement is a no-op. Patterns match the same as for file name matching, except that slashes and leading dots are not significant.

{ list }

Execute list. The braces are used simply for grouping.

( list )

Execute list in a subshell. All substitutions also occur in the subshell and so cannot affect the parent shell.

Functions1

Functions are defined with a syntax vaguely reminiscent of C. More precisely:

function-name()
{
 statement-list
}

or, alternately

function function-name()
{
  statement-list
}

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