Core Syntax

words are passed as arguments to the invoked command. The command name is used to locate the command (builtin, internal,external) . The
value of a simple-command.

A pipeline is a sequence of one or more commands separated
by |. The standard output of each command but the last is
connected by a Pipe to the standard input of the next
command. Each command is run as a separate thread (if builtin or internal) or as a seperate process (if external). The
shell waits for the last command to terminate. The exit
status of a pipeline is the exit status of the last command
in the pipeline.
A list is a sequence of one or more pipelines separated by
;, &, &&, or

, and optionally terminated by ; or &. Of these four symbols, ; and & have equal precedence, which is lower than that of && and

. The symbols && and

also have equal precedence. A semicolon (;) causes sequential execution of the preceding pipeline, that is, the shell waits for the pipeline to finish before executing any com- mands following the semicolon. An ampersand (&) causes asyn- chronous execution of the preceding pipeline, that is, the shell does not wait for that pipeline to finish. The symbol && (

) causes the list following it to be executed only if the preceding pipeline returns a zero (non-zero) exit status. An arbitrary number of newlines may appear in a list, instead of semicolons, to delimit commands. A command is either a simple-command or one of the follow- ing. Unless otherwise stated, the value returned by a com- mand is that of the last simple-command executed in the com- mand. for name [ in word ... ] do list done Each time a for command is executed, name is set to the next word taken from the in word list. If in word ... is omitted, then the for command executes the do list once for each positional parameter that is set (see Parameter Substitution section below). Execution ends when there are no more words in the list. case word in [ pattern [ | pattern ] ) list ;; ] ... esac A case command executes the list associated with the first pattern that matches word. The form of the pat- terns is the same as that used for file-name generation (see File Name Generation section), except that a slash, a leading dot, or a dot immediately following a slash need not be matched explicitly. if list ; then list ; [ elif list ; then list ; ] ... [ else list ; ] fi The list following if is executed and, if it returns a zero exit status, the list following the first then is executed. Otherwise, the list following elif is executed and, if its value is zero, the list following the next then is executed. Failing that, the else list is executed. If no else list or then list is executed, then the if command returns a zero exit status. while list do list done A while command repeatedly executes the while list and, if the exit status of the last command in the list is zero, executes the do list; otherwise the loop ter- minates. If no commands in the do list are executed, then the while command returns a zero exit status; until may be used in place of while to negate the loop termination test. (list) Execute list in a sub-shell. { list;} list is executed in the current (that is, parent) shell. The { must be fol- lowed by a space. name () { list;} Define a function which is referenced by name. The body of the function is the list of commands between { and }. The { must be followed by a space. Execution of func- tions is described below (see Execution section). The { and } are unnecessary if the body of the function is a command as defined above, under Commands. The following words are only recognized as the first word of a command and when not quoted: if then else elif fi case esac for while until do done { } Comments Lines A word beginning with # causes that word and all the follow- ing characters up to a newline to be ignored. Command Substitution The shell reads commands from the string between two grave accents (``) and the standard output from these commands may be used as all or part of a word. Trailing newlines from the standard output are removed. No interpretation is done on the string before the string is read, except to remove backslashes (\) used to escape other characters. Backslashes may be used to escape a grave accent (`) or another backslash (\) and are removed before the com- mand string is read. Escaping grave accents allows nested command substitution. If the command substitution lies within a pair of double quotes (" ...` ...` ... "), a backslash used to escape a double quote (\") will be removed; otherwise, it will be left intact. If a backslash is used to escape a newline character (\new- line), both the backslash and the newline are removed (see the later section on Quoting). In addition, backslashes used to escape dollar signs (\$) are removed. Since no parameter substitution is done on the command string before it is read, inserting a backslash to escape a dollar sign has no effect. Backslashes that precede characters other than \, `, ", newline, and $ are left intact when the command string is read.