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6 Shell Grammar
6.1 Simple Commands & Pipelines
A simple command is a sequence of optional parameter assignments followed by blank-separated words, with optional redirections interspersed. For a description of assignment, see the beginning of Parameters.
The first word is the command to be executed, and the remaining words, if any, are arguments to the command. If a command name is given, the parameter assignments modify the environment of the command when it is executed. The value of a simple command is its exit status, or 128 plus the signal number if terminated by a signal. For example,
echo foo
is a simple command with arguments.
A pipeline is either a simple command, or a sequence of two or more
simple commands where each command is separated from the next by ‘|
’
or ‘|&
’. Where commands are separated by ‘|
’, the standard output of
the first command is connected to the standard input of the next. ‘|&
’
is shorthand for ‘2>&1 |
’, which connects both the standard output and
the standard error of the command to the standard input of the next. The
value of a pipeline is the value of the last command, unless the
pipeline is preceded by ‘!
’ in which case the value is the logical
inverse of the value of the last command. For example,
echo foo | sed 's/foo/bar/'
is a pipeline, where the output (‘foo
’ plus a newline) of the first
command will be passed to the input of the second.
If a pipeline is preceded by ‘coproc
’, it is executed as a coprocess;
a two-way pipe is established between it and the parent shell. The shell
can read from or write to the coprocess by means of the ‘>&p
’ and
‘<&p
’ redirection operators or with ‘print -p
’ and ‘read -p
’. A
pipeline cannot be preceded by both ‘coproc
’ and ‘!
’. If job control
is active, the coprocess can be treated in other than input and output
as an ordinary background job.
A sublist is either a single pipeline, or a sequence of two or more
pipelines separated by ‘&&
’ or ‘||
’. If two pipelines are separated
by ‘&&
’, the second pipeline is executed only if the first succeeds
(returns a zero status). If two pipelines are separated by ‘||
’, the
second is executed only if the first fails (returns a nonzero status).
Both operators have equal precedence and are left associative. The value
of the sublist is the value of the last pipeline executed. For example,
dmesg | grep panic && print yes
is a sublist consisting of two pipelines, the second just a simple
command which will be executed if and only if the grep
command returns
a zero status. If it does not, the value of the sublist is that return
status, else it is the status returned by the print
(almost certainly
zero).
A list is a sequence of zero or more sublists, in which each sublist
is terminated by ‘;
’, ‘&
’, ‘&|
’, ‘&!
’, or a newline. This
terminator may optionally be omitted from the last sublist in the list
when the list appears as a complex command inside ‘(
...)
’ or
‘{
...}
’. When a sublist is terminated by ‘;
’ or newline, the
shell waits for it to finish before executing the next sublist. If a
sublist is terminated by a ‘&
’, ‘&|
’, or ‘&!
’, the shell executes
the last pipeline in it in the background, and does not wait for it to
finish (note the difference from other shells which execute the whole
sublist in the background). A backgrounded pipeline returns a status of
zero.
More generally, a list can be seen as a set of any shell commands whatsoever, including the complex commands below; this is implied wherever the word ‘list’ appears in later descriptions. For example, the commands in a shell function form a special sort of list.
6.2 Precommand Modifiers
A simple command may be preceded by a precommand modifier, which will
alter how the command is interpreted. These modifiers are shell builtin
commands with the exception of nocorrect
which is a reserved word.
-
The command is executed with a ‘-
’ prepended to its argv[0]
string.
builtin
The command word is taken to be the name of a builtin command, rather than a shell function or external command.
command
[ -pvV
]
The command word is taken to be the name of an external command, rather
than a shell function or builtin. If the POSIX_BUILTINS
option is set,
builtins will also be executed but certain special properties of them
are suppressed. The -p
flag causes a default path to be searched
instead of that in $path
. With the -v
flag, command
is similar to
whence
and with -V
, it is equivalent to whence -v
.
exec
[ -cl
] [ -a
argv0
]
The following command together with any arguments is run in place of the
current process, rather than as a sub-process. The shell does not fork
and is replaced. The shell does not invoke TRAPEXIT
, nor does it
source zlogout
files. The options are provided for compatibility with
other shells.
The -c
option clears the environment.
The -l
option is equivalent to the -
precommand modifier, to treat
the replacement command as a login shell; the command is executed with a
-
prepended to its argv[0]
string. This flag has no effect if used
together with the -a
option.
The -a
option is used to specify explicitly the argv[0]
string (the
name of the command as seen by the process itself) to be used by the
replacement command and is directly equivalent to setting a value for
the ARGV0
environment variable.
nocorrect
Spelling correction is not done on any of the words. This must appear before any other precommand modifier, as it is interpreted immediately, before any parsing is done. It has no effect in non-interactive shells.
noglob
Filename generation (globbing) is not performed on any of the words.
6.3 Complex Commands
A complex command in zsh is one of the following:
if
list
then
list
[ elif
list
then
list
] ... [
else
list
] fi
The if
list
is executed, and if it returns a zero exit status, the
then
list
is executed. Otherwise, the elif
list
is executed and
if its status is zero, the then
list
is executed. If each elif
list
returns nonzero status, the else
list
is executed.
for
name
... [ in
word
... ] term
do
list
done
Expand the list of word
s, and set the parameter name
to each of them
in turn, executing list
each time. If the ‘in
word
’ is omitted,
use the positional parameters instead of the word
s.
The term
consists of one or more newline or ;
which terminate the
word
s, and are optional when the ‘in
word
’ is omitted.
More than one parameter name
can appear before the list of word
s. If
N
name
s are given, then on each execution of the loop the next N
word
s are assigned to the corresponding parameters. If there are more
name
s than remaining word
s, the remaining parameters are each set to
the empty string. Execution of the loop ends when there is no remaining
word
to assign to the first name
. It is only possible for in
to
appear as the first name
in the list, else it will be treated as
marking the end of the list.
for ((
[expr1
] ;
[expr2
] ;
[expr3
] )) do
list
done
The arithmetic expression expr1
is evaluated first (see Arithmetic
Evaluation). The
arithmetic expression expr2
is repeatedly evaluated until it evaluates
to zero and when non-zero, list
is executed and the arithmetic
expression expr3
evaluated. If any expression is omitted, then it
behaves as if it evaluated to 1.
while
list
do
list
done
Execute the do
list
as long as the while
list
returns a zero
exit status.
until
list
do
list
done
Execute the do
list
as long as until
list
returns a nonzero exit
status.
repeat
word
do
list
done
word
is expanded and treated as an arithmetic expression, which must
evaluate to a number n
. list
is then executed n
times.
The repeat
syntax is disabled by default when the shell starts in a
mode emulating another shell. It can be enabled with the command
‘enable -r repeat
’
case
word
in
[ [(
] pattern
[ |
pattern
] ... )
list
(;;
|;&
|;|
) ] ... esac
Execute the list
associated with the first pattern
that matches
word
, if any. The form of the patterns is the same as that used for
filename generation. See Filename
Generation.
Note further that, unless the SH_GLOB
option is set, the whole pattern
with alternatives is treated by the shell as equivalent to a group of
patterns within parentheses, although white space may appear about the
parentheses and the vertical bar and will be stripped from the pattern
at those points. White space may appear elsewhere in the pattern; this
is not stripped. If the SH_GLOB
option is set, so that an opening
parenthesis can be unambiguously treated as part of the case syntax, the
expression is parsed into separate words and these are treated as strict
alternatives (as in other shells).
If the list
that is executed is terminated with ;&
rather than ;;
,
the following list is also executed. The rule for the terminator of the
following list ;;
, ;&
or ;|
is applied unless the esac
is
reached.
If the list
that is executed is terminated with ;|
the shell
continues to scan the pattern
s looking for the next match, executing
the corresponding list
, and applying the rule for the corresponding
terminator ;;
, ;&
or ;|
. Note that word
is not re-expanded; all
applicable pattern
s are tested with the same word
.
select
name
[ in
word
... term
] do
list
done
where term
is one or more newline or ;
to terminate the word
s.
Print the set of word
s,
each preceded by a number. If the in
word
is omitted, use the
positional parameters. The PROMPT3
prompt is printed and a line is
read from the line editor if the shell is interactive and that is
active, or else standard input. If this line consists of the number of
one of the listed word
s, then the parameter name
is set to the
word
corresponding to this number. If this line is empty, the
selection list is printed again. Otherwise, the value of the parameter
name
is set to null. The contents of the line read from standard input
is saved in the parameter REPLY
. list
is executed for each selection
until a break or end-of-file is encountered.
(
list
)
Execute list
in a subshell. Traps set by the trap
builtin are reset
to their default values while executing list
.
{
list
}
Execute list
.
{
try-list
} always {
always-list
}
First execute try-list
. Regardless of errors, or break
or continue
commands encountered within try-list
, execute always-list
. Execution
then continues from the result of the execution of try-list
; in other
words, any error, or break
or continue
command is treated in the
normal way, as if always-list
were not present. The two chunks of code
are referred to as the ‘try block’ and the ‘always block’.
Optional newlines or semicolons may appear after the always
; note,
however, that they may not appear between the preceding closing brace
and the always
.
An ‘error’ in this context is a condition such as a syntax error which
causes the shell to abort execution of the current function, script, or
list. Syntax errors encountered while the shell is parsing the code do
not cause the always-list
to be executed. For example, an erroneously
constructed if
block in try-list
would cause the shell to abort
during parsing, so that always-list
would not be executed, while an
erroneous substitution such as ${*foo*}
would cause a run-time error,
after which always-list
would be executed.
An error condition can be tested and reset with the special integer
variable TRY_BLOCK_ERROR
. Outside an always-list
the value is
irrelevant, but it is initialised to -1
. Inside always-list
, the
value is 1 if an error occurred in the try-list
, else 0. If
TRY_BLOCK_ERROR
is set to 0 during the always-list
, the error
condition caused by the try-list
is reset, and shell execution
continues normally after the end of always-list
. Altering the value
during the try-list
is not useful (unless this forms part of an
enclosing always
block).
Regardless of TRY_BLOCK_ERROR
, after the end of always-list
the
normal shell status $?
is the value returned from try-list
. This
will be non-zero if there was an error, even if TRY_BLOCK_ERROR
was
set to zero.
The following executes the given code, ignoring any errors it causes. This is an alternative to the usual convention of protecting code by executing it in a subshell.
{
# code which may cause an error
} always {
# This code is executed regardless of the error.
(( TRY_BLOCK_ERROR = 0 ))
}
# The error condition has been reset.
When a try
block occurs outside of any function, a return
or a
exit
encountered in try-list
does not cause the execution of
always-list
. Instead, the shell exits immediately after any EXIT
trap has been executed. Otherwise, a return
command encountered in
try-list
will cause the execution of always-list
, just like break
and continue
.
function
word
... [ ()
] [ term
] {
list
}
word
... ()
[ term
] {
list
}
word
... ()
[ term
] command
where term
is one or more newline or ;
. Define a function which is
referenced by any one of word
. Normally, only one word
is provided;
multiple word
s are usually only useful for setting traps. The body of
the function is the list
between the {
and }
. See
Functions.
If the option SH_GLOB
is set for compatibility with other shells, then
whitespace may appear between the left and right parentheses when there
is a single word
; otherwise, the parentheses will be treated as
forming a globbing pattern in that case.
In any of the forms above, a redirection may appear outside the function body, for example
func() { ... } 2>&1
The redirection is stored with the function and applied whenever the function is executed. Any variables in the redirection are expanded at the point the function is executed, but outside the function scope.
time
[ pipeline
]
The pipeline
is executed, and timing statistics are reported on the
standard error in the form specified by the TIMEFMT
parameter. If
pipeline
is omitted, print statistics about the shell process and its
children.
[[
exp
]]
Evaluates the conditional expression exp
and return a zero exit status
if it is true. See Conditional
Expressions for a
description of exp
.
6.4 Alternate Forms For Complex Commands
Many of zsh’s complex commands have alternate forms. These are non-standard and are likely not to be obvious even to seasoned shell programmers; they should not be used anywhere that portability of shell code is a concern.
The short versions below only work if sublist
is of the form ‘{
list
}
’ or if the SHORT_LOOPS
option is set. For the if
, while
and until
commands, in both these cases the test part of the loop must
also be suitably delimited, such as by ‘[[
...
]]
’ or ‘((
...
))
’, else the end of the test will not be recognized. For the for
,
repeat
, case
and select
commands no such special form for the
arguments is necessary, but the other condition (the special form of
sublist
or use of the SHORT_LOOPS
option) still applies.
-
if
list
{
list
}
[elif
list
{
list
}
] ... [else {
list
}
]
An alternate form ofif
. The rules mean thatif [[ -o ignorebraces ]] { print yes }
works, but
if true { # Does not work! print yes }
does not, since the test is not suitably delimited.
-
if
list
sublist
A short form of the alternateif
. The same limitations on the form oflist
apply as for the previous form. -
for
name
...(
word
...)
sublist
A short form offor
. -
for
name
... [in
word
... ]term
sublist
whereterm
is at least one newline or;
. Another short form offor
. -
for ((
[expr1
];
[expr2
];
[expr3
]))
sublist
A short form of the arithmeticfor
command. -
foreach
name
...(
word
...)
list
end
Another form offor
. -
while
list
{
list
}
An alternative form ofwhile
. Note the limitations on the form oflist
mentioned above. -
until
list
{
list
}
An alternative form ofuntil
. Note the limitations on the form oflist
mentioned above. -
repeat
word
sublist
This is a short form ofrepeat
. -
case
word
{
[ [(
]pattern
[|
pattern
] ...)
list
(;;
|;&
|;|
) ] ...}
An alternative form ofcase
. -
select
name
[in
word
...term
]sublist
whereterm
is at least one newline or;
. A short form ofselect
. -
function
word
... [()
] [term
]sublist
This is a short form offunction
.
6.5 Reserved Words
The following words are recognized as reserved words when used as the
first word of a command unless quoted or disabled using disable -r
:
do done esac then elif else fi for case if while function repeat time until select coproc nocorrect foreach end ! [[ { } declare export float integer local readonly typeset
Additionally, ‘}
’ is recognized in any position if neither the
IGNORE_BRACES
option nor the IGNORE_CLOSE_BRACES
option is set.
6.6 Errors
Certain errors are treated as fatal by the shell: in an interactive
shell, they cause control to return to the command line, and in a
non-interactive shell they cause the shell to be aborted. In older
versions of zsh, a non-interactive shell running a script would not
abort completely, but would resume execution at the next command to be
read from the script, skipping the remainder of any functions or shell
constructs such as loops or conditions; this somewhat illogical
behaviour can be recovered by setting the option CONTINUE_ON_ERROR
.
Fatal errors found in non-interactive shells include:
- Failure to parse shell options passed when invoking the shell
- Failure to change options with the
set
builtin - Parse errors of all sorts, including failures to parse mathematical expressions
- Failures to set or modify variable behaviour with
typeset
,local
,declare
,export
,integer
,float
- Execution of incorrectly positioned loop control structures
(
continue
,break
) - Attempts to use regular expression with no regular expression module available
- Disallowed operations when the
RESTRICTED
options is set - Failure to create a pipe needed for a pipeline
- Failure to create a multio
- Failure to autoload a module needed for a declared shell feature
- Errors creating command or process substitutions
- Syntax errors in glob qualifiers
- File generation errors where not caught by the option
BAD_PATTERN
- All bad patterns used for matching within case statements
- File generation failures where not caused by
NO_MATCH
or similar options - All file generation errors where the pattern was used to create a multio
- Memory errors where detected by the shell
- Invalid subscripts to shell variables
- Attempts to assign read-only variables
- Logical errors with variables such as assignment to the wrong type
- Use of invalid variable names
- Errors in variable substitution syntax
- Failure to convert characters in
$’
...’
expressions
If the POSIX_BUILTINS
option is set, more errors associated with shell
builtin commands are treated as fatal, as specified by the POSIX
standard.
6.7 Comments
In non-interactive shells, or in interactive shells with the
INTERACTIVE_COMMENTS
option set, a word beginning with the third
character of the histchars
parameter (‘#
’ by default) causes that
word and all the following characters up to a newline to be ignored.
6.8 Aliasing
Every eligible word in the shell input is checked to see if there is
an alias defined for it. If so, it is replaced by the text of the alias
if it is in command position (if it could be the first word of a simple
command), or if the alias is global. If the replacement text ends with a
space, the next word in the shell input is always eligible for purposes
of alias expansion.
An alias is defined using
the alias
builtin; global aliases may be defined using the -g
option
to that builtin.
A word is defined as:
- Any plain string or glob pattern
- Any quoted string, using any quoting method (note that the quotes must be part of the alias definition for this to be eligible)
- Any parameter reference or command substitution
- Any series of the foregoing, concatenated without whitespace or other tokens between them
- Any reserved word (
case
,do
,else
, etc.) - With global aliasing, any command separator, any redirection
operator, and ‘
(
’ or ‘)
’ when not part of a glob pattern
Alias expansion is done on the shell input before any other expansion
except history expansion. Therefore, if an alias is defined for the word
foo
, alias expansion may be avoided by quoting part of the word, e.g.
\foo
. Any form of quoting works, although there is nothing to prevent
an alias being defined for the quoted form such as \foo
as well.
When POSIX_ALIASES
is set, only plain unquoted strings are eligible
for aliasing. The alias
builtin does not reject ineligible aliases,
but they are not expanded.
For use with completion, which would remove an initial backslash
followed by a character that isn’t special, it may be more convenient to
quote the word by starting with a single quote, i.e. ’foo
; completion
will automatically add the trailing single quote.
6.8.1 Alias difficulties
Although aliases can be used in ways that bend normal shell syntax, not every string of non-white-space characters can be used as an alias.
Any set of characters not listed as a word above is not a word, hence no
attempt is made to expand it as an alias, no matter how it is defined
(i.e. via the builtin or the special parameter aliases
described in
The zsh/parameter
Module). However, as
noted in the case of POSIX_ALIASES
above, the shell does not attempt
to deduce whether the string corresponds to a word at the time the alias
is created.
For example, an expression containing an =
at the start of a command
line is an assignment and cannot be expanded as an alias; a lone =
is
not an assignment but can only be set as an alias using the parameter,
as otherwise the =
is taken part of the syntax of the builtin command.
It is not presently possible to alias the ‘((
’ token that introduces
arithmetic expressions, because until a full statement has been parsed,
it cannot be distinguished from two consecutive ‘(
’ tokens introducing
nested subshells. Also, if a separator such as &&
is aliased, \&&
turns into the two tokens \&
and &
, each of which may have been
aliased separately. Similarly for \<<
, \>|
, etc.
There is a commonly encountered problem with aliases illustrated by the following code:
alias echobar='echo bar'; echobar
This prints a message that the command echobar
could not be found.
This happens because aliases are expanded when the code is read in; the
entire line is read in one go, so that when echobar
is executed it is
too late to expand the newly defined alias. This is often a problem in
shell scripts, functions, and code executed with ‘source
’ or ‘.
’.
Consequently, use of functions rather than aliases is recommended in
non-interactive code.
Note also the unhelpful interaction of aliases and function definitions:
alias func='noglob func'
func() {
echo Do something with $*
}
Because aliases are expanded in function definitions, this causes the following command to be executed:
noglob func() {
echo Do something with $*
}
which defines noglob
as well as func
as functions with the body
given. To avoid this, either quote the name func
or use the
alternative function definition form ‘function func
’. Ensuring the
alias is defined after the function works but is problematic if the code
fragment might be re-executed.
6.9 Quoting
A character may be quoted (that is, made to stand for itself) by
preceding it with a ‘\
’. ‘\
’ followed by a newline is ignored.
A string enclosed between ‘$’
’ and ‘’
’ is processed the same way as
the string arguments of the print
builtin, and the resulting string is
considered to be entirely quoted. A literal ‘’
’ character can be
included in the string by using the ‘\’
’ escape.
All characters enclosed between a pair of single quotes (’’
) that is
not preceded by a ‘$
’ are quoted. A single quote cannot appear within
single quotes unless the option RC_QUOTES
is set, in which case a pair
of single quotes are turned into a single quote. For example,
print ''''
outputs nothing apart from a newline if RC_QUOTES
is not set, but one
single quote if it is set.
Inside double quotes (""
), parameter and command substitution occur,
and ‘\
’ quotes the characters ‘\
’, ‘‘
’, ‘"
’, ‘$
’, and the
first character of $histchars
(default ‘!
’).
This document was generated on February 15, 2020 using
texi2html 5.0.
Zsh version 5.8, released on February 14, 2020.