path: root/Doc/Zsh/grammar.yo

texinode(Shell Grammar)(Redirection)(Files)(Top)
chapter(Shell Grammar)
cindex(shell grammar)
cindex(grammar, shell)
startmenu()
menu(Simple Commands & Pipelines)
menu(Precommand Modifiers)
menu(Complex Commands)
menu(Alternate Forms For Complex Commands)
menu(Reserved Words)
menu(Errors)
menu(Comments)
menu(Aliasing)
menu(Quoting)
endmenu()
texinode(Simple Commands & Pipelines)(Precommand Modifiers)()(Shell Grammar)
sect(Simple Commands & Pipelines)
cindex(simple commands)
cindex(commands, simple)
A em(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
ifnzman(noderef(Parameters))\
ifzman(zmanref(zshparam)).

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,

example(echo foo)

is a simple command with arguments.

cindex(pipeline)
A em(pipeline) is either a simple command, or a sequence of two or more
simple commands where each command is separated from the next by `tt(|)'
or `tt(|&)'.  Where commands are separated by `tt(|)', the standard
output of the first command is connected to the
standard input of the next.  `tt(|&)' is shorthand for `tt(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
`tt(!)' in which case the value is the logical inverse of the value of the
last command.
For example,

example(echo foo | sed 's/foo/bar/')

is a pipeline, where the output (`tt(foo)' plus a newline) of the first
command will be passed to the input of the second.

findex(coproc)
cindex(coprocess)
If a pipeline is preceded by `tt(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 `tt(>&p)'
and `tt(<&p)' redirection operators or with `tt(print -p)' and `tt(read -p)'.
A pipeline cannot be preceded by both `tt(coproc)' and `tt(!)'.
If job control is active, the coprocess can be treated in other than input
and output as an ordinary background job.

cindex(sublist)
A em(sublist) is either a single pipeline, or a sequence of two or more
pipelines separated by `tt(&&)' or `tt(||)'.  If two pipelines are separated
by `tt(&&)', the second pipeline is executed only if the first succeeds
(returns a zero status).  If two pipelines are separated by `tt(||)', 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,

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 tt(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 tt(print) (almost certainly zero).

cindex(list)
A em(list) is a sequence of zero or more sublists, in which each sublist
is terminated by `tt(;)', `tt(&)', `tt(&|)', `tt(&!)', 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 `tt(LPAR())...tt(RPAR())'
or `tt({)...tt(})'.  When a
sublist is terminated by `tt(;)' or newline, the shell waits for it to
finish before executing the next sublist.  If a sublist is terminated
by a `tt(&)', `tt(&|)', or `tt(&!)',
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.

texinode(Precommand Modifiers)(Complex Commands)(Simple Commands & Pipelines)(Shell Grammar)
sect(Precommand Modifiers)
cindex(precommand modifiers)
cindex(modifiers, precommand)
With the exception of tt(nocorrect), which is a reserved word that
affects further parsing when it is found in command position, each
of the following builtin commands is a em(precommand modifier) which
may precede a simple command to alter how that command is interpreted.

startitem()
findex(-)
item(tt(-))(
The command is executed with a `tt(-)' prepended to its
tt(argv[0]) string.
)
findex(builtin)
item(tt(builtin))(
The command word is taken to be the name of a builtin command,
rather than a shell function or external command.
)
findex(command)
item(tt(command) [ tt(-pvV) ])(
The command word is taken to be the name of an external command,
rather than a shell function or builtin.   If the tt(POSIX_BUILTINS) option
is set, builtins will also be executed but certain special properties
of them are suppressed. The tt(-p) flag causes a default path to be
searched instead of that in tt($path). With the tt(-v) flag, tt(command)
is similar to tt(whence) and with tt(-V), it is equivalent to tt(whence
-v).
)
findex(exec)
item(tt(exec) [ tt(-cl) ] [ tt(-a) var(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 tt(TRAPEXIT), nor does it
source tt(zlogout) files.
The options are provided for compatibility with other shells.

The tt(-c) option clears the environment.

The tt(-l) option is equivalent to the tt(-) precommand modifier, to
treat the replacement command as a login shell; the command is executed
with a tt(-) prepended to its tt(argv[0]) string.  This flag has no effect
if used together with the tt(-a) option.

The tt(-a) option is used to specify explicitly the tt(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 tt(ARGV0) environment variable.
)
findex(nocorrect)
item(tt(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.
)
findex(noglob)
item(tt(noglob))(
Filename generation (globbing) is not performed on any of the words.
)
enditem()
texinode(Complex Commands)(Alternate Forms For Complex Commands)(Precommand Modifiers)(Shell Grammar)
sect(Complex Commands)
cindex(complex commands)
cindex(commands, complex)
A em(complex command) in zsh is one of the following:

startitem()
findex(if)
cindex(if construct)
item(tt(if) var(list) tt(then) var(list) [ tt(elif) var(list) tt(then) var(list) ] ... [ tt(else) var(list) ] tt(fi))(
The tt(if) var(list) is executed, and if it returns a zero exit status,
the tt(then) var(list) is executed.
Otherwise, the tt(elif) var(list) is executed and if its status is zero,
the tt(then) var(list) is executed.
If each tt(elif) var(list) returns nonzero status, the tt(else) var(list)
is executed.
)
findex(for)
cindex(for loops)
cindex(loops, for)
item(tt(for) var(name) ... [ tt(in) var(word) ... ] var(term) tt(do) var(list) tt(done))(
Expand the list of var(word)s, and set the parameter
var(name) to each of them in turn, executing var(list)
each time.  If the `tt(in) var(word)' is omitted,
use the positional parameters instead of the var(word)s.
If any var(name) has been declared as a named reference,
the corresponding var(word) is treated as the name of a
parameter and var(name) is made a reference to that.
Note that for the positional parameters, this treats the
value of each positional as parameter name rather than
creating a reference to the position.

The var(term) consists of one or more newline or tt(;)
which terminate the var(word)s, and are optional when the
`tt(in) var(word)' is omitted.

More than one parameter var(name) can appear before the list of
var(word)s.  If var(N) var(name)s are given, then on each execution of the
loop the next var(N) var(word)s are assigned to the corresponding
parameters.  If there are more var(name)s than remaining var(word)s, the
remaining parameters are each set to the empty string.  Execution of the
loop ends when there is no remaining var(word) to assign to the first
var(name).  It is only possible for tt(in) to appear as the first var(name)
in the list, else it will be treated as marking the end of the list.
)
item(tt(for LPAR()LPAR()) [var(expr1)] tt(;) [var(expr2)] tt(;) [var(expr3)] tt(RPAR()RPAR() do) var(list) tt(done))(
The arithmetic expression var(expr1) is evaluated first (see
noderef(Arithmetic Evaluation)).  The arithmetic expression
var(expr2) is repeatedly evaluated until it evaluates to zero and
when non-zero, var(list) is executed and the arithmetic expression
var(expr3) evaluated.  If any expression is omitted, then it behaves
as if it evaluated to 1.
)
findex(while)
cindex(while loops)
cindex(loops, while)
item(tt(while) var(list) tt(do) var(list) tt(done))(
Execute the tt(do) var(list) as long as the tt(while) var(list)
returns a zero exit status.
)
findex(until)
cindex(until loops)
cindex(loops, until)
item(tt(until) var(list) tt(do) var(list) tt(done))(
Execute the tt(do) var(list) as long as tt(until) var(list)
returns a nonzero exit status.
)
findex(repeat)
cindex(repeat loops)
cindex(loops, repeat)
item(tt(repeat) var(word) tt(do) var(list) tt(done))(
var(word) is expanded and treated as an arithmetic expression,
which must evaluate to a number var(n).
var(list) is then executed var(n) times.

The tt(repeat) syntax is disabled by default when the
shell starts in a mode emulating another shell.  It can be enabled
with the command `tt(enable -r repeat)'
)
findex(case)
cindex(case selection)
cindex(selection, case)
item(tt(case) var(word) tt(in) [ [tt(LPAR())] var(pattern) [ tt(|) var(pattern) ] ... tt(RPAR()) var(list) (tt(;;)|tt(;&)|tt(;|)) ] ... tt(esac))(
Execute the var(list) associated with the first var(pattern)
that matches var(word), if any.  The form of the patterns
is the same as that used for filename generation.  See
noderef(Filename Generation).

Note further that, unless the tt(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 tt(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 var(list) that is executed is terminated with tt(;&) rather than
tt(;;), the following list is also executed.  The rule for
the terminator of the following list tt(;;), tt(;&) or tt(;|) is
applied unless the tt(esac) is reached.

If the var(list) that is executed is terminated with tt(;|) the
shell continues to scan the var(pattern)s looking for the next match,
executing the corresponding var(list), and applying the rule for
the corresponding terminator tt(;;), tt(;&) or tt(;|).
Note that var(word) is not re-expanded; all applicable var(pattern)s
are tested with the same var(word).
)
findex(select)
cindex(user selection)
cindex(selection, user)
item(tt(select) var(name) [ tt(in) var(word) ... var(term) ] tt(do) var(list) tt(done))(
where var(term) is one or more newline or tt(;) to terminate the var(word)s.
vindex(REPLY, use of)
Print the set of var(word)s, each preceded by a number.
If the tt(in) var(word) is omitted, use the positional parameters.
The tt(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 var(word)s, then the parameter var(name)
is set to the var(word) corresponding to this number.
If this line is empty, the selection list is printed again.
Otherwise, the value of the parameter var(name) is set to null.
The contents of the line read from standard input is saved
in the parameter tt(REPLY).  var(list) is executed
for each selection until a break or end-of-file is encountered.
)
cindex(subshell)
item(tt(LPAR()) var(list) tt(RPAR()))(
Execute var(list) in a subshell.  Traps set by the tt(trap) builtin
are reset to their default values while executing var(list); an
exception is that ignored signals will continue to be ignored
if the option tt(POSIXTRAPS) is set.
)
item(tt({) var(list) tt(}))(
Execute var(list).
)
findex(always)
cindex(always blocks)
cindex(try blocks)
item(tt({) var(try-list) tt(} always {) var(always-list) tt(}))(
First execute var(try-list).  Regardless of errors, or tt(break) or
tt(continue) commands encountered within var(try-list),
execute var(always-list).  Execution then continues from the
result of the execution of var(try-list); in other words, any error,
or tt(break) or tt(continue) command is treated in the
normal way, as if var(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 tt(always);
note, however, that they may em(not) appear between the preceding
closing brace and the tt(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 var(always-list) to be executed.  For example,
an erroneously constructed tt(if) block in var(try-list) would cause the
shell to abort during parsing, so that var(always-list) would not be
executed, while an erroneous substitution such as tt(${*foo*}) would
cause a run-time error, after which var(always-list) would be executed.

An error condition can be tested and reset with the special integer
variable tt(TRY_BLOCK_ERROR).  Outside an var(always-list) the value is
irrelevant, but it is initialised to tt(-1).  Inside var(always-list), the
value is 1 if an error occurred in the var(try-list), else 0.  If
tt(TRY_BLOCK_ERROR) is set to 0 during the var(always-list), the error
condition caused by the var(try-list) is reset, and shell execution
continues normally after the end of var(always-list).  Altering the value
during the var(try-list) is not useful (unless this forms part of an
enclosing tt(always) block).

Regardless of tt(TRY_BLOCK_ERROR), after the end of var(always-list) the
normal shell status tt($?) is the value returned from var(try-list).
This will be non-zero if there was an error, even if tt(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.

example({
    # 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 tt(try) block occurs outside of any function, 
a tt(return) or a tt(exit) encountered in var(try-list) does em(not) cause
the execution of var(always-list).  Instead, the shell exits immediately
after any tt(EXIT) trap has been executed.
Otherwise, a tt(return) command encountered in var(try-list) will cause the
execution of var(always-list), just like tt(break) and tt(continue).

COMMENT(The semantics of calling 'exit' in try-list inside a function are
deliberately left unspecified, because historically there was a mismatch between
the documented and implemented behaviours.  Cf. 20076, 21734/21735, 45075.)
)
findex(function)
xitem(tt(function) [ tt(-T) ] var(word) ... [ tt(()) ] [ var(term) ] tt({) var(list) tt(}))
xitem(var(word) ... tt(()) [ var(term) ] tt({) var(list) tt(}))
item(var(word) ... tt(()) [ var(term) ] var(command))(
where var(term) is one or more newline or tt(;).
Define a function which is referenced by any one of var(word).
Normally, only one var(word) is provided; multiple var(word)s
are usually only useful for setting traps.
The body of the function is the var(list) between
the tt({) and tt(}).  See noderef(Functions).

The options of tt(function) have the following meanings:

startitem()
item(-T)(
Enable tracing for this function, as though with tt(functions -T).  See the
documentation of the tt(-f) option to the tt(typeset) builtin, in
ifzman(zmanref(zshbuiltins))\
ifnzman(noderef(Shell Builtin Commands)).
)
enditem()

If the option tt(SH_GLOB) is set for compatibility with other shells, then
whitespace may appear between the left and right parentheses when
there is a single var(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

example(func+LPAR()RPAR() { ... } 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.
)
cindex(timing)
findex(time)
item(tt(time) [ var(pipeline) ])(
The var(pipeline) is executed, and timing statistics are
reported on the standard error in the form specified
by the tt(TIMEFMT) parameter.
If var(pipeline) is omitted, print statistics about the
shell process and its children.
)
cindex(conditional expression)
findex([[)
item(tt([[) var(exp) tt(]]))(
Evaluates the conditional expression var(exp)
and return a zero exit status if it is true.
See noderef(Conditional Expressions)
for a description of var(exp).
)
enditem()
texinode(Alternate Forms For Complex Commands)(Reserved Words)(Complex Commands)(Shell Grammar)
sect(Alternate Forms For Complex Commands)
cindex(alternate forms for complex commands)
cindex(commands, alternate forms for complex)
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 var(sublist) is of the form `tt({)
var(list) tt(})' or if the tt(SHORT_LOOPS) option is set.  For the tt(if),
tt(while) and tt(until) commands, in both these cases the test part of the
loop must also be suitably delimited, such as by `tt([[) var(...) tt(]])' or
`tt(LPAR()LPAR()) var(...) tt(RPAR()RPAR())',
else the end of the test will not be recognized.  For the
tt(for), tt(repeat), tt(case) and tt(select) commands no such special form
for the arguments is necessary, but the other condition (the special form
of var(sublist) or use of the tt(SHORT_LOOPS) option) still applies.
The tt(SHORT_REPEAT) option is available to enable the short version only
for the tt(repeat) command.

startitem()
item(tt(if) var(list) tt({) var(list) tt(}) [ tt(elif) var(list) tt({) var(list) tt(}) ] ... [ tt(else {) var(list) tt(}) ])(
An alternate form of tt(if).  The rules mean that

example(if [[ -o ignorebraces ]] {
  print yes
})

works, but

example(if true {  # Does not work!
  print yes
})

does em(not), since the test is not suitably delimited.
)
item(tt(if) var(list) var(sublist))(
A short form of the alternate tt(if).  The same limitations on the form of
var(list) apply as for the previous form.
)
item(tt(for) var(name) ... tt(LPAR()) var(word) ... tt(RPAR()) var(sublist))(
A short form of tt(for).
)
item(tt(for) var(name) ... [ tt(in) var(word) ... ] var(term) var(sublist))(
where var(term) is at least one newline or tt(;).
Another short form of tt(for).
)
item(tt(for LPAR()LPAR()) [var(expr1)] tt(;) [var(expr2)] tt(;) [var(expr3)] tt(RPAR()RPAR()) var(sublist))(
A short form of the arithmetic tt(for) command.
)
findex(foreach)
item(tt(foreach) var(name) ... tt(LPAR()) var(word) ... tt(RPAR()) var(list) tt(end))(
Another form of tt(for).
)
item(tt(while) var(list) tt({) var(list) tt(}))(
An alternative form of tt(while).  Note the limitations on the form of
var(list) mentioned above.
)
item(tt(until) var(list) tt({) var(list) tt(}))(
An alternative form of tt(until).  Note the limitations on the form of
var(list) mentioned above.
)
item(tt(repeat) var(word) var(sublist))(
This is a short form of tt(repeat).
)
item(tt(case) var(word) tt({) [ [tt(LPAR())] var(pattern) [ tt(|) var(pattern) ] ... tt(RPAR()) var(list) (tt(;;)|tt(;&)|tt(;|)) ] ... tt(}))(
An alternative form of tt(case).
)
item(tt(select) var(name) [ tt(in) var(word) ... var(term) ] var(sublist))(
where var(term) is at least one newline or tt(;).
A short form of tt(select).
)
item(tt(function) var(word) ... [ tt(+LPAR()+RPAR()) ] [ var(term) ] var(sublist))(
This is a short form of tt(function).
)
enditem()
texinode(Reserved Words)(Errors)(Alternate Forms For Complex Commands)(Shell Grammar)
sect(Reserved Words)
cindex(reserved words)
findex(disable, use of)
The following words are recognized as reserved words when used as the first
word of a command unless quoted or disabled using tt(disable -r):

tt(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, `tt(})' is recognized in any position if neither the
tt(IGNORE_BRACES) option nor the tt(IGNORE_CLOSE_BRACES) option is set.
texinode(Errors)(Comments)(Reserved Words)(Shell Grammar)
sect(Errors)
cindex(errors, handling of)
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 tt(CONTINUE_ON_ERROR).

Fatal errors found in non-interactive shells include:

startitemize()
itemiz(Failure to parse shell options passed when invoking the shell)
itemiz(Failure to change options with the tt(set) builtin)
itemiz(Parse errors of all sorts, including failures to parse
mathematical expressions)
itemiz(Failures to set or modify variable behaviour with tt(typeset),
tt(local), tt(declare), tt(export), tt(integer), tt(float))
itemiz(Execution of incorrectly positioned loop control structures
(tt(continue), tt(break)))
itemiz(Attempts to use regular expression with no regular expression
module available)
itemiz(Disallowed operations when the tt(RESTRICTED) options is set)
itemiz(Failure to create a pipe needed for a pipeline)
itemiz(Failure to create a multio)
itemiz(Failure to autoload a module needed for a declared shell feature)
itemiz(Errors creating command or process substitutions)
itemiz(Syntax errors in glob qualifiers)
itemiz(File generation errors where not caught by the option tt(BAD_PATTERN))
itemiz(All bad patterns used for matching within case statements)
itemiz(File generation failures where not caused by tt(NOMATCH) or
similar options)
itemiz(All file generation errors where the pattern was used to create a
multio)
itemiz(Memory errors where detected by the shell)
itemiz(Invalid subscripts to shell variables)
itemiz(Attempts to assign read-only variables)
itemiz(Logical errors with variables such as assignment to the wrong type)
itemiz(Use of invalid variable names)
itemiz(Errors in variable substitution syntax)
itemiz(Failure to convert characters in tt($')...tt(') expressions)
enditemize()

If the tt(POSIX_BUILTINS) option is set, more errors associated with
shell builtin commands are treated as fatal, as specified by the POSIX
standard.

texinode(Comments)(Aliasing)(Errors)(Shell Grammar)
sect(Comments)
cindex(comments)
pindex(INTERACTIVE_COMMENTS, use of)
vindex(histchars, use of)
In non-interactive shells, or in interactive shells with the
tt(INTERACTIVE_COMMENTS) option set, a word beginning
with the third character of the tt(histchars) parameter
(`tt(#)' by default) causes that word and all the following
characters up to a newline to be ignored.
texinode(Aliasing)(Quoting)(Comments)(Shell Grammar)
sect(Aliasing)
cindex(aliasing)
Every eligible em(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.

It is an error for the function name, var(word), in the sh-compatible function
definition syntax `var(word) tt(+LPAR()+RPAR()) ...' to be a word that resulted
from alias expansion, unless the tt(ALIAS_FUNC_DEF) option is set.

findex(alias, use of)
cindex(aliases, global)
An alias is defined using the tt(alias) builtin; global aliases
may be defined using the tt(-g) option to that builtin.

A em(word) is defined as:

startitemize()
itemiz(Any plain string or glob pattern)
itemiz(Any quoted string, using any quoting method (note that the quotes
must be part of the alias definition for this to be eligible))
itemiz(Any parameter reference or command substitution)
itemiz(Any series of the foregoing, concatenated without whitespace or
other tokens between them)
itemiz(Any reserved word (tt(case), tt(do), tt(else), etc.))
itemiz(With global aliasing, any command separator, any redirection
operator, and `tt(LPAR())' or `tt(RPAR())' when not part of a glob pattern)
enditemize()

Alias expansion is done on the shell input before any other expansion
except history expansion.  Therefore, if an alias is defined for the
word tt(foo), alias expansion may be avoided by quoting part of the
word, e.g. tt(\foo).  Any form of quoting works, although there is
nothing to prevent an alias being defined for the quoted form such as
tt(\foo) as well.

In particular, note that quoting must be used when using tt(unalias) to remove
global aliases:

example(% alias -g foo=bar
% unalias foo
unalias: no such hash table element: bar
% unalias \foo
% )

When tt(POSIX_ALIASES) is set, only plain unquoted strings are eligible
for aliasing.  The tt(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. tt('foo); completion will
automatically add the trailing single quote.

subsect(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 tt(aliases) described in
ifnzman(noderef(The zsh/parameter Module))\
ifzman(the section THE ZSH/PARAMETER MODULE in zmanref(zshmodules))).
However, as noted in the case of tt(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 tt(=) at the start of
a command line is an assignment and cannot be expanded as an alias;
a lone tt(=) is not an assignment but can only be set as an alias
using the parameter, as otherwise the tt(=) is taken part of the
syntax of the builtin command.

It is not presently possible to alias the `tt(LPAR()LPAR())' token that
introduces arithmetic expressions, because until a full statement has been
parsed, it cannot be distinguished from two consecutive `tt(LPAR())'
tokens introducing nested subshells.
Also, if a separator such as tt(&&) is aliased,
tt(\&&) turns into the two tokens tt(\&) and tt(&), each of which may
have been aliased separately.  Similarly for tt(\<<), tt(\>|), etc.

There is a commonly encountered problem with aliases
illustrated by the following code:

example(alias echobar='echo bar'; echobar)

This prints a message that the command tt(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 tt(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 `tt(source)'
or `tt(.)'.  Consequently, use of functions rather than aliases is
recommended in non-interactive code.

texinode(Quoting)()(Aliasing)(Shell Grammar)
sect(Quoting)
cindex(quoting)
A character may be em(quoted) (that is, made
to stand for itself) by preceding it with a `tt(\)'.
`tt(\)' followed by a newline is ignored.

A string enclosed between `tt($')' and `tt(')' is
processed the same way as the string arguments of the
tt(print) builtin, and the resulting string is considered to be
entirely quoted.  A literal `tt(')' character can be included in the
string by using the `tt(\')' escape.

pindex(RC_QUOTES, use of)
All characters enclosed between a pair of single quotes (tt('')) that
is not preceded by a `tt($)' are quoted.  A single quote cannot appear
within single quotes unless the option tt(RC_QUOTES) is set, in which case
a pair of single quotes are turned into a single quote.  For example,

example(print '''')

outputs nothing apart from a newline if tt(RC_QUOTES) is not set, but one
single quote if it is set.

Inside double quotes (tt("")), parameter and
command substitution occur, and `tt(\)' quotes the characters
`tt(\)', `tt(`)', `tt(")', `tt($)', and the first character
of tt($histchars) (default `tt(!)').