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) A simple command may be preceded by a em(precommand modifier), which will alter how the command is interpreted. These modifiers are shell builtin commands with the exception of tt(nocorrect) which is a reserved word. 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. 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(!)').