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# 5. Shell Grammar

> 5.1 Simple Commands & Pipelines |  |
> 5.2 Precommand Modifiers |  |
> 5.3 Complex Commands |  |
> 5.4 Alternate Forms For Complex Commands |  |
> 5.5 Reserved Words |  |
> 5.6 Comments |  |
> 5.7 Aliasing |  |
> 5.8 Quoting |  |

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## 5.1 Simple Commands & Pipelines

A simple command is a sequence of optional parameter assignments followed by blank-separated words, with optional redirections interspersed. 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 value). If two pipelines are separated by ``||`', the second is executed only if the first fails (returns a nonzero value). 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 value. If it does not, the value of the sublist is that return value, else it is the value 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.

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## 5.2 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.

`noglob`
  Filename generation (globbing) is not performed on any of the words.

`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.

`exec`
  The command is executed in the parent shell without forking.

`command`
  The command word is taken to be the name of an external command, rather than a shell function or builtin.

`builtin`
  The command word is taken to be the name of a builtin command, rather than a shell function or external command.

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## 5.3 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 value is zero, the `then` list is executed. If each `elif` list returns nonzero, the `else` list is executed.

`for` name ... [ `in` word ... ] term `do` list `done`
  where term is at least one newline or `;`. Expand the list of words, 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 words.

  More than one parameter name can appear before the list of words. If N names are given, then on each execution of the loop the next `N` words are assigned to the corresponding parameters. If there are more names than remaining words, 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 10. 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.

`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 13.8 Filename Generation. If the list that is executed is terminated with `;&` rather than `;;`, the following list is also executed. This continues until either a list is terminated with `;;` or the `esac` is reached.

`select` name [ `in` word ... term ] `do` list `done`
  where term is one or more newline or `;` to terminate the words. Print the set of words, 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 words, 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`, `continue`, or `return` 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`, `continue`, or `return` 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 preceeding 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 `always-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.

  An `exit` command encountered in `try-list` does not cause the execution of always-list. Instead, the shell exits immediately after any `EXIT` trap has been executed.

`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 words are usually only useful for setting traps. The body of the function is the list between the `{` and `}`. See 8. Functions.

  If the option `SH_GLOB` is set for compatibility with other shells, then whitespace may appear between 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.

`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 11. Conditional Expressions for a description of exp.

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## 5.4 Alternate Forms For Complex Commands

Many of zsh's complex commands have alternate forms. These particular versions of complex commands should be considered deprecated and may be removed in the future. The versions in the previous section should be preferred instead.

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 of `if`. The rules mean that

      if [[ -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 alternate `if'. The same limitations on the form of list apply as for the previous form.

`for` name ... `(` word ... `)` sublist
  A short form of `for`.

`for` name ... [ `in` word ... ] term sublist
  where term is at least one newline or `;`. Another short form of `for`.

`for ((` [expr1] `;` [expr2] `;` [expr3] `))` sublist
  A short form of the arithmetic `for` command.

`foreach` name ... `(` word ... `)` list `end`
  Another form of `for`.

`while` list `{` list `}`
  An alternative form of `while`. Note the limitations on the form of list mentioned above.

`until` list `{` list `}`
  An alternative form of `until`. Note the limitations on the form of list mentioned above.

`repeat` word sublist
  This is a short form of `repeat`.

`case` word `{` [ [`(`] pattern [ `|` pattern ] ... `)` list (`;;`|`;&`) ] ... `}`
  An alternative form of `case`.

`select` name [ `in` word term ] sublist
  where term is at least one newline or `;`. A short form of `select`.

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## 5.5 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 ! [[ { }`

Additionally, ``}`' is recognized in any position if the `IGNORE_BRACES` option is not set.

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## 5.6 Comments

In noninteractive 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.

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## 5.7 Aliasing

Every token 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 text ends with a space, the next word in the shell input is treated as though it were in command position 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.

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`. But there is nothing to prevent an alias being defined for `\foo` as well.

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## 5.8 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 ``$`'.

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This document was generated by Peter Stephenson on November, 30 2005 using texi2html