= Syntax
:encoding: UTF-8
:lang: en
//:title: Yash manual - Syntax
:description: This page defines yash's command syntax and semantics.

The shell reads, parses, and executes command line by line. If there is more
than one command on a line, all the commands are parsed before executed. If a
command is continued to next lines, the shell reads more enough lines to
complete the command. On a syntax error, the shell neither reads nor executes
any more commands.

[[tokens]]
== Tokens and keywords

A command is composed of one or more tokens. In the shell syntax, a
dfn:[token] is a word that is part of a command. Normally, tokens are
separated by whitespaces, that is, the space or tab character. Whitespaces
inside a command substitution or a parameter expansion, however, do not
separate tokens.

The following symbols have special meanings in the shell syntax and in most
cases separate tokens:

 ; & | < > ( ) [newline]

The following symbols do not separate tokens, but have syntactic meanings:

 $ ` \ " ' * ? [ # ~ = %

The following tokens are treated as dfn:[keywords] depending on the context in
which they appear:

 ! { } case do done elif else esac fi
 for function if in then until while

A token is treated as a keyword when:

* it is the first token of a command,
* it follows another keyword (except +case+, +for+, and +in+), or
* it is a non-first token of a command and is supposed to be a keyword to
  compose a composite command.

If a token begins with `#`, then the `#` and any following characters up to
the end of the line are treated as a dfn:[comment], which is completely
ignored in syntax parsing.

[[quotes]]
== Quotations

If you want whitespaces, separator characters, or keywords described above to
be treated as a normal characters, you must quote the characters using
appropriate quotation marks. Quotation marks are not treated as normal
characters unless they are themselves quoted. You can use the following three
quotation marks:

* A backslash (+\+) quotes a character that immediately follows.
  +
  The only exception about a backslash is the case where a backslash is
  followed by a newline. In this case, the two characters are treated as a
  dfn:[line continuation] rather than a newline being quoted. The two
  characters are removed from the input and the two lines surrounding the line
  continuation are concatenated into a single line.
* A pair of single-quotation marks (+'+) quote any characters between them
  except another single-quotation. Note that newlines can be quoted using
  single-quotations.
* Double-quotation marks (+"+) are like single-quotations, but they have a few
  exceptions: Parameter expansion, command substitution, and arithmetic
  expansion are interpreted as usual even between double-quotations. A
  backslash between double-quotations is treated as a quotation mark only when
  it is followed by +$+, +`+, +"+, +\+, or a newline; other backslashes are
  treated as normal characters.

[[aliases]]
== Aliases

Tokens that compose a command are subject to dfn:[alias substitution]. A token
that matches the name of an alias that has already been defined is substituted
with the value of the alias before the command is parsed.

Tokens that contain quotations are not alias-substituted since an alias name
cannot contain quotation marks. Keywords and command separator characters are
not alias-substituted either.

There are two kinds of aliases: normal aliases and global aliases. A
dfn:[normal alias] can only substitute the first token of a command while a
dfn:[global alias] can substitute any part of a command. Global aliases are
yash extension that is not defined in POSIX.

If a token is alias-substituted with the value of a normal alias that ends
with a whitespace, the next token is exceptionally subject to alias
substitution for normal aliases.

The results of alias substitution are again subject to alias substitution for
other aliases (but not for the aliases that have been already applied).

You can define aliases using the link:_alias.html[alias built-in] and remove
using the link:_unalias.html[unalias built-in].

[[simple]]
== Simple commands

A command that does not start with a keyword token is a dfn:[simple command].
Simple commands are executed as defined in link:exec.html#simple[Execution of
simple commands].

If the first and any number of following tokens of a simple command have the
form +{{name}}={{value}}+, they are interpreted as
link:params.html#variables[variable] assignments.
A variable name must consist of one or more alphabets, digits and/or
underlines (+_+) and must not start with a digit.
The first token that is not a variable assignment is considered as a command
name and all the following tokens (whether or not they have the form
+{{name}}={{value}}+) as command arguments.

A variable assignment of the form +{{var}}=({{tokens}})+ is interpreted as
assignment to an link:params.html#arrays[array].
You can write any number of tokens between a pair of parentheses. Tokens can
be separated by not only spaces and tabs but also newlines.

[[pipelines]]
== Pipelines

A dfn:[pipeline] is a sequence of one or more <<simple,simple commands>>,
<<compound,compound commands>>, and/or <<funcdef,function definitions>> that
are separated by +|+.

A pipeline that has more than one subcommand is executed by executing each
subcommand of the pipeline in a subshell simultaneously. The standard output
of each subcommand except the last one is redirected to the standard input of
the next subcommand. The standard input of the first subcommand and the
standard output of the last subcommand are not redirected. The exit status of
the pipeline is that of the last subcommand.

A pipeline can be prefixed by +!+, in which case the exit status of the
pipeline is _reversed_: the exit status of the pipeline is 1 if that of the
last subcommand is 0, and 0 otherwise.

[NOTE]
When the execution of a pipeline finishes, at least the execution of the last
subcommand has finished since the exit status of the last subcommand defines
that of the whole pipeline. The execution of other subcommands, however, may
not have finished then. On the other hand, the execution of the pipeline may
not finish soon after that of the last subcommand finished because the shell
may choose to wait for the execution of other subcommands to finish.

[NOTE]
The POSIX standard allows executing any of subcommands in the current shell
rather than subshells, though yash does not do so.

[[and-or]]
== And/or lists

An dfn:[and/or list] is a sequence of one or more <<pipelines,pipelines>>
separated by +&&+ or +||+.

An and/or list is executed by executing some of the pipelines conditionally.
The first pipeline is always executed. The other pipelines are either executed
or not executed according to the exit status of the previous pipelines.

- If two pipelines are separated by +&&+ and the exit status of the first
  pipeline is zero, the second pipeline is executed.
- If two pipelines are separated by +||+ and the exit status of the first
  pipeline is not zero, the second pipeline is executed.
- In other cases, the execution of the and/or list ends: the second and any
  remaining pipelines are not executed.

The exit status of an and/or list is that of the last pipeline that was
executed.

Normally, an and/or list must be terminated by a semicolon, ampersand, or
newline. See <<async,Command separators and asynchronous commands>>.

[[async]]
== Command separators and asynchronous commands

The whole input to the shell must be composed of any number of <<and-or,and/or
lists>> separated by a semicolon or ampersand.
A terminating semicolon can be omitted if it is followed by +;;+, +)+, or a
newline.
Otherwise, an and/or list must be terminated by a semicolon or ampersand.

If an and/or list is terminated by a semicolon, it is executed synchronously:
the shell waits for the and/or list to finish before executing the next and/or
list. If an and/or list is terminated by an ampersand, it is executed
asynchronously: after the execution of the and/or list is started, the next
and/or list is executed immediately. An asynchronous and/or list is always
executed in a link:exec.html#subshell[subshell] and its exit status is zero.

If the shell is not doing link:job.html[job control],
the standard input of an asynchronous and/or list is automatically redirected
to /dev/null. Signal handlers of the and/or list for the SIGINT and SIGQUIT
signals are set to ``ignore'' the signal so that the execution of the and/or
list cannot be stopped by those signals.
(In the link:posix.html[POSIXly-correct mode], the standard input is
redirected if and only if the shell is link:interact.html[interactive],
regardless of whether job control is on. Moreover, the SIGINT and SIGQUIT
signals are ignored even if job control is on.)

When the execution of an asynchronous and/or list is started, the shell
remembers its process ID. You can obtain the ID by referencing the
link:params.html#sp-exclamation[+!+ special parameter]. You can obtain the
current and exit status of the asynchronous list as well by using the
link:_jobs.html[jobs] and link:_wait.html[wait] built-ins.

[[compound]]
== Compound commands

Compound commands provide you with programmatic control of shell command
execution.

[[grouping]]
=== Grouping

A grouping is a list of commands that is treated as a <<simple,simple
command>>.

Normal grouping syntax::
  +{ {{command}}...; }+

Subshell grouping syntax::
  +({{command}}...)+

The +{+ and +}+ tokens are keywords, which must be separated from other
tokens. The +(+ and +)+ tokens, however, are special separators that need not
to be separated.

In the normal grouping syntax, the commands in a grouping are executed in the
current shell. In the subshell grouping syntax, the commands are executed in a
new link:exec.html#subshell[subshell].

In the link:posix.html[POSIXly-correct mode], a grouping must contain at least
one command. If the shell is not in the POSIXly-correct mode, a grouping may
contain no commands.

The exit status of a grouping is that of the last command in the grouping. If
the grouping contains no commands, its exit status is that of the last
executed command before the grouping.

[[if]]
=== If command

The if command performs a conditional branch.

Basic if command syntax::
  +if {{condition}}...; then {{body}}...; fi+

Syntax with the else clause::
  +if {{condition}}...; then {{body}}...; else {{body}}...; fi+

Syntax with the elif clause::
  +if {{condition}}...; then {{body}}...; elif {{condition}}...; then {{body}}...; fi+

Syntax with the elif clause::
  +if {{condition}}...; then {{body}}...; elif {{condition}}...; then {{body}}...; else {{body}}...; fi+

For all the syntaxes, the execution of an if command starts with the execution
of the {{condition}} commands that follows the +if+ token. If the exit status
of the condition commands is zero, the condition is considered as ``true''. In
this case, the {{body}} commands that follows the +then+ token are executed
and the execution of the if command finishes. If the exit status of the
condition commands is non-zero, the condition is considered as ``false''. In
this case, the {{condition}} commands for the next elif clause are executed
and the exit status is tested in the same manner as above. If there is no elif
clause, the {{body}} commands that follow the +else+ token are executed and
the execution of the if command finishes. If there is no else clause either,
the execution of the if command just ends.

An if command may have more than one elif-then clause.

The exit status of an if command is that of the {{body}} commands that were
executed. The exit status is zero if no {{body}} commands were executed, that
is, all the conditions were false and there was no else clause.

[[while-until]]
=== While and until loops

The while loop and until loop are simple loops with condition.

While loop syntax::
  +while {{condition}}...; do {{body}}...; done+

Until loop syntax::
  +until {{condition}}...; do {{body}}...; done+

If the shell is not in the link:posix.html[POSIXly-correct mode], you can omit
the {{condition}} and/or {{body}} commands of a while/until loop.

The execution of a while loop is started by executing the {{condition}}
commands. If the exit status of the {{condition}} commands is zero, the shell
executes the {{body}} commands and returns to the execution of the
{{condition}} commands. The {{condition}} and {{body}} commands are repeatedly
executed until the exit status of the {{condition}} commands is non-zero.

[NOTE]
The {{body}} commands are not executed at all if the first execution of the
{{condition}} commands yields a non-zero exit status.

An until loop is executed in the same manner as a while loop except that the
condition to repeat the loop is reversed: the {{body}} commands are executed
when the exit status of the {{condition}} commands is non-zero.

The exit status of a while/until loop is that of the last executed {{body}}
command. The exit status is zero if the {{body}} commands are empty or were
not executed at all.

[[for]]
=== For loop

The for loop repeats commands with a variable assigned one of given values in
each round.

For loop syntax::
  +for {{varname}} in {{word}}...; do {{command}}...; done+
  +
  +for {{varname}} do {{command}}...; done+

The {{word}} list after the +in+ token may be empty, but the semicolon (or
newline) before the +do+ token is required even in that case. The {{word}}s
are not treated as keywords, but you need to <<quotes,quote>> separator
characters (such as +&+ and +|+) to include them as part of a {{word}}. If you
omit the +in+ token and the following {{word}}s, you must also omit the
semicolon before the +do+ token. However, the shell does not complain about
the existence of the semicolon if not in the link:posix.html[POSIXly-correct
mode]. The {{command}} list may be empty if not in the POSIXly-correct mode.

The execution of a for loop is started by expanding the {{word}}s in the same
manner as in the execution of a <<simple,simple command>>. If the +in+
and {{word}} tokens are omitted, the shell assumes the {{word}} tokens to be
+"$@"+. Next, the following steps are taken for each word expanded (in the
order the words were expanded):

. Assign the word to the variable whose name is {{varname}}.
. Execute the {{command}}s.

Each word is assigned as a link:exec.html#localvar[local variable] except in
the POSIXly-correct mode.
If the expansion of the {{word}}s yielded no words as a result, the
{{command}}s are not executed at all.

The exit status of a for loop is that of the last executed {{command}}. The
exit status is zero if the {{command}}s are not empty and not executed at all.
If the {{command}}s are empty, the exit status is that of the last executed
command before the for loop.

[[case]]
=== Case command

The case command performs a pattern matching to select commands to execute.

Case command syntax::
  +case {{word}} in {{caseitem}}... esac+

Case item syntax::
  +({{patterns}}) {{command}}...;;+

The {{word}} between the +case+ and +in+ tokens must be exactly one word. The
{{word}} is not treated as a keyword, but you need to <<quotes,quote>>
separator characters (such as +&+ and +|+) to include them as part of the
{{word}}. Between the +in+ and +esac+ tokens you can put any number of case
items (may be none). You can omit the first +(+ token of a case item and the
last +;;+ token before the +esac+ token. If the last {{command}} of a case
item is terminated by a semicolon, you can omit the semicolon as well. The
{{command}}s in a case item may be empty.

The {{patterns}} in a case item are one or more tokens each separated by a +|+
token.

The execution of a case command starts with subjecting the {{word}} to
link:expand.html[the four expansions]. Next, the following steps are taken for
each case item (in the order of appearance):

. For each word in the {{patterns}}, expand the word in the same manner as the
  {{word}} and test if the expanded pattern link:pattern.html[matches] the
  expanded word. (If a pattern is found that matches the word, the remaining
  patterns are not expanded nor tested, so some of the {{patterns}} may not be
  expanded. Yash expands and tests the patterns in the order of appearance,
  but it may not be the case for other shells.)
. If one of the {{patterns}} was found to match the {{word}} in the previous
  step, the {{command}}s in this case item are executed and the execution of
  the whole case item ends. Otherwise, proceed to the next case item.

The exit status of a case command is that of the {{command}}s executed. The
exit status is zero if no
{{command}}s were executed, that is, there were no case items, no matching
pattern was found, or no commands were associated with the matching pattern.

In the link:posix.html[POSIXly-correct mode], the first pattern in a case item
cannot be +esac+ (even if you do not omit the +(+ token).

[[funcdef]]
== Function definition

The function definition command defines a link:exec.html#function[function].

Function definition syntax::
  +{{funcname}} ( ) {{compound_command}}+
  +
  +function {{funcname}} {{compound_command}}+
  +
  +function {{funcname}} ( ) {{compound_command}}+

In the first syntax without the +function+ keyword, {{funcname}} cannot
contain any special characters such as semicolons and quotation marks. In the
second and third syntax, which cannot be used in the
link:posix.html[POSIXly-correct mode], {{funcname}} is subjected to
link:expand.html[the four expansions] when executed.

When a function definition command is executed, a function whose name is
{{funcname}} is defined with its body being {{compound_command}}.

A function definition command cannot be directly link:redir.html[redirected].
Any redirections that follow a function definition are associated with
{{compound_command}} rather than the whole function definition command.
In +func() { cat; } >/dev/null+, for example, it is not +func() { cat; }+ but
+{ cat; }+ that is redirected.

The exit status of a function definition is zero if the function was defined
without errors, and non-zero otherwise.

// vim: set filetype=asciidoc textwidth=78 expandtab:
