diff options
Diffstat (limited to 'doc/bashref.texi')
| -rw-r--r-- | doc/bashref.texi | 81 |
1 files changed, 72 insertions, 9 deletions
diff --git a/doc/bashref.texi b/doc/bashref.texi index ff9c2f41..eb37bc37 100644 --- a/doc/bashref.texi +++ b/doc/bashref.texi @@ -2628,26 +2628,27 @@ expansion as described below. Command substitution allows the output of a command to replace the command itself. -Command substitution occurs when a command is enclosed as follows: +The standard form of command substitution occurs when a command is +enclosed as follows: @example $(@var{command}) @end example @noindent -or +or (deprecated) @example -`@var{command}` +`@var{command}`. @end example @noindent -Bash performs the expansion by executing @var{command} in a subshell environment -and replacing the command substitution with the standard output of the -command, with any trailing newlines deleted. +Bash performs the expansion by executing @var{command} in a subshell +environment and replacing the command substitution with the standard +output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting. The command substitution @code{$(cat @var{file})} can be replaced by the equivalent but faster @code{$(< @var{file})}. -When the old-style backquote form of substitution is used, +With the old-style backquote form of substitution, backslash retains its literal meaning except when followed by @samp{$}, @samp{`}, or @samp{\}. The first backquote not preceded by a backslash terminates the @@ -2655,11 +2656,73 @@ command substitution. When using the @code{$(@var{command})} form, all characters between the parentheses make up the command; none are treated specially. +There is an alternate form of command substitution: + +@example +$@{@var{C} @var{command}; @} +@end example + +@noindent +which executes @var{command} in the current execution environment. +This means that side effects of @var{command} take effect immediately +in the current execution environment and persist in the current +environment after the command completes (e.g., the @code{exit} builtin +will exit the shell). + +The character @var{C} following the open brace must be a space, tab, +newline, @samp{(}, or @samp{|}, and the close brace must be in a position +where a reserved word may appear (i.e., preceded by a command terminator +such as semicolon). +Bash allows the close brace to be joined to the remaining characters in +the word without being followed by a shell metacharacter as a reserved +word would usually require. + +This type of command substitution superficially resembles executing an +unnamed shell function: local variables are created as when a shell +function is executing, and the @code{return} builtin forces +@var{command} to complete; +however, the rest of the execution environment, +including the positional parameters, is shared with the caller. + +If the first character following the open brace is a @samp{(}, +@var{command} is executed in a subshell, and @var{command} must be +terminated by a @samp{)}. This is similar to the @code{(} compound +command (@pxref{Command Grouping}). +If the first character is a @samp{|}, the construct expands to the +value of the @code{REPLY} shell variable after @var{command} executes, +without removing any trailing newlines, +and the standard output of @var{command} remains the same as in the +calling shell. +Bash creates @code{REPLY} as an initially-unset local variable when +@var{command} executes, and restores @code{REPLY} to the value it had +before the command substitution after @var{command} completes, +as with any local variable. + +For example, this construct expands to @samp{12345}, and leaves the +shell variable @code{X} unchanged in the current execution environment: + +@example +$@{ local X=12345 ; echo $X; @} +@end example + +@noindent +(not declaring @code{X} as local would modify its value in the current +environment, as with normal shell function execution), +while this construct does not require any output to expand to +@samp{12345}: + +@example +$@{| REPLY=12345; @} +@end example + +@noindent +and restores @code{REPLY} to the value it had before the command substitution. + Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes. -If the substitution appears within double quotes, word splitting and -filename expansion are not performed on the results. +If the substitution appears within double quotes, Bash does not perform +word splitting and filename expansion on the results. @node Arithmetic Expansion @subsection Arithmetic Expansion |