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GNU Bash Reference Manual
by Chet Ramey and Brian Fox
Paperback (6"x9"), 180 pages
ISBN 0954161777
RRP £19.95 ($29.95)

"An essential resource .... the most detailed coverage available for all aspects of Bash" --- Linux User and Developer Magazine (Issue 37, Mar 2004) Get a printed copy>>>

3.5.4 Command Substitution

Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows:

$(command)

or

`command`

Bash performs the expansion by executing command 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 $(cat file) can be replaced by the equivalent but faster $(< file).

When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by ‘$’, ‘`’, or ‘\’. The first backquote not preceded by a backslash terminates the command substitution. When using the $(command) form, all characters between the parentheses make up the command; none are treated specially.

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.

ISBN 0954161777GNU Bash Reference ManualSee the print edition