Is command substitution $(foo) bashism?

问题:

There are two different syntaxes for command substitution,

FOO=$(echo bar)

and

FOO=`echo bar`

As far as I know, the first method is defined in Bash, while the second is defined in sh.

Consider the following use of command substitution in an sh script.

#!/bin/sh
FOO=$(echo bar)

Does that fall under the definition of bashism?

bashisms, i.e. features not defined by POSIX (won't work in dash, or general /bin/sh).

回答1:

It's the same thing. So no it's not a bashism nor related to bash only.

Command Substitution
Command substitution allows the output of a command to be substituted in place of the command name itself. Command substitution occurs when the command is enclosed as follows:

$(command)

or ( ''backquoted'' version):

`command`

The shell expands the command substitution by executing command in a subshell environment and replacing the command substitution with the standard output of the command, removing sequences of one or more newlines at the end of the substitution. (Embedded newlines before the end of the output are not removed; however, during field splitting, they may be translated into spaces, depending on the value of IFS and quoting that is in effect.)

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Resources :

• man dash
• The Open Group Base Specifications - Shell Command Language thanks to @Ned Deily for this good resource :)

回答2:

Actually, the $(...) command substitution syntax is defined by POSIX, though it's not part of the earlier SVID sh standard. So as long as you don't care about running on pre-POSIX systems, it should be fine.