CODEWITHSUNDEEP
shellposix
Mark Galeck
Mark Galeck

Reputation: 6395

can command substitution in a POSIX shell be used only in place of a command name?

The POSIX shell standard at

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_07_04

says in section 2.6.3:

Command substitution allows the output of a command to be substituted in place of the command name itself

This would seem to imply that it is only guaranteed to work, if you substitute for the whole command name; if you substitute for a part of it, or something else, then it may or may not work.

Experimenting:

$ echo ;

$ $(echo echo) ;

$

So far so good...

$ e$(echo cho) ;

$ echo $(echo ';')
;
$ echo $(echo foobar)
foobar

The first and third example above seem to "work" but the second "does not work". Is this all simply undocumented and random behavior, as the standard seems to imply, and in reality for some other POSIX shell, none of these three are guaranteed to "work"?

(By "work", I mean "produce the same result as if the results of the substitution were typed in the command itself on the terminal")

Upvotes: 2

Views: 302

Answers (1)

pah
pah

Reputation: 4778

The reason why this:

$ echo $(echo ';')

Does not output the same as this:

$ echo ;

Is the same reason why this:

$ ;
bash: syntax error near unexpected token `;'

Does not output the same as this:

$ `echo ';'`
;: command not found

The latest case, the output from command substitution (the (?)expected-to-be(?) token ;) is interpreted as a command, because it is passed [and interpreted] as a command in a subshell, and not as a command interpreter built-in token.

As my interpretation, this isn't against the standard.

EDIT: Answering the question

You stated:

This would seem to imply that it is only guaranteed to work, if you substitute for the whole command name; if you substitute for a part of it, or something else, then it may or may not work.

And the POSIX standard states:

The shell shall expand the command substitution by executing command in a subshell environment (see Shell Execution Environment) and replacing the command substitution (the text of command plus the enclosing "$()" or backquotes) with the standard output of the command, removing sequences of one or more <newline> characters at the end of the substitution.

The standard seems clear regarding to what happens. This is not a question of what's present in a command containing a command substitution, only the command that's inside the enclosing $() or or ``.

Upvotes: 0

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