Altering a variable outside the scope of a loop influenced by a subshell

Posted by Nico Golde in
Sunday, April 8. 2007

I recently had a nice discussion on a mailinglist about #!/bin/bash vs #!/bin/sh as the shebang line in shell scripts.
In general I support to use #!/bin/sh, maybe there are some corner-cases in which it makes sense to use #!/bin/bash, I am not a shell programmer.
But since the guy from shellscripts.org (btw why shellscripts and not bashscripts?) told me that I have to use a bashism to
alter a variable outside the scope of a loop because of the created subshell and in my opinion this is a common misunderstanding I blog this.

foo=123
echo 456 | while read line ; do
foo=$line
done
echo $foo

results in 123 because the loop has its own subshell and no access to foo outside of its scope.

foo=123
while read line ; do
foo=$line
done < <( echo 456 )
echo $foo

is the proposed bashism to solve this issue, it will result in 456.

echo 456 | {
foo=123
while read line; do
foo=$line
done;}
echo $foo

Also results in 456 and is sh-compliant (or at-least runs in ksh93 which is the reference implementation for the POSIX standard.
What also surprised me is that this is not running in bash, dash but in zsh

I reported this as a bug in bash and dash for Debian, please correct me if I am wrong.

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I tested your solution with solaris sh and it doesn't work, however I don't know the POST compliance level of the solaris sh.

#1 Yann on 2007-04-09 00:25 (Reply)

s/POST/POSIX/

#1.1 Yann on 2007-04-09 00:27 (Reply)

I had this same problem a few years ago, but I saw that bash did create a subshell for |{ } constructs, and concluded that I was mistaken

Also, from that fact, I couldn't understand the difference between ( ) and { } in most cases...

#2 Anonymous (Homepage) on 2007-04-09 04:16 (Reply)

The blog topic is wrong, your problem is not the loop but the fact that the loop is running in a pipe construct. And that is documented behavior.

------------------
dash(1):
Note that unlike some other shells, each process in the pipeline is a child of the invoking shell (unless it is
a shell builtin, in which case it executes in the current shell -- but any effect it has on the environment is wiped).
------------------

This is also POSIX compliant, POSIX does allow both to execute pipe components in subshells or the the same shell. "2.12 Shell Execution Environment" says:
-----------------------------
Changes made to the subshell environment shall not affect the shell environment. Command substitution, commands that are grouped with parentheses, and asynchronous lists shall be executed in a subshell environment. Additionally, each command of a multi-command pipeline is in a subshell environment; as an extension, however, any or all commands in a pipeline may be executed in the current environment.
-----------------------------

cu andreas

#3 Andreas Metzler on 2007-04-09 10:49 (Reply)

Yes the title is a bit bad I have to admit and I know this is intended behaviour.

#4 nion on 2007-04-09 14:36 (Reply)

thanks for that...

#4.1 sh newb on 2008-05-22 01:43 (Reply)

Hi,

The workaround you suggest in your 3rd code snippet should be like this (the final 'echo' inside the block):

[snip]
printf '456\n789\n' | {
foo=123
while read line; do
foo=$line
done
echo $foo
}
[snap]

I have not seen it the way you put it, before.
I don't think this is a bug. Neither for dash, nor for bash.

And in reply to Yann:
Solaris does come with a POSIXly compliant shell, but it's in
one of Solaris' thousand obscure $PATH parts. '/bin/sh' on
Solaris is a shell that's pretty close to the original Bourne
Shell. That shell even forces a subshell on redirections:

[snip]
foo=bar
while read line; do
foo=$line
done < /etc/hosts
echo $foo
[snap]

...will output 'bar' with Solaris' '/bin/sh'. However, the
corrected workaround I presented above should work in that
shell as well.

Regards, Frank

#5 ft (Homepage) on 2007-04-10 01:02 (Reply)

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