Question

I have noticed that Solaris 10’s Bourne shell, /bin/sh (and also /sbin/sh) forks a subshell when using indirection (<). I have tried a pile of other Bourne-ish shells, including:

• The POSIX /usr/xpg4/bin/sh shell on Solaris 10
• /bin/bash, /bin/ksh on Solaris 10
• /bin/sh on AIX 5
• /bin/sh on Debian Linux 5

and none of these exhibit this behavior.

I’m amazed I haven’t been bitten by this before. For example, in the saner shells (ie all those listed above) the following script outputs “1”:

$ cat foo
#!/bin/sh
x=0
while read y ; do
  x=1
done </etc/passwd
echo $x
$ ./foo
0
$

Solaris 10’s /bin/sh returns 0 because the assignment x=1 occurs in the subshell caused by the indirection: when the subshell exits that assignment is lost. (If I remove </etc/passwd and read from stdin instead then “1” is output, as expected).

Is there some age-old reason that the “traditional” Solaris sh has this property? Or is this a bug?

Answer 1

I would say this is a violation of the POSIX standard.

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. All other commands shall be executed in the current shell environment.

Source: Shell Command Language, section 2.12.