difference between sh and bash when symlink is used

Question

I have a shell script which uses process substitution

The script is:

  #!/bin/bash
  while read line
  do
    echo "$line"
  done < <( grep "^abcd$" file.txt )

When I run the script using sh file.sh I get the following output

$sh file.sh
file.sh: line 5: syntax error near unexpected token `<'
file.sh: line 5: `done < <( grep "^abcd$" file.txt )'

When I run the script using bash file.sh, the script works.

Interestingly, sh is a soft-link mapped to /bin/bash.

$ which bash
/bin/bash
$ which sh
/usr/bin/sh
$ ls -l /usr/bin/sh
lrwxrwxrwx 1 root root 9 Jul 23  2012 /usr/bin/sh -> /bin/bash
$ ls -l /bin/bash
-rwxr-xr-x 1 root root 648016 Jul 12  2012 /bin/bash

I tested to make sure symbolic links are being followed in my shell using the following:

$ ./a.out
hello world
$ ln -s a.out a.link
$ ./a.link
hello world
$ ls -l a.out
-rwx--x--x 1 xxxx xxxx 16614 Dec 27 19:53 a.out
$ ls -l a.link
lrwxrwxrwx 1 xxxx xxxx 5 May 14 14:12 a.link -> a.out

I am unable to understand why sh file.sh does not execute as /bin/bash file.sh since sh is a symbolic link to /bin/bash.

Any insights will be much appreciated. Thanks.

Answer 1

When invoked as sh, bash enters posix mode after the startup files are read. Process substitution is not recognized in posix mode. According to posix, <(foo) should direct input from the file named (foo). (Well, that is, according to my reading of the standard. The grammar is ambiguous in many places.)

EDIT: From the bash manual:

The following list is what’s changed when ‘POSIX mode’ is in effect:
...
Process substitution is not available.