Why does process substitution not work in a shell script?

Question

I am trying to comm command in shell script but getting an error:

a.sh: command substitution: line 1: syntax error near unexpected token `('
a.sh: command substitution: line 1: `comm -12 <( sort /home/xyz/a.csv1 | uniq) <( sort /home/abc/tempfile | uniq) | wc -l'

snippet of code-

temp=`comm -12 <( sort /home/xyz/a.csv1 | uniq) <( sort /home/abc/tempfile | uniq) | wc -l`
echo $temp

Answer 1

It isn't entirely clear yet, but the chances are very high that you either have an incorrect shebang line at the top of the script:

#!/bin/sh

or you are using sh script.sh instead of bash script.sh while testing it, or you have SHELL=/bin/sh or something similar set in the environment. Your failure is on the process substitution code. When Bash is run as sh (in POSIX mode), then process substitution is not available:

30. Process substitution is not available.

You need to write:

#!/bin/bash

temp=$(comm -12 <(sort -u /home/xyz/a.csv1) <(sort -u /home/abc/tempfile) | wc -l)
echo $temp

or even simply:

#!/bin/bash

comm -12 <(sort -u /home/xyz/a.csv1) <(sort -u /home/abc/tempfile) | wc -l

which will achieve the same effect as the capture followed by the echo. When testing, use bash -x script.sh or bash script.sh.

---

Deciphering the indecipherable comment

In an indecipherable comment, the information appears to include:

BASH=/bin/sh
BASHOPTS=cmdhist:extquote:force_fignore:hostcomplete:interactive_comments:progco‌mp:promptvars:sourcepath
BASH_ALIASES=()
BASH_ARGC=()
BASH_ARGV=()
BASH_CMDS=()
BASH_LINENO=([0]="0")
BASH_SOURCE=([0]="a.sh")
BASH_VERSINFO=([0]="4" [1]="1" [2]="2" [3]="1" [4]="release" [5]="x86_64-redhat-linux-gnu")
BASH_VERSION='4.1.2(1)-release'
CVS_RSH=ssh
SHELL=/bin/bash
SHELLOPTS=braceexpand:hashall:interactive-comments:posix
SHLVL=2

Note that BASH=/bin/sh and SHELLOPTS=braceexpand:hashall:interactive-comments:posix. Either or both of these might be a major part of the problem.