Search code examples
bashshellpipelineio-redirection

How do I print STDERR from a subshell?

Using the answer found in How can you diff two pipelines in Bash? I have written some shell scripts that I want to compare the output of:

diff <(script1 | script2) <(script3 | script4)

However, any errors printed to STDERR in any of the scripts in the subshell pipelines disappear. How can I get them to print in my outer level script (that contains the diff)?

Solution

  • The error messages from scripts 1..4 shouldn't vanish down a black hole; you only redirected their standard output as the files given to diff.

    For example, given these files:

    $ cat script1
#!/bin/bash
echo $0 stdout
echo $0 stderr >&2
$ cat script2
#!/bin/bash
echo $0 stdout
cat -
echo $0 stderr >&2
$ cat script3
#!/bin/bash
echo $0 stdout
echo $0 stderr >&2
$ cat script4
#!/bin/bash
echo $0 stdout
cat -
echo $0 stderr >&2
$

    The output from your command line is:

    $ diff <(script1 | script2) <(script3 | script4)
./script1 stderr
./script3 stderr
./script2 stderr
./script4 stderr
1,2c1,2
< ./script2 stdout
< ./script1 stdout
---
> ./script4 stdout
> ./script3 stdout
$