1

I am on LinuxMint (have tried CentOS too) and have been trying to use the printenv command for a environment variable which may or may not exist. So I want it to print the value of that name if it is present, else print nothing but return with a exit code 0.

However, if I invoke it with a non-existent environment name, it exits with error code 1. The man pages or the help for printenv doesn't say anything about this behaviour.

Here's how I tried it:

> /usr/bin/printenv foo

That just returns, then I do:

> echo $?

which returns:

1

The version of printenv is:

printenv (GNU coreutils) 8.25
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by David MacKenzie and Richard Mlynarik.

Does anyone have an alternate way to achieve what I'm after? Ultimately, I just need to set a bash variable to contain the value of an environment variable if that environment variable exists, else have that bash variable empty:

mybashv=`printenv foo`
Improve this question

2 Answers 2

Reset to default
2

The info documentation for printenv says the following about exit codes:

 0 if all variables specified were found
 1 if at least one specified variable was not found
 2 if a write error occurred

This looks consistent with what you observed.

If I understand correctly, you want to hide this status code. For any command, this can be achieved by appending || true, e.g.:

$ var=`printenv foo || true`
$ echo $?
0
Improve this answer
5
  • Thank you! That || true option worked. As for the documentation you pasted, can you tell me where/how you found it? I checked man printenv and printenv --help and even looked online and did not find that information. Commented Dec 1, 2016 at 14:01
  • @Watson see the manual or info '(coreutils) printenv invocation' (as documented in the manpage). Commented Dec 1, 2016 at 14:11
  • @dhag Would you mind explaining what a write error is (exit code 2)? What device / file does printenv try to write to where it could go wrong? Does this relate to stdout / stderr, for example when printenv is executed in the background? Commented Jun 22, 2020 at 12:34
  • 1
    @Binarus: Many things could happen that would cause a write error: stdout is closed ((exec 1<&-; printenv) => 2), redirect to a file system where the write fails (printenv >/dev/full => 2), for example. Commented Jun 22, 2020 at 14:50
  • I see. Thank you very much. Commented Jun 22, 2020 at 14:52
2

Returning a non-zero exit status (failure) when failing to print the content of an environment variable sounds like the right thing to do to me. That matches the behaviour of the tcsh builtin of the same name and of historical implementations on BSDs.

Not doing that would make it difficult to differentiate between a variable that is set to an empty value and one that is not set.

If you want to avoid the effect of set -e/set -o errexit, use:

var=$(printenv VAR) || : Do not care if it fails

Or:

if var=$(printenv VAR); then
  if [ -z "$var" ]; then
    echo "VAR is set but empty or consists only of newline characters"
  else
    echo "VAR is set and non-empty"
  fi
else
  echo "VAR is not in the environment"
fi

Also note that all shells initialise shell variables from the corresponding environment variables (for those that can be mapped to shell variables, and some shells strip the ones that can't), so you could as well use $VAR in your script to refer to their value (note however that in the Bourne, csh or tcsh shells (the only shells where one may still want to use those `...` of yours), the value of the shell and env var may diverge if you modify the shell variable without exporting it (or assigning it with setenv for (t)csh)).

Improve this answer

You must log in to answer this question.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.