Bash – Turn on xtrace with Environment Variable

bashenvironment-variablestrace

Bash has a sometimes-useful feature whereby if you turn on the "-x" option (I believe the symbolic name is xtrace), Bash outputs each line of script as it executes it.

I know of two ways to enable this behavior:

In the script itself, say set -x
On the command line, pass the -x option to Bash.

Is there any way of turning this option on via environment variables?

(In particular, I'm not invoking Bash myself, so I can't pass any options to it, and the script of interest is inside a compressed archive which I don't really feel like rebuilding. If I could set an environment variable, it would presumably be inherited by all child processes…)

The manpage says something about BASHOPTS, but when I try it Bash says that's read-only. (Thanks for not mentioning that in the manpage.)
Similarly, SHELLOPTS also seems to be read-only.
You can select which FD is used with BASH_XTRACEFD. But I still need to turn tracing on in the first place.

Best Answer

Use env to ignore the readonly flags.

env SHELLOPTS=xtrace ./yourscript

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Bash Variables – Difference Between Environment Variable and Shell Variable

Your second assignment TEST="SHELL_TEST" doesn't un-export the variable. It's still marked as "to be inherited by children". And the value inherited by the child is the value currently set in the parent.
In other words, your second assignment doesn't revert the status of TEST to a shell variable, it's still an environment variable according to that terminology.

You'd have to un-export it for it to become unset in children processes:

$ typeset +x TEST
$ sh -c 'echo $TEST'

$

Unsetting environment variable with an empty name

Environment variables aren't supposed to have an empty name, so many utilities don't support them.

The env command from GNU coreutils supports setting the environment variable with an empty name but not unsetting it. That's a bug.

$ env '=wibble' env |grep wibble                                 
=wibble
$ env '=wibble' env -u '' env
env: cannot unset `': Invalid argument

Common shells can't unset the empty name either. That's ok, since the empty name isn't supposed to be used as an environment variable, and can't be used as a shell variable. Zsh is the only buggy one in the lot: it pretends to do the job but in fact does nothing.

$ env '=wibble' dash -c 'unset ""'
dash: 1: unset: : bad variable name
$ env '=wibble' bash -c 'unset ""'
bash: line 0: unset: `': not a valid identifier
$ env '=wibble' ksh -c 'unset ""'
ksh[1]: unset: : invalid variable name
$ env '=wibble' mksh -c 'unset ""'
mksh: : is read only
$ env '=wibble' posh -c 'unset ""'
posh: unset:  is read only
$ env '=wibble' zsh -c 'unset ""'
$ env '=wibble' zsh -c 'unset ""; env' | grep wibble
=wibble

Python, as you've noticed, bugs out when it finds the empty name for an environment variable.

Perl has no such problem, so it may be a solution for you. Note that you have to execute a new shell to use an external process to change the environment.

perl -e 'delete $ENV{""}; exec $ARGV[0] @ARGV' "$SHELL" "-$-"

Best Answer

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Bash Variables – Difference Between Environment Variable and Shell Variable

Unsetting environment variable with an empty name

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