locating the source of environment variables



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350 words

Recently, I was working on a project when I realized that my username was being assigned to a variable that I wasn’t expecting.

I wanted to understand where my shell was sourcing the variable from, which led me to this StackExchange conversation.

Stéphane Chazelas provided two solutions depending on your shell:

  • In bash: PS4='+$BASH_SOURCE
  • In zsh: zsh -xl

Running Zsh, I proceeded with that. As Stéphane mentions, this “simulates a login shell and show[s] everything that is done […] along with the name of the file currently being interpreted.”

The stream has a lot of very interesting information, however, I was not able to exit or stop the process once started.

Therefor, if using it in the future, I will avoid invoking it within a session where I am actively working, and instead open a new session to see how my environment is being booted.

The stream of information that this produces is also quite a bit. To help wade through the deluge of information, another user suggested merging the stderr and stdout outputs with zsh -xl 2 to then grep as usual. This sort of works. The way I made it work was, again in a new session, to search for what I was looking for immediately. For example:

$ zsh -xl 2>&1 | grep MY
+/Users/stephen/.bash_profile:117> export MY_DB_USER=sweiss

Here, I’m able to see that my environment is picking up the variable MY_DB_USER from my .bash_profile at line 117.

The way the -xl emulates a login is because the -x is the XTRACE and -l is LOGIN

LOGIN (-l, ksh: -l) This is a login shell. If this option is not explicitly set, the shell becomes a login shell if the first character of the argv[0] passed to the shell is a ‘-’.

XTRACE (-x, ksh: -x) Print commands and their arguments as they are executed. The output is preceded by the value of $PS4, formatted as described in Prompt Expansion.

These definitions, and other options, can be found in the ZSH docs.

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