TIL: Redirect Bash `set -x` to file

27 May, 2025

Sometimes you will need to debug commands that are being called as sub-commands to another process. Case-in-point are external git diff helpers for special files, and in my case the rails encrypted credentials file.

I wanted to intercept calls to this command and write out the arguments that this script was being called with to a log file.

Here's what I did:

1. Create the diff script and save to ~/rails_differ

   #!/bin/bash
   
   exec 10>/home/chris/rails_differ.log
   BASH_XTRACEFD=10
   set -x
   
   bundle exec rails credentials:diff "$@"
   

   • The key here is that you can set the BASH_XTRACEFD=19, the file descriptor 19 is arbitrary, as long as you do not use the special FDs 0 (STDIN), 1 (STDOUT),2 (STDERR), it should be fine.
   • 19>/home/chris/rails_differ.log opens the file descriptor and points it to the log
   • set -x enables command tracing

2. Update .git/config in the repository from

   [diff "rails_credentials"]
   textconv = bin/rails credentials:diff
   

   to

   [diff "rails_credentials"]
   textconv = /home/chris/rails_differ
   

3. And of course make sure your .gitattributes contains

   config/credentials/*.yml.enc diff=rails_credentials
   config/credentials.yml.enc diff=rails_credentials
   

4. Now when I run

   git diff -- config/credentials/test.yml.enc
   

5. Then by cat ~/rails_differ.log I can see

   + bundle exec rails credentials:diff config/credentials/test.yml.enc
   

#til