diff of config files showing only uncommented lines

Question

Consider the need to use 'diff' to compare two files with many commented lines, such as system configuration files. (For example, the files could be /etc/cups/cups-files.conf.pacnew and /etc/cups/cups-files.conf.)

I would like to ignore differences of any lines that are commented out such as these:

< #DocumentRoot /usr/share/cups/doc
---
> #DocumentRoot /usr/share/cups/doc-root

I only want to see differences of active (uncommented) lines.

(I would also like to ignore whitespace differences. I mention that for completeness in case the solution for the above interferes with using the the -w option.)

• Ignacio Vazquez-Abrams: Other than -I?
• Stephen Kitt: @Ignacio -I only works if you have no mixed hunks – e.g. if you use -I '^#', but add one commented line followed by a non-commented line, you’ll see both in the diff output because they’re part of the same hunk.

Answer 1

You can filter the commented lines before comparing the files:

diff -u -w <(grep -v '^[[:space:]]*#' /etc/cups/cups-files.conf.pacnew) <(grep -v '^[[:space:]]*#' /etc/cups/cups-files.conf)

That way diff will only see uncommented lines, and will only compare those.

• MountainX: can this be easily modified to work when the comment symbol is not in column 0? (That would be true in the case of leading whitespace.)
• Hiko Haieto: This does not account for lines that may have comments at the end of them. A line that contains DocumentRoot /usr/share/cups/doc #some comment here would be excluded, regardless of whether the "active" part is different between files.
• Stephen Kitt: @MountainX see the update.
• Stephen Kitt: @Hiko no, it only excludes lines consisting solely of a comment.
• Ignacio Vazquez-Abrams: @HikoHaieto: Default config files are almost never commented in that manner.