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.)

  • Other than -I? Feb 17, 2018 at 21:59
  • @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. Feb 17, 2018 at 22:07

1 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.

  • 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.)
    – MountainX
    Feb 17, 2018 at 22:10
  • 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. Feb 17, 2018 at 22:11
  • @MountainX see the update. Feb 17, 2018 at 22:12
  • @Hiko no, it only excludes lines consisting solely of a comment. Feb 17, 2018 at 22:12
  • @HikoHaieto: Default config files are almost never commented in that manner. Feb 17, 2018 at 22:12

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