Shell – Silent result with two identical files in diff: how to show them

diff()file-comparisonshell-script

When I set the -s parameter, diff also print files, that are different.

diff -s $FIRST_FILE $SECOND_FILE

Best Answer

One possible solution may be:

diff -s $FIRST_FILE $SECOND_FILE > /dev/null
if [ $? -eq 0 ]; then
    echo "The files are identical"
fi

NOTE: It changed the question text.

The Test

Three files are involved in my test:
File test1:

    text

File test2:

    text
    #comment

File test3:

    changed text
    #comment

The commands:

$ # comparing files with comment-only changes
$ diff -u -I '#.*' test{1,2}
$ # comparing files with both comment and regular changes
$ diff -u -I '#.*' test{2,3}
--- test2       2011-07-20 16:38:59.717701430 +0200
+++ test3       2011-07-20 16:39:10.187701435 +0200
@@ -1,2 +1,2 @@
-text
+changed text
 #comment

The alternative way

Since there is no answer so far explaining how to use the -I option correctly, I'll provide an alternative which works in bash shells:

diff -u -B <(grep -vE '^\s*(#|$)' test1)  <(grep -vE '^\s*(#|$)' test2)

diff -u - unified diff
- -B - ignore blank lines
<(command) - a bash feature called process substitution which opens a file descriptor for the command, this removes the need for a temporary file
grep - command for printing lines (not) matching a pattern
- -v - show non-matching lines
- E - use extended regular expressions
- '^\s*(#|$)' - a regular expression matching comments and empty lines
  - ^ - match the beginning of a line
  - \s* - match whitespace (tabs and spaces) if any
  - (#|$) match a hash mark, or alternatively, the end of a line

Diff Command – Why It Reports Two Files Differ When They Are the Same

Odd .. can you try cmp? You may want to use the '-b' option too.

cmp man page - Compare two files byte by byte.

This is one of the nice things about Unix/Linux .. so many tools :)

Best Answer

Related Solutions

How to diff files ignoring comments (lines starting with #)

The Test

The alternative way

Diff Command – Why It Reports Two Files Differ When They Are the Same

Related Question