Parsing result of Diff in Shell Script - linux

Question

I want to compare two files and see if they are the same or not in my shell script, my way is:
diff_output=`diff ${dest_file} ${source_file}`
if [ some_other_condition -o ${diff_output} -o some_other_condition2 ]
then
....
fi
Basically, if they are the same ${diff_output} should contain nothing and the above test would evaluate to true.
But when I run my script, it says
[: too many arguments
On the if [....] line.
Any ideas?

Answer 1

Do you care about what the actual differences are, or just whether the files are different? If it's the latter you don't need to parse the output; you can check the exit code instead.
if diff -q "$source_file" "$dest_file" > /dev/null; then
: # files are the same
else
: # files are different
fi
Or use cmp which is more efficient:
if cmp -s "$source_file" "$dest_file"; then
: # files are the same
else
: # files are different
fi

Answer 2

There's an option provided precisely for doing this: -q (or --quiet). It tells diff to just let the exit code indicate whether the files were identical. That way you can do this:
if diff -q "$dest_file" "$source_file"; then
# files are identical
else
# files differ
fi
or if you want to swap the two clauses:
if ! diff -q "$dest_file" "$source_file"; then
# files differ
else
# files are identical
fi
If you really wanted to do it your way (i.e. you need the output) you should do this:
if [ -n "$diff_output" -o ... ]; then
...
fi
-n means "test if the following string is non-empty. You also have to surround it with quotes, so that if it's empty, the test still has a string there - you're getting your error because your test evaluates to some_other_condition -o -o some_other_condition2, which as you can see isn't going to work.

Answer 3

diff is for comparing files line by line for processing the differences tool like patch . If you just want to check if they are different, you should use cmp:
cmp --quiet $source_file $dest_file || echo Different

Answer 4

diff $FILE $FILE2
if [ $? = 0 ]; then
echo “TWO FILES ARE SAME”
else
echo “TWO FILES ARE SOMEWHAT DIFFERENT”
fi

Answer 5

Check files for difference in bash
source_file=abc.txt
dest_file=xyz.txt
if [[ -z $(sdiff -s $dest_file $source_file) ]]; then
echo "Files are same"
else
echo "Files are different"
fi
Code tested on RHEL/CentOS Linux (6.X and 7.X)