Compare the heads of two files in bash

Question

I can do diff filea fileb to see the difference between files. I can also do head -1 filea to see the first line of filea or fileb. How can I combine these commands to show the difference between the first line of filea and the first line of fileb?

Answer 1

If your shell supports process substitution, try:

diff <(head -n 1 filea) <(head -n 1 fileb)

Answer 2

If you are only comparing the first line of each file, maybe you care about word-level changes within the line, using dwdiff:

dwdiff <(head -n 1 filea) <(head -n 1 fileb)

dwdiff has some nice options, like -c to colorize the changed words.

Or, using sed instead of head:

dwdiff <(sed 1q filea) <(sed 1q fileb)

which the manual for head suggests is more portable than head, as the syntax for head has changed over time head -1 vs head -n 1

Also, you could just eyeball the changes, with a bit less typing:

head -n1 -q filea fileb

which displays the two lines, one below the other, for easy visual comparison.

Answer 3

You can use:

if [ "`head -1 file1`" == "`head -1 file2`" ]; then echo "the same"; fi