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?

score 23 · Accepted Answer · answered Apr 23 '15 at 12:31

23

If your shell supports process substitution, try:

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

answered Apr 23 '15 at 12:31

cuonglm

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I had already seen the process substition command somewhere, but didn't know what it did. Thanks ! – edi9999 Apr 23 '15 at 14:47

score 6 · Answer 2 · answered Apr 23 '15 at 14:59

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.

score 0 · Answer 3 · answered Apr 23 '15 at 12:32

0

You can use:

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

answered Apr 23 '15 at 12:32

Lambert

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Note that it would answer "the same" if file1 is an empty file (or not readable) and file2's first line is empty. – Stéphane Chazelas Apr 23 '15 at 12:54
I did not say that this method is 'idiot proof' but if you quickly want to match the first line of two files... – Lambert Apr 23 '15 at 12:58

Compare the heads of two files in bash

3 Answers3