Saying that I have two files: a.txt and b.txt.
The content of a.txt:
hello world
The content of b.txt:
hello world
something else
Of course I can use vimdiff to check their difference, I can make sure that a.txt is a subset of b.txt, which means that b.txt must contain all of lines existing in a.txt (just like the example above).
My question is how to record lines which exists in b.txt but doesn't exist in a.txt into a file?
comm -1 -3 a.txt b.txt > c.txt
The -1 excludes lines that are only in a.txt, and the -3 excludes lines that are in both. Thus only the lines exclusively in b.txt are output (see man comm or comm --help for details). The output is redirected to c.txt
If you want the difference between the two files, use diff rather than comm. e.g.
diff -u a.txt b.txt > c.txt
comm looks like a very handy command during file comparison. But diff -u is giving a bunch of other information as well.
diff is context and instructions (e.g. - to delete a line, + to add a line) that allow b.txt to be reconstructed from a.txt and c.txt - i.e. c.txt is a patch file, understood by the patch program. Also simple enough for people to easily understand. BTW, diff has several different output styles, -u is the "unified diff" format. There are also various specialised version of diff - e.g. to do side-by-side comparisons, or compare 3 files at once, or find word differences within a line. colordiff is also handy for colourising diff output.
a.txt and b.txt files are not sorted you will get a warning. To avoid the warning and the sorting use the --nocheck-order flag.
If you dont care for subset, you can use just
diff a.txt b.txt|grep ">"|cut -c 3- > foo.txt
.
$ cat a.txt
hello world
$ cat b.txt
hello world
something else
$ diff a.txt b.txt|grep ">"|cut -c 3- > foo.txt
$ cat foo.txt
something else
Limitation: this is not a real file diff, more like a set of the lines diff (but you may need exactly this).
All differences between a.txt and b.txt:
sort a.txt b.txt | uniq -u > c.txt
Lines which missing from a.txt (ignoring lines which missing from b.txt):
sort a.txt a.txt b.txt | uniq -u > c.txt
Explanation: after cat and sort the 2 files together you have the subset lines duplicated, uniq -u shows only the uniq lines, those are the lines which are only present in one of the file. Duplicating one of the input (a.txt above) suppresses all the lines exist in that file on the output.
Duplication in any of the files ruins the output of the commands above, if you have duplicates in your files you have to remove those first and run the commands above on the newly created files:
sort a.txt | uniq | aa.txt
sort b.txt | uniq | bb.txt
You can check the result, this 2 command should give you the same checksum:
sort b.txt c.txt | uniq | sha256sum
sort a.txt c.txt | uniq | sha256sum
If one of the file is a superset of the other (so it has all lines of the other plus (maybe) more) then you can simplify a bit. Like in your example the b.txt is the superset, so this 2 commands should give you the same checksum too:
sort b.txt | sha256sum
sort a.txt c.txt | sha256sum
b.txt - a.txt :
sort a.txt a.txt b.txt | uniq -u > foo.txt
a.txt ?
b.txt, and will remove any lines which are repeated in b.txt even if they aren’t present in a.txt.
Commented
Feb 11, 2022 at 12:57
a.txtis a subset ofb.txt. I've already known thata.txtis a subset ofb.txt. My question is how to print the difference into a file.