multiple command output into a single program:

diff -u <(ls -c1 dir_1) <(ls -c1 dir_2)

Will show you a diff of files in the root of dir_1 and dir_2

You can use diff to see the differences between two files, but it can be useful to see what is the same and more clearly how they differ. This is where comm comes in useful.

comm tells you what information is common to two lists and what information appears uniquely in one or the other.

$ find . -type f -print -exec cat {} \;
./1.txt
a
b
c
./2.txt
a
c
e

$ comm 1.txt 2.txt 
                a
b
                c
        e

The first column shows lines only in the first file, the second column lines from the second file and the third column lines from both.

This can be made easier still by adding a bit of perl:

$ comm 1.txt 2.txt | perl -pe 's/^/1: /g;s/1: \t/2: /g;s/2: \t/A: /g;' | sort
1: b
2: e
A: a
A: c

vimdiff <(svn cat -r ##ver## foo) foo

will put you in the usual vimdiff output, but comparing between ##ver## and your current version of foo

Suppose we have to find the differences between local file "/tmp/localfile" and remote file "/tmp/remotefile" located on remote host 123.4.5.6

This is how can do it:

$ ssh user@123.4.5.6 "cat /tmp/remotefile" | diff - /tmp/localfile

And using vimdiff:

$ vimdiff scp://user@123.4.5.6//tmp/remotefile /tmp/localfile

(of course we would need ssh to work using public key authentication so that we can do remote commands execution without being prompted for passwords).

A quick script to compare files from two directories (for example a backup and working directory).

#!/bin/bash

cr='*'
if [ -z $3 ]; then cr=$3; fi

for f in `find $1/$3 -type f | sed "s|$1/||"`
do
  printf "===!%-76s" "$f!" | tr ' !' '= '; echo
  diff $1/$f $2/$f | sed -e "s/^</$1: /" -e "s/^>/$2: /"
done

Usage: [script name] directory1 directory2 to check all files
[script name] directory1 directory2 *html to check files of type html.