Diff and Cmp (Find the difference)

DIFF & CMP
( Find the difference )

Comparing two files to see if they are identical is not always easy using a GUI text-editor and having them side by side. But of course Linux has a command for that to make things easier. Actually there are two commands: "diff" and "cmp" but you will see that the first one is so much easier to use then the second one.

Okay let us set up an example to explain things . . . I place two almost identical files ( colors1.txt and colors2.txt ) in a "test" directory.

First we cd to that directory:

CODE

$ cd /home/bruno/test

Now first I will show you the use of diff without any options:

CODE

$ diff colors1.txt colors2.txt

6c6
< # indigo
---
> # indigo-blue
11d10
< # sienna
12a12
> # darkblue
15c15
< # tomato
---
> # tomato-red

We have to admit that the output is at least confusing . . . so now we add the -u ( unified ) option:

CODE

$ diff -u colors1.txt colors2.txt

--- colors1.txt 2005-04-01 20:45:21.954828407 +0200
+++ colors2.txt 2005-04-01 20:44:50.152742422 +0200
@@ -3,13 +3,13 @@
# darkred
# deeppink
# firebrick
-# indigo
+# indigo-blue
# limegreen
# royalblue
# sandybrown
# seagreen
-# sienna
# silver
+# darkblue
# skyblue
# teal
-# tomato
+# tomato-red

Now we start to see clearly . . . . . the differences from color1.txt to color2.txt are marked with "-" and "+" . . . . In the first file it says "indigo" in the second one "indigo-blue" the second one has no sienna but has the darkblue the first one has not . . etc. etc.

Okay now we will make it even better to read:

CODE

$ diff -y -W 70 colors1.txt colors2.txt

# chocolate
# crimson
# darkred
# deeppink
# firebrick
# indigo          |
# limegreen
# royalblue
# sandybrown
# seagreen
# sienna       <
# silver
   >
# skyblue
# teal
# tomato    |

# chocolate
# crimson
# darkred
# deeppink
# firebrick
# indigo-blue
# limegreen
# royalblue
# sandybrown
# seagreen

# silver
# darkblue
# skyblue
# teal
# tomato-red

See this ?? . . . . The "|" indicates there is a difference and the ">" and "<" tells us what is left out or added !! The -y option stands for "Side by side" . . and the -W 70 sets the width for the column.

Now we have found the best way to compare the two files I will show you the other command: "cmp" The cmp command is a whole other story. Let me first give an example:

CODE

$ cmp colors1.txt colors2.txt

colors1.txt colors2.txt differ: byte 64, line 6

That is all the output it gives . . . that is because cmp stops at the first byte that differs and then exits, writing the line and the byte that causes it to stop. Not really a clear output is it ?

Sure we can give cmp the -l option so it does not stop at the first byte that differs but give all the bytes that are different . . but then you will see that ALL the bytes after byte 64 are different and you will only get a real long list even from the short example we use:

CODE

$ cmp -l colors1.txt colors2.txt

64 12 55
65 43 142
66 40 154
67 154 165
68 151 145
69 155 12
70 145 43
71 147 40
72 162 154
73 145 151
74 145 155
<<---------------------
I did remove byte 75 to 140 to shorten it a bit
---------------------->>
141 43 153
142 40 171
143 164 142
144 145 154
145 141 165
146 154 145
151 157 145
152 155 141
153 141 154
154 164 12
155 157 43
156 12 40
cmp: EOF on colors1.txt

So it will be clear to you that I prefer using diff . . . . . and the next time we will be using it together with another command we already know to check out the differences of the content of 2 directories. In the 3rd episode on diff I will post a script Ragnar sent me a while ago : "dirdiff" . . . . see you then !

Bruno

-- Apr 12 2005 ( Revised Dec 10 2005 ) --