What is cmp?
5 cmp Examples
Two provide examples for this command, lets consider two files :
$ cat file1.txt Hi My name is Himanshu
$ cat file2.txt Hi My name is Himanshu Arora
1. Directly compare using cmp
If we straight away try to compare the two files mentioned above, then we see :
$ cmp file1.txt file2.txt file1.txt file2.txt differ: byte 23, line 1
So we see that cmp utility instantly recognized the difference between contents in two files.
2. Skip same number of initial bytes from both input files
If we want to skip some initial bytes of input files and then start the comparison then -i option can be used
$ cmp -i 5 file1.txt file2.txt file1.txt file2.txt differ: byte 18, line 1
So we see that the initial 5 bytes were skipped and hence cmp complains that byte 18 differs (which in example 1 was byte 23)
3. Skip different number of initial bytes from both input files
Another case could be when we would want to skip ‘x’ bytes of input in file1 and ‘y’ bytes of input in file2. This could be achieved as follows :
$ cmp -i 5:2 file1.txt file2.txt file1.txt file2.txt differ: byte 2, line 1
So we see that we can give the option -i with ‘x:y’ kind of input for this case. The output shows that this time the difference occurred very early at the second byte.
4. Display bytes that differ
If in the example 3 above we would also like to know the values of individual differing bytes being compared then the -l option can be used
$ cmp -l -i 5:2 file1.txt file2.txt 2 156 115 3 141 171 4 155 40 5 145 156 6 40 141 7 151 155 8 163 145 10 110 151 11 151 163 12 155 40 13 141 110 14 156 151 15 163 155 16 150 141 17 165 156 18 12 163 cmp: EOF on file1.txt
So we see can easily observe the difference in byte2 above.
5. Upper limit on number of bytes to compare
Also, we can put a limit on number of bytes to compare. This can be used when we want to compare only a specified number of bytes and not the complete file. This can be achieved through -n option which is followed by the number of bytes.
So in the above example 4 if we want to put an upper limit on the bytes to be compared, it can be done as follows :
$ cmp -l -n 10 -i 5:2 file1.txt file2.txt 2 156 115 3 141 171 4 155 40 5 145 156 6 40 141 7 151 155 8 163 145 10 110 151
Syntax and Options
cmp [OPTION]... FILE1 [FILE2 [SKIP1 [SKIP2]]]
|Short Option||Long Option||Option Description|
|-b||–print-bytes||Print differing bytes.|
|-i SKIP||–ignore-initial=SKIP||Skip the first SKIP bytes of input|
|-i SKIP1:SKIP2||–ignore-initial=SKIP1:SKIP2||Skip the first SKIP1 bytes of FILE1 and the first SKIP2 bytes of FILE2.|
|–no-preserve-root||do not treat `/’ specially (the default)|
|-l||–verbose||Output byte numbers and values of all differing bytes.|
|-n LIMIT||–bytes=LIMIT||Compare at most LIMIT bytes.|
|-s||–quiet –silent||Output nothing; yield exit status only.|
|-v||–version||Output version info.|
|–help||Output this help.|