Linux: Using diff to compare files
Purpose:
diff reports the differences between two files.
Description
diff [options] <i>file1</i> <i>file2</i>
Simple Usage Example
File test1:
Test!
A common line.
Really common line.
One more common line.
Only here.
File test2:
Test!
A almost common line.
Really common line.
Really not common line.
One further line.
One more common line.
For example
$ diff test1 test2
will output:
2c2
< A common line.
—
> A almost common line.
3a4,5
> Really not common line.
> One further line.
5d6
< Only here.
What does that mean? First of all this output can be used as a script for ed (see example ). Lines
from first file are preceded by a less then symbol ( < ) and lines from
second file by a greater then symbol ( > ).
A dashed line ( — ) is used to separate output from the two files.
The letters can be used to convert file1 into file2:
c Replace lines from file1 with those from file2.
d Delete lines from file1.
a Add lines from file2 to file1.
The two files have three differences:
Line 2 differs.
Lines 4 and 5 of test2 are not present in test1. These lines would need to
be added after line 3 of test1 for the files to be the same.
Since you normally think in terms of converting the first file into the second file it is better to say that line 5 would need to be deleted from test1 for the files to be the same.
Useful Options
Option
Description
-b
Ignores repeated blanks (e.g. <space><space> is the same as <space>) and blanks at the end of lines.
-w
Ignore all spaces and tabs (e.g. 1 or 2 is equivalent to 1or2).
-i
Ignore case (e.g. howdy, HOWDY and HoWdY are equivalent).
-c
Use the context output format. Context output includes three lines before and after those that are normally printed to give “context” for the differences.
-u
Use the unified output format, easy readable with file information.
-C n
Like -c but include n lines of context output.
-e
Produce a script file that can be used by ed to convert file1 to file2. This option is not used as often as the patch command to convert file1 to file2.
-h
Do a faster but less accurate comparison. This does not work well if the files are very different and cannot be used with the -e option.
Directory related Options
Option
Description
-l
Output is formatted so that each file comparison occurs on a new page. Other comparisons are listed on a final page.
-r
Recursively compare all files in common subdirectories.
-s
Include a listing of all identical files in the output.
Examples
Comparing Directories
diff /tmp/oldFolder/ /tmp/newfolder/
Try it out! The output is self-explaining.
Changed files in folder tree
lists all files that have changed in a folder tree
diff -uwrq /tmp/oldFolder/ /tmp/newfolder/
Ignore Case and Repeated Blanks
Report the differences between poem1 and poem2 using the -i option to ignore the differences between upper and lower case charactersand the -b
option which ignores all repeated blanks and blanks at the end of lines.
diff -ib test1 test2
Patch with Diff
The recommended way …
Using output redirection
diff test1 test2 > diff_for_patch
patch test1 diff_for_patch
test1 is converted into test2 regarding the diff results, wich you can
influence by using options.
With
diff -r dir1 dir2 > dir2.patch
you can create a patch over all files in a directory which you can apply using
cd dir1 ; patch -p1 < dir2.patch
Ed with Diff
The -e option creates a script that gives directives to the ed text editor to convert file1 into file2.
diff -e test1 test2 > diff_for_ed
( cat diff_for_ed && echo w )| ed – test1
( ) for subshells, | – piping, ‘echo w’ is appended to ed input to make ed write the file
Diff with Context and more verbose Output
diff -c test1 test2
You can use the -C n option for n context lines. Output related to file1 is
preceded by stars (***) and file2 by dashes (—). Differences are separated by a long row of stars (***************).
In output the following symbols are used:
! Indicates corresponding lines in the two files that differ.
+ Indicates lines that exist in file2 but not file1.
– Indicates lines that exist in file1 but not file2.
Diff with Script
Using the Bourne shell:
#!/bin/sh
# use -h option for faster, less accurate comparison
diff -h $1 $2 &gt; /dev/null
# Asking for exit status using $? for bourne shell, 0 means equal
if [ $? -eq 0 ]; then
echo identical
elif [ $? -eq 1 ]; then
echo different
else
echo an error occurred
fi
By: K Rekk
