fgrep files

Learn how to use fgrep to compare the contents of two files with each other. The command can be used to find the contents present in one file but not in the other file. This does not require the file contents to be sorted.

Find contents of file1 that are also present in file2

fgrep -x -f file2 file1

This command takes a line from the file specified next to ‘-f’ and searches for the same in the other file.
-x makes it match the lines exactly(For example, with normal match 123 matches 12345. But with -x, it does not match).
Find the contents of file1 that are not present in file2

fgrep -x -v -f file2 file1

The -v option negates the match.

Examples for fgrep files match

For below examples, let’s assume we have two files with contents as below.

Data1.txt

orange
apple
blue
pineapple
red
watermelon

Now the second file Data2.txt is as below

white
red
apple
green
yellow

Example 1:
Find all elements of data1.txt that are also present in data2.txt(In other words, find the records present in both the files)

~# fgrep -x -f data2.txt data1.txt
apple
red

Example 2:
Find all elements of data1.txt that are NOT present in data2.txt

~# fgrep -v -x -f data1.txt data2.txt
 white
 green
 yellow

‘-x’ helps to match whole line, instead of a partial match. If we remove -x, the output for example 1 would be

fgrep -f data2.txt data1.txt
apple
pineapple
red

fgrep returns the whole file even though only few lines match

This happens if the file specified with -f option has a blank line. Remove blank lines from the file and then fgrep should print only matched lines.

fgrep -v returns empty when there should be some lines not present in the other file

This again happens for the same reason. Verify the first file does not have any blank lines.