In linux, the most vanilla version of 'uniq' eliminates consecutive repetitions of a line when a text file is piped through it.
Plain Uniq
For instance, if this is the file test.txt
00
00
01
01
00
00
02
02
This is the output on passing it through the 'uniq' command, either via pipes or as input via STDIN.
Command: uniq < test.txt
00
01
00
02
Since the first two lines of the original file are the same (00) and same for the next two (01) again followed by two repetitions of 00 and two repetitions of 02 - the 'uniq' command replaces consecutive repetitions by only one line in each case.
Uniq with counts
uniq -c < test.txt
This one also indicates the count of repetitions for each of the lines it collapses.
If this is the test file (say, testCounts.txt)
00
00
01
01
00
00
02
02
03
aa
aa
aa
uniq -c < input00.txt
2 00
2 01
2 00
2 02
1 03
3 aa
The first number is the count of the number of repeated occurrences in the original file.
Printing only duplicate lines
The '-d' option prints only those lines which are followed by one or more repetitions immediately after them.
uniq -d < testCounts.txt
OR
cat testCounts.txt | uniq -d
OR
uniq -d testCounts.txt
Printing only unique lines
The '-u' option prints only those lines which are succeeded and preceded by different lines.
uniq -u < testCounts.txt
OR
cat testCounts.txt | uniq -u
OR
uniq -u testCounts.txt
It is also possible to
- limit comparison only to the first N characters (using the -w option)
- avoid comparing first N characters (using the -s option)
- ignore variations in case between lines (the -i option)
- avoid comparing the first N fields using the -f option.
(This may be useful while processing TSV files when you'd like to ignore the first column, if it has serial numbers.)
You might find these examples interesting and useful.