Sort command in Unix/Linux

sort command is used to sort or merge lines of text and binary files. Comparisons are based on one or more sort keys extracted from each line of input, and are performed lexicographically. By default, if keys are not given, sort uses entire lines for comparison. In this post I am going to explore commonly used sort command with examples.

Syntax

sort [OPTION]... [FILE]...

Make man your friend, for reference or in doubt use the command $man sort

Examples

Consider the text file rk.txt as input with the following content

Rahul
rahul
Aman
Nitish
01rk

1. Sort in lexicographical order

$sort rk.txt
output:
01rk
Aman
Nitish
Rahul
rahul

By default sort uses entire line for comparison and sorts based on lexicographical order (number < alphabet and Uppercase < Lowercase).

2. Sort in reverse order

-r flag is used to sort in reverse order

$sort -r rk.txt
output:
rahul
Rahul
Nitish
Aman
01rk

3. Check whether the file is already sorted

Use -c flag to check whether the file is already sorted.

$sort -c rk.txt
output:
sort: rk.txt:3: disorder: Aman

Since the file is not sorted it prints line number with disorder message. If the file is sorted it does not print anything.

4. Writing output to a file

Note: input file does not change with sort command, content of input file remains in same order as before
To save output of sort command in a file either use > redirection operator or `-o’ flag

$sort rk.txt > output.txt
$sort -o output.txt rk.txt

The both the above command output of sort will be saved in output.txt file.

6. Sorting numeric values

-n flag is used to sort numeric values, Lets override the content of rk.txt with following values

12
1
4
5.23
23.2
$sort -n rk.txt
output:
1
4
5.23
12
23.2
$sort -nr rk.txt
output:
23.2
12
5.23
4
1

7. Merge sorted files

Consider files a.txt and b.txt with following contents

a.txt
Aman
Rahul
b.txt
Amit
Nishant
$sort -m a.txt b.txt
output:
Aman
Amit
Nishant
Rahul

Note: If the input files are not sorted, the order of lines in output is not defined.

8. Sort and remove duplicates

Consider a.txt with following content

Rahul
Aman
Rahul
Rohit

-u command is used to sort and remove duplicated.

$sort -u a.txt
output:
Aman
Rahul
Rohit

9. Sort based on a specific column/field of each line

-k flag with column/field number is used to sort on based on that column.
Consider a.txt with following content.

Rahul 2
Rohit 21
Aman 10
Amit 15

Following command will sort content of a.txt based on second column

$sort -n -k 2 a.txt
output:
Rahul 2
Aman 10
Amit 15
Rohit 21

Above command can also be written as $sort -nk2 a.txt. note -n flag is used for numeric sorting. By default white spaces used as field separator, for other types of delimiter use -t flag.

Consider b.txt with following content.

Rahul,2
Rohit,21
Aman,10
Amit,15

Following command will sort content of b.txt on second column.

$sort -t ',' -n -k 2 b.txt
output:
Rahul,2
Aman,10
Amit,15
Rohit,21

10. Ignore case while sorting

Use -f flag to ignore case, consider a.txt with following content

rahul
Rohit
Amit
Aman
$sort -f a.txt
output:
Aman
Amit
rahul
Rohit