Linux sort Command with Examples

Introduction

The sort command is a tool for sorting file contents and printing the result in standard output. Reordering a file’s contents numerically or alphabetically and arranging information in ascending or descending order improves readability.

In this tutorial, you will learn to use the Linux sort command and see useful examples.

Prerequisites

• A system running Linux.
• Access to a terminal.

Linux sort Syntax

The sort command has the following syntax:

sort [options] filename

Running the sort command without [options] outputs the file’s contents and applies the default sorting rules for the current locale.

The main rules are:

• Lines starting with a number go before the lines beginning with a letter, arranged in ascending order (1-10).
• Lines starting with letters are sorted in ascending alphabetical order (A-Z).

For example, the following file has multiple random lines:

Run sort to arrange the file’s contents in the default manner:

The example above shows how the sort command arranges lines by default when no options are specified.

Linux sort Options

Specify an option to change the sort command sorting rules. The most used options are listed in the table below:

Short option form	Long option form	Description
-b	--ignore-leading-blanks	Causes sort to ignore leading blanks.
-d	--dictionary-order	Causes sort to consider only blanks and alphanumeric characters.
-f	--ignore-case	Ignores the default case sorting rule and changes all lowercase letters to uppercase before comparison.
/	--files0-from=F	Reads input from the files specified by NUL-terminated names in the specified F file. If F is -, sort reads names from standard input.
-M	--month-sort	Sorts lines according to months (Jan-Dec).
-h	--human-numeric-sort	Compares human-readable numbers (e.g., 2K 1G).
-n	--numeric-sort	Compares data according to string numerical values.
-R	--random-sort	Sorts data by a random hash of keys but groups identical keys together.
/	--random-source=FILE	Gets random bytes from the specified FILE.
-r	--reverse	Reverses the comparison results.
/	--sort=WORD	Sort data according to the specified WORD: general-numeric -g, human-numeric -h, month -M, numeric -n, random -R, version -V.
-c	--check, --check=diagnose-first	Checks if the input is already sorted but doesn’t sort it.
/	--debug	Annotates the part of the line used for sorting.
-k	--key=KEYDEF	Sort data using the specified KEYDEF, which gives the key location and type.
-m	--merge	Causes sort to merge already sorted files.
-o	--output=FILE	Redirects the output to FILE instead of printing it in standard output.
-t	--field-separator=SEP	Uses the specified SEP separator instead of non-blank to blank transition.
-z	--zero-terminated	Causes sort to use NUL as the line delimiter instead of the newline character.
/	--help	Displays the help file with full options list and exits.
/	--version	Outputs the program version and exits.

Linux sort Examples

Below are examples of using the sort command to arrange file contents in different ways.

Example 1: Save Output to File

The sort command only displays a file’s contents after arranging them, but it doesn’t change the file. However, the -o option allows you to save the sort command output to a file.

For example, the following command sorts the data in the default.txt file and saves the output to the sorted.txt file:

sort -o sorted.txt default.txt

Running cat outputs the new file’s contents.

Another way to save the sort command output is to redirect the output to a file:

sort default.txt > sorted.txt

Example 2: Check for Sorting in File

Specify the -c option to check if a file’s contents have already been sorted. If the contents are already sorted, there’s no output from the sort command.

However, when sort finds an unsorted line, it outputs a message reporting the first line is out of place.

For example:

sort -c default.txt

The command outputs a message reporting the number and contents of the first unsorted line.

Example 3: Sort Multiple Files

There are two ways to sort multiple files:

1. List filenames as arguments separated with a space.

Sort multiple files simultaneously by specifying the respective file names as arguments, separated by whitespace. The files’ contents are joined together, sorted, and printed in standard output.

For example:

sort default1.txt default2.txt

The command concatenates, sorts, and prints both files’ contents.

2. Pipe the find command output to sort.

Another way to sort multiple files simultaneously is to pipe the find command output to sort and use the --files0-from= option in the sort command.

Specify the -print0 option in find to end file name with the NUL character and ensure the program properly reads the file list.

For example:

find -name "default?.txt" -print0 | sort --files0-from=-

The example above concatenates and sorts the default1.txt and default2.txt files.

Example 4: Sort by Fields

By default, sort compares the entire line’s contents and decides how to sort it. To compare according to a data subset, specify which fields to compare using the -k option.

The -k option takes the following syntax:

-k [ FStart [ .CStart ] ] [ Modifier ] [ , [ FEnd [ .Cend ] ][ Modifier ] ]

• FStart. The beginning of the line.
• CStart. The first column in the field.
• Fend. The end of the line.
• CEnd. The last column of the field.

The sort key includes all characters beginning with the field specified by the FStart variable and the column specified by the CStart variable. The key ends with the field specified by FEnd and the column specified by CEnd.

Not specifying Fend assumes the line’s last character as the end. Without specifying CEnd, the last character in the FEnd field is assumed.

If there is any space between the fields, sort considers them as separate fields.

For example, the lines in the following file contain three fields:

Sort the data based on the second field by running:

sort -k 2,2 school.txt

The command sorts the data according to the second field and ignores the first field.

Example 5: Sort in Reverse Order

Use the -r option to sort the data in reverse order, i.e., in reverse alphabetical order or highest to lowest numbers.

For example, the following file is sorted with the default sorting rules:

sort default.txt

Specifying the -r option reverses the sorting order:

sort -r default.txt

Example 6: Remove Duplicate Entries

The -u option allows sort to remove duplicate entries from a file. For example, the following file has several duplicate entries:

Specifying the -u option sorts the data and removes the duplicate lines:

sort -u test.txt

Example 7: Sort by Months

Specify the -M option to arrange data according to months listed in a file.

To demonstrate sorting by months, create a file populated by the ls command output:

ls -l > month-sorting.txt

Sort the data from the ls -l output by months by running the following command:

sort -Mk6 month-sorting.txt

The command sorts the file contents based on the sixth field containing the file creation month.

Example 8: Randomly Sort Data

Use the -R option to randomly arrange data without applying any sorting criteria.

For example, the following file contains several lines sorted numerically:

Mix up the line order with the following command:

sort -R ordered.txt

The output shows that data is now randomly sorted.

Example 9: Specify a Delimiter

sort separates fields in a line using whitespace (spaces or tabs) as field delimiters. Specify the -t option to change the field delimiter to a different character.

For example, the following file contains multiple fields, including a person’s name, job title, and place of residence:

Sorting the file by place of residence without changing the delimiter doesn’t work because some cities have a space in the middle of their name. The space makes sort interpret the city name as two fields instead of one.

Change the delimiter to a comma (,) to properly sort the file’s contents:

sort -t, -k4 school.txt

Changing the delimiter produces the desired output.

Example 10: Sort a Stream Output

Another way to supply input to the sort command is to use the pipe (|) operator. Piping into sort allows it to arrange another command’s output and, optionally, save the results into a file.

The following example shows how to pipe the ls command output into sort and arrange the contents by file size:

ls -l /home/$USER | sort -nk5

The -nk5 options specify numeric sorting in the fifth field.

Conclusion

This tutorial showed how to use the sort command to rearrange a file’s contents effectively. Easily arrange large data sets in an ascending or descending order using sort.

For more Linux commands and a downloadable PDF cheat sheet, refer to our Linux commands cheat sheet.