How to sort a file in-place in Linux?

In Linux, the sort command is a powerful utility for arranging file contents in a specific order. By default, it sorts lines alphabetically based on ASCII values. However, the basic sort command only displays sorted output without modifying the original file. To sort a file in-place (modifying the original file), we need specific techniques.

Understanding the Sort Command

The sort command operates line by line, treating each line as a record. Key characteristics include −

• Sorts one line at a time from the input

• Displays file contents similar to the cat command when applied to files

• Case-insensitive by default (though this can be changed with options)

• Does not modify the original file unless explicitly redirected

Basic Syntax

sort filename

Example − Basic Sort Operation

Consider a file with unsorted content −

cat somefile.txt

Is this text added
this file contains
a new text stream
and i am going to edit
that stream
yes i can do that
ask jeffrey

Using the basic sort command −

sort somefile.txt

Is this text added
a new text stream
and i am going to edit
ask jeffrey
that stream
this file contains
yes i can do that

Important: The original file remains unchanged. This can be verified by running cat somefile.txt again.

Methods for In-Place Sorting

Method 1: Using the -o Option

The most straightforward way to sort a file in-place is using the -o option −

sort -o somefile.txt somefile.txt

This command safely sorts the file and writes the result back to the same file.

Method 2: Using Temporary Files

Create a sorted copy and replace the original −

sort somefile.txt > temp_sorted.txt && mv temp_sorted.txt somefile.txt

Method 3: Using Command Substitution

sort somefile.txt | sponge somefile.txt

Note: The sponge command (from moreutils package) reads input completely before writing output.

Common Sort Options

Verification Example

After sorting in-place with sort -o somefile.txt somefile.txt −

cat somefile.txt

Is this text added
a new text stream
and i am going to edit
ask jeffrey
that stream
this file contains
yes i can do that

The file content is now permanently sorted.

Conclusion

To sort a file in-place in Linux, use sort -o filename filename as the safest method. This approach ensures the original file is modified with sorted content while maintaining data integrity. Alternative methods include temporary files or specialized tools like sponge for more complex scenarios.