How to swap two files in Linux command line?

As system administrators or DevOps professionals, we often need to exchange file contents. For example, you might have a backup file /etc/passwd.backup that you want to restore to /etc/passwd, while preserving the current content by moving it to the backup file. This operation is called file swapping exchanging the actual content of two files, not their locations.

When we talk about swapping two files on Linux, we mean exchanging their content while keeping their names and locations unchanged. This tutorial demonstrates practical methods to accomplish this task using command line tools.

Example Files Setup

Let's create two sample files to demonstrate the swapping process:

$ echo "Hi Welcome in Linux!" > f1.txt
$ echo "Hi Welcome in Linux shell scripting" > f2.txt
$ cat f1.txt
Hi Welcome in Linux!
$ cat f2.txt  
Hi Welcome in Linux shell scripting

Goal: After swapping, f1.txt should contain "Hi Welcome in Linux shell scripting" and f2.txt should contain "Hi Welcome in Linux!"

Method 1: Using mv Command with Temporary File

The mv command can rename and move files. We'll use it for renaming to implement the classic three-step swap algorithm using a temporary file:

# Step 1: Move first file to temporary name
mv f1.txt temp.txt

# Step 2: Move second file to first file's name  
mv f2.txt f1.txt

# Step 3: Move temporary file to second file's name
mv temp.txt f2.txt

This approach follows the standard variable swapping pattern:

temp = x
x = y  
y = temp

Verification

$ cat f1.txt
Hi Welcome in Linux shell scripting
$ cat f2.txt
Hi Welcome in Linux!

Method 2: Shell Script for Automated Swapping

Create a reusable script to automate the swapping process:

#!/bin/bash

if (( $# == 2 )); then
   # Create temporary file in same directory as first file
   TMPFILE=$(mktemp $(dirname "$1")/swap_temp.XXXXXX)
   
   # Perform three-step swap
   mv "$1" "$TMPFILE" && mv "$2" "$1" && mv "$TMPFILE" "$2"
   
   echo "Successfully swapped contents of $1 and $2"
else
   echo "Error: Two valid file paths required"
   echo "Usage: $0 file1 file2"
   exit 1
fi

Script Explanation

• $# Number of arguments passed to the script

• mktemp Creates a unique temporary file in the same directory

• && Ensures each mv command succeeds before proceeding

• Error handling for incorrect usage

Usage

$ chmod +x swap_files.sh
$ ./swap_files.sh f1.txt f2.txt
Successfully swapped contents of f1.txt and f2.txt

Method 3: One-liner with mktemp

For quick swaps, you can use this one-liner command:

$ TEMP=$(mktemp) && mv file1 "$TEMP" && mv file2 file1 && mv "$TEMP" file2

Important Considerations

• File permissions Ensure you have write permissions for both files and their directory

• Atomic operations The && operator ensures all steps complete successfully

• Temporary files mktemp creates unique temporary files to avoid conflicts

• Same filesystem mv works fastest when files are on the same filesystem

Conclusion

Swapping file contents in Linux can be accomplished using the mv command with a temporary file approach. The shell script method provides automation and error handling, making it suitable for frequent use. Both methods preserve file permissions and efficiently exchange content without data loss.