1. Overview
Comparing remote and local files without copying them is a common task in the software development process. Transferring large files over the internet can be time-consuming and requires significant storage space on the local machine.
The diff command in Linux is a popular tool for this task. It allows users to compare two files line by line and display the differences. In this tutorial, we’ll learn how to compare two files using the diff command provided that one file is present locally while the other file is on a remote server.
2. Comparing a Local File With a Remote File
In order to compare a local file with a remote file, we need a combination of diff and ssh commands. We need to use the ssh command to establish a secure shell connection with the remote server. Let’s take an example where we have a local file named file1.txt and a remote file named file2.txt located in the home directory of the remote server.
Let’s compare these two files by executing the diff command in the terminal:
$ diff file1.txt <(ssh user@remote_server 'cat file2.txt')
The above command uses the ssh command to connect to the remote server. The cat command will display the content of the remote file. The <() syntax is used to treat the output of the ssh command as a file and pass it to the diff command.
Using the diff command in combination with the ssh command, we can compare remote and local files without copying them. It’s important to ensure that the remote server is secure and properly configured before using this method.
2.1. Alternate Method
Alternatively, we can use a similar command to compare a local and a remote file:
$ ssh user@remote-server "cat file2.txt" | diff file1.txt -
First, we use the ssh command to establish a secure shell connection with the remote host. Further, we execute the cat command to output the contents of the remote file file2.txt. Then, the | symbol (pipe) redirects the output of the ssh command to the diff command. The diff command compares the output of the ssh command with the local file file1.txt. Lastly, the – symbol is used to indicate that the remote file should be read from standard input.
The output of this command will be the same as the previous examples, indicating the differences between the remote and local files. This command is similar to the previous example that used the <() syntax to pass the output of the ssh command to the diff command. However, in this case, the | symbol is used to achieve the same result.
3. Conclusion
In this article, we learned how to compare a local file with a remote file without copying them. We used two different combinations of the diff and ssh commands to achieve our goal. Also, we can use both methods interchangeably depending on personal preference. When working with huge files, these techniques are extremely helpful as they save us both time and local storage space.