When you are working with files on the command line, you might need to check their modification time and content integrity.
Linux has a powerful command line which allows you to explore multiple aspects of files and filesystems.
In case you need to check if file was modified, you can follow these two approaches:
How to Check if a File Was Modified by Checking Modification Time
When a file is edited, its timestamp changes to match the modification time.
We can view the last modified time of a file using long listing( ls -l).
In the output below, we can see that the file was modified on Jul 19 13:22.
zaira@Zaira:~$ ls -lrt | grep calculator.py
-rw-r--r-- 1 zaira zaira 263 Jul 19 13:22 calculator.pyJul 19 13:22How to Check if a File Was Modified by Checking File Size
If we know the previous size of the file, we can compare it with the current file size to see if was changed.
We can view the file size using long listing( ls -l). The 5th column shows the size of the file in bytes.
zaira@Zaira:~$ ls -lrt | grep calculator.py
-rw-r--r-- 1 zaira zaira 263 Jul 19 13:22 calculator.pyThe methods mentioned above usually get the task done, but there is an advanced method to check file integrity using a hash. The method is called 'checksum' and the corresponding command for that in Linux is cksum.
What is Checksum in Linux?
Sometimes the data gets corrupted during transmission or storage. To ensure that the data remains consistent, we can use checksum.
Checksum is the result of an algorithm called a cryptographic hash function. It's applied to blocks of the data in the file.
In networking, you can use checksum to compare the hash value at sender and receiver ends. If the hash value is same, it implies that your copy of the file is genuine and error free.
Some commonly used cryptographic hash functions include MD5 and SHA-1.
Next we will see how we can calculate the hash in Linux.
How to Find the Checksum in Linux using cksum
cksum is a command found in *nix-like operating systems that generates a checksum value for a file or stream of data.
According to the man page of cksum, the command prints CRC (cyclic redundancy check) checksum and byte counts of each FILE.
To learn more about the CRC algorithm, refer to this page.
Syntax of cksum
The cksum command takes the filename as an argument and generates its hash value. The basic syntax is as follows:
cksum [FILE]How to Use cksum
Let's suppose we have a file named calculator.py. We can calculate its checksum like this:
zaira@Zaira:~$ cksum calculator.py
1991291549 262 calculator.pyIn the output, we get three columns:
- The first column is the hash value.
- The second value is the amount of data in bytes for the given file.
- The third column is the file name.
Even a slight modification changes the hash value. Let's see how that looks with an example.
Let's modify our original file calculator.py by adding an extra line at the end:
zaira@Zaira:~$ echo >> "this file is now changed" >> calculator.pyLet's calculate the checksum again and see if the hash value has changed:
zaira@Zaira:~$ cksum calculator.py
331872555 263 calculator.pyThe first column is the hash value and it has changed since we appended the text.
Now we know that the file has changed as the checksum hash values are no longer the same.
We can use the same method to compare files with the same name, size, and modification time across different machines to ensure that both files are the same.
Conclusion
There are cases when you need to compare the files across systems, specially when they are transferred from one location to the other. We can use a combination of the three methods to verify if our file is intact:
- Viewing the file modification time.
- Verifying the file size.
- Generating and comparing the hash value using
cksum.
I hope you found this tutorial helpful. Thank you for reading till the end.
What’s your favorite thing you learned from this tutorial? Let me know on Twitter!
You can also read my other posts here.