sha256sum

The sha256sum command in Unix and Linux is used to compute and check SHA-256 hash values. SHA-256 is part of the SHA-2 (Secure Hash Algorithm 2) family, which generates a 256-bit (32-byte) hash value, commonly used to verify data integrity and authenticate files.

Basic Usage

The basic syntax for the sha256sum command is:

sha256sum [OPTION]... [FILE]...

• FILE: The file(s) for which to compute the SHA-256 checksum. If no file is specified, sha256sum reads from standard input.

Examples

Calculating SHA-256 Checksum

To calculate the SHA-256 checksum of a file:

sha256sum file.txt

This command outputs the SHA-256 hash value followed by the file name.

Example output:

e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855  file.txt

Verifying SHA-256 Checksum

To verify the integrity of a file using a previously generated checksum:

1. Create a checksum file:

sha256sum file.txt > file.txt.sha256

2. Verify the file against the checksum:

sha256sum -c file.txt.sha256

This command checks the hash value in file.txt.sha256 against the hash value of file.txt and reports if the file is unchanged.

Example output:

file.txt: OK

Options

Reading Checksum from File

To read and verify checksums from a file:

sha256sum -c checksums.sha256

The checksums.sha256 file should contain lines with the format <checksum> <filename>.

Quiet Mode

To run in quiet mode and only display a message for files that do not match:

sha256sum -c -q file.txt.sha256

Status Mode

To output only the status for verification without printing the detailed results:

sha256sum -c --status file.txt.sha256

This command returns an exit status of 0 if all files are verified correctly, 1 if any files are mismatched, and 2 for other errors.

Binary Mode

To read files in binary mode:

sha256sum --binary file.txt

Text Mode

To read files in text mode (default):

sha256sum --text file.txt

Practical Use Cases

Verifying Downloaded Files

When downloading software or large files, it is common to verify the integrity of the downloaded files using the provided SHA-256 checksum:

sha256sum -c software_download.sha256

Creating Checksums for Multiple Files

To generate SHA-256 checksums for all files in a directory:

sha256sum * > all_files.sha256

Checking Data Integrity

To ensure data integrity when transferring files between systems, compute and verify checksums before and after transfer:

On the source system:

sha256sum datafile > datafile.sha256

On the destination system:

sha256sum -c datafile.sha256

Summary

The sha256sum command is a robust tool for computing and verifying SHA-256 hash values, ensuring the integrity and authenticity of files. By using sha256sum, you can safeguard against data corruption and verify that files have not been tampered with. Understanding its options and use cases can significantly enhance your data management and security practices.