Linux command line offers several tools for checking and verifying a file’s integrity. One such tool is sha1sum, which we will be discussing here in this tutorial using some easy to understand examples. But before we do that, it’s worth mentioning that all examples here have been tested on an Ubuntu 16.04 LTS machine.
Linux sha1sum command
The sha1sum command is used to compute and check SHA1 message digest. Following is its syntax:
sha1sum [OPTION]… [FILE]…
And here’s how the man page describes this tool:
Print or check SHA1 (160-bit) checksums.
With no FILE, or when FILE is -, read standard input.
Following are some Q&A-styled examples that should give you a better idea on how this tool works.
Q1. How to use sha1sum command?
Basic usage is fairly simple – just run the command with a filename as input.
Here’s the output the above command produced on my system:
One the left is the message digest computed by the tool. Here’s how the output can be comprehended:
The default mode is to print a line with checksum, a space, a
character indicating input mode ('*' for binary, ' ' for text or
where binary is insignificant), and name for each FILE.
Q2. How to use sha1sum to verify a file’s integrity?
For this, first save the message digest produced by the command in a .sha1 file. For example, here’s how we did it in our case:
sha1sum test.txt > test.sha1
Now, with both test.txt and test.sha1 in the same directory, use the -c command line option to verify the file’s integrity.
sha1sum -c test.sha1
Here’s the output produced:
In case the check fails, the tool produces following output:
Q3. How to force sha1sum to read file in binary mode?
By default, the sha1sum command reads a file in text mode. However, you can force the tool to read in binary mode as well, something which you can do using the -b option.
sha1sum -b [filename]
Q4. What all sub-options are available while verifying checksums?
There are multiple preferences that you can set while verifying checksums using sha1sum. Here’s the list:
don't fail or report status for missing files
don't print OK for each successfully verified file
don't output anything, status code shows success
exit non-zero for improperly formatted checksum lines
warn about improperly formatted checksum lines
As you’d agree, the sha1sum utility doesn’t offer many features. Most of its command line options we have already discussed here. Once you’re done practicing these, head to the tool’s man page for more info.