sha1sum:tldr:f7a9a

The command sha1sum --ignore-missing --check --quiet ${filename-sha1} performs a checksum verification for a file using the SHA-1 algorithm, while also utilizing some additional options.

Here is an explanation of each component of the command:

1. sha1sum: It is the command used to calculate the SHA-1 hash value of a file. The hash value is a fixed-size unique alphanumeric representation of the file's content.

2. --ignore-missing: This option instructs the command to ignore missing files. If the filename specified in ${filename-sha1} does not exist, it will not report an error.

3. --check: This option is used to verify the checksum of one or more files. Essentially, it compares the hash values provided in ${filename-sha1} with the calculated SHA-1 hashes of the corresponding files, and produces output indicating the verification results.

4. --quiet: This option causes the command to suppress non-error output. It will only display error messages or the result of the verification, making the output more concise.

5. ${filename-sha1}: This is a placeholder for the name of a file that contains a list of SHA-1 hash values. Each line in the file should follow the format <SHA-1 hash> <file name>. The command uses this file to compare the actual hash values of the files against the expected values, performing the checksum verification.

In summary, this command reads a file with SHA-1 hashes (${filename-sha1}), checks if the corresponding files exist, calculates the SHA-1 hash of each file, and then compares it against the expected hash value. It outputs the verification result, disregarding any missing files and producing minimal output with the --quiet option.