b3sum:tldr:b36af

The command b3sum is a program used for computing and verifying checksums of files. The given command is using the b3sum program with the following options and arguments:

• --check: This option is used to verify the checksums of the specified file.
• --quiet: This option is used to suppress the normal output and only print error messages if any.
• ${filename-b3}: This is a shell parameter expansion syntax used to substitute the value of the variable filename if it exists, and if not, it substitutes it with the string "b3".

Therefore, the command checks the checksum of the file specified by the value of the variable filename, or if the variable is not set, it checks the checksum of a file named "b3". The process is performed silently without printing normal output, only displaying error messages if any.