Calculate or check cryptographic SHA checksums.

# To calculate the SHA1 checksum for a file:
shasum <filename>

# To calculate the SHA256 checksum for a file:
shasum --algorithm 256 <filename>

# To calculate the SHA512 checksum for multiple files:
shasum --algorithm 512 <filename1> <filename2>

# To check a file with a list of sums against the directory's files:
shasum --check <list_file>

# To calculate the SHA1 checksum from 'stdin':
<somecommand> | shasum