Input sources
GNU parallel reads input from input sources. These can be files, the command line, and stdin (standard input or a pipe).
A single input source
Input can be read from the command line:
parallel echo ::: A B C
Output (the order may be different because the jobs are run in parallel):
A
B
C
The input source can be a file:
parallel -a abc-file echo
Output: Same as above.
STDIN (standard input) can be the input source:
cat abc-file | parallel echo
Output: Same as above.
Multiple input sources
GNU parallel can take multiple input sources given on the command line. GNU parallel then generates all combinations of the input sources:
parallel echo ::: A B C ::: D E F
Output (the order may be different):
A D
A E
A F
B D
B E
B F
C D
C E
C F
Source: GNU Parallel tutorial