The command

The command can be a script, a binary or a Bash function if the function is exported using export -f:

If the command is complex, it often improves readability to make it into a function.

The replacement strings

GNU parallel has some replacement strings to make it easier to refer to the input read from the input sources.

If the input is mydir/mysubdir/myfile.myext then:

When a job is started it gets a sequence number that starts at 1 and increases by 1 for each new job. The job also gets assigned a slot number. This number is from 1 to the number of jobs running in parallel. It is unique between the running jobs, but is re-used as soon as a job finishes.

The positional replacement strings

The replacement strings have corresponding positional replacement strings. If the value from the 3rd input source is mydir/mysubdir/myfile.myext:

So the number of the input source is simply prepended inside the {}'s.