Replacement Strings

A replacement string is where an input goes inside your command. {} is the input itself; the others cut it into the pieces file work usually needs. All of these were run for real in transcript t02, over two inputs chosen to show the cuts:

$ parallel echo '{} :: {/} :: {//} :: {.} :: {/.}' ::: samples/north/notes-01.txt archive/backup.tar.gz
samples/north/notes-01.txt :: notes-01.txt :: samples/north :: samples/north/notes-01 :: notes-01
archive/backup.tar.gz :: backup.tar.gz :: archive :: archive/backup.tar :: backup.tar

String	Meaning	samples/north/notes-01.txt becomes
{}	the input, whole	samples/north/notes-01.txt
{/}	basename — directory removed	notes-01.txt
{//}	dirname — only the directory	samples/north
{.}	extension removed	samples/north/notes-01
{/.}	basename, extension removed	notes-01

Two details the table makes visible:

• Only the last extension goes. archive/backup.tar.gz under {.} is archive/backup.tar, not archive/backup. Parallel removes one suffix, which is what you want for .tar.gz files most of the time.
• The pieces compose. Building an output name is usually {/.} for the stem plus a new ending of your own: convert {} {.}.png, or with a redirection, tr a-z A-Z < {} > {/.}.upper.

Positional strings: {1} {2} …

With more than one input source (chapter 7), {1} is the input from the first source, {2} from the second, and the cutting forms follow: {1/} is the basename of the first, {2.} the second without extension.

$ parallel -k echo '{1} goes with {2}: {1/} + {2.}' ::: red blue ::: report.txt data.csv
red goes with report.txt: red + report
red goes with data.csv: red + data
blue goes with report.txt: blue + report
blue goes with data.csv: blue + data

Two more worth knowing

• {#} — the job's sequence number, the same number the job log uses. Handy for naming temporary outputs that must not collide: tmp-{#}.
• {= … =} — a Perl expression evaluated per input, for transformations the five cutters can't do (changing case, arithmetic). It is powerful and easy to abuse; the manual's examples are the place to learn it, and most everyday work never needs it.

The quoting rule

Parallel quotes each replacement for the shell, so a filename containing spaces or an apostrophe arrives at your command intact — one argument, not several. The command template itself is a shell command line, though: pipes, && and redirections you write in it are real shell syntax. That is a feature (chapter 10 uses it) and the source of the classic beginner's bug of putting an unquoted | or > in a template and watching it act on the whole run instead of per job. Chapter 14 has the symptom and the fix.