parallel

CLI tool to execute shell commands in parallel.

Loosely based on GNU parallel command.

Installation

Using npm:

$ npm install -g parallel

Usage

parallel [options] [command [arguments]] < list_of_arguments
parallel [options] [command [arguments]] (::: arguments)...
cat ... | parallel --pipe [options] [command [arguments]]

Options

-j, --jobs <n>          Max processes to run in parallel (0 for ∞) [default CPUs]
-n, --max-args <args>   Number of input lines per command line [default 1]
-X, --xargs             Multiple arguments with context replace
-d, --delimiter <delim> Input items are terminated by delim [default \n]
-0, --null              Use NUL as delimiter
-q, --quote             Quote each input line in case they contain special caracters
--quote-all             Quote each input line in case they contain special caracters (alias for --quote)
--trim                  Removes spaces, tabs and new lines around the input lines
-t, --print-commands    Print the jobs which parallel is running to stderr
-C, --colsep <regex>    Column separator for positional placeholders [default " "]
-a, --arg-file <file>   Use file as input source instead of stdin
-p, --pipe              Spread input lines to jobs via their stdin
--block <size>          Size of each block in --pipe mode (e.g., 1M, 10K)
-D, --dry-run           Print commands to run without running them
--tag                   Prefix each line of output with the argument that generated it
--shuf                  Randomize the order of jobs
-k, --keep-order        Keep same order as input
--joblog <file>         Log job details (start time, runtime, exit code, command) to file
--bg                    Run commands in background and exit
--delay <secs>          Wait before starting new jobs, secs can be less than 1
--timeout <secs>        If the command runs for longer than secs it will get killed with SIGTERM
--halt-on-error         Kill all jobs and exit if any job exits with a code other than 0
-v, --verbose           Print job commands and timing information to stderr
-s, --shell             Wrap command with shell (supports escaped pipes, redirection, etc.) [experimental]
--help                  Print this message and exit
--version               Print the comand version and exit

Placeholders

{}     the input line
{.}    the input line without extension
{/}    the basename of the input line
{//}   the dirname of the input line
{/.}   the basename of the input line without extension
{#}    the sequence number of the job to run, [1,]
{%}    the job slot number [1, --jobs]
{##}   total number of jobs to be run
{..}   the input line without two extensions (e.g., file.tar.gz → file)
{...}  the input line without up to three extensions (e.g., file.tar.gz.backup → file)
{/..}  the basename without two extensions (e.g., path/file.tar.gz → file)
{/...} the basename without three extensions (e.g., path/file.tar.gz.backup → file)
{+/}   the number of "/" in the input line
{+.}   the number of "." in the input line
{+..}  the extensions removed by {..} (e.g., file.tar.gz → .tar.gz)
{+...} the extensions removed by {...} (e.g., file.tar.gz.bak → .tar.gz.bak)
{n}    nth input column, followed by any operator above (f.e {2/.})

Non-GNU placeholders

{ext}  the extension of the input line
{trim} the input line with leading/trailing whitespace removed
{v}    lower case the value
{^}    upper case the value
{t}    current date-time as a number
{T}    current date-time in ISO format
{d}    current date in ISO format
{r}    random number between 100000 and 999999
{md5}  MD5 hash of the input line
{len}  the length of the input line in characters
{wc}   the word count of the input line

Input from command-line arguments

Input can be provided as command-line arguments preceeded by a :::. Each argument will be considered a separate input line. If you include several :::, parallel will use all the permutations between them as input lines.

You can also read arguments from files using :::: followed by filenames. This allows you to combine multiple input sources.

While GNU´s version also permutates stdin and input files, this version won't. You can also combine multiple input files with :::: to create permutations.

Check examples (8), (10), (11), and (12) to see command-line input in action.

Examples

Basic Operations

(1) Download files simultaneously

cat urls.txt | parallel curl -L {} -o downloads/{/}

(2) Convert video files using all CPU cores

parallel ffmpeg -i {} -c:v libx264 converted/{.}.mp4 ::: *.avi

(3) Compress large log files efficiently

find /var/log -name "*.log" -size +100M | parallel gzip {}

Placeholders & File Processing

(4) Demonstrate path manipulation placeholders

echo -e "/home/user/document.pdf\n/tmp/archive.tar.gz" | \
  parallel echo "Full: {} | Dir: {//} | File: {/} | Name: {/.} | Ext: {ext}"

(5) Multi-extension removal (GNU --plus compatibility)

echo -e "project.tar.gz\nfile.min.js.map" | \
  parallel echo "File: {} | Remove 1: {.} | Remove 2: {..} | Remove 3: {...}"

(6) Count characters in paths and filenames

echo -e "/deep/nested/path/file.min.js\nshallow.txt" | \
  parallel echo "File: {} | Slashes: {+/} | Dots: {+.} | Length: {len}"

Column Processing & Data Manipulation

(7) Process CSV data with column placeholders

echo -e "John,28,Engineer\nSarah,32,Designer" | \
  parallel -C ',' echo "Employee: {1} ({2} years old) works as {3}"

(8) Clean whitespace from messy input

printf "  Alice  \n\t  Bob\t\n" | parallel echo "Original: '{}' | Cleaned: '{trim}'"
# Or
printf "  Alice  \n\t  Bob\t\n" | parallel --trim echo Cleaned: {}

(9) Transform text case and count words

echo -e "Hello World\nFOO BAR" | parallel echo "Text: {} | Lower: {v} | Upper: {^} | Words: {wc}"

Job Management & Control

(10) Preserve output order despite varying job times

seq 5 | parallel --keep-order --shell "sleep \$((6 - {})); echo 'Job {} done'"

(11) Limit concurrency and log job details

parallel -j 2 --joblog build.log echo 'Built {}' ::: app1 app2 app3

(12) Tag output lines with their input source

echo -e "google.com\namazon.com" | parallel --tag ping -c 1 {}

Advanced Features

(13) Process large files in manageable chunks

cat huge_dataset.csv | parallel --pipe --block 10M wc -c

(14) Group multiple arguments per command

echo -e "file1\nfile2\nfile3\nfile4" | parallel -X -j 1 echo "Processing batch:"

(15) Randomize execution order for testing

seq 10 | parallel --shuf --dry-run echo 'Processing {}'

(16) Generate combinations using structured input

echo -e "backup:database\narchive:config\nclone:source" | \
  parallel -C ':' echo "Operation {1} on {2}"

(17) Use built-in time and random placeholders

parallel echo 'Job {} at {T} (ID: {r})' ::: task1 task2

Command-line options

Once a command-line parameter that is not an option is found, then the "command" starts. parallel supports command-line options in all these formats (all equivalent):

• --trim --jobs 2
• --trim --jobs=2
• -t -j 2
• -tj 2
• -tj2

Exit code

Just like GNU parallel does, the exit code will be the amount of jobs that failed (up to 101). It means that if any job fails, "global" exit code will be non-zero as well. You can add --halt-on-error to abort as soon as one job fails.

Differences with GNU parallel

✅ GNU Parallel Compatible Features

• Full placeholder compatibility: {..}, {...}, {/..}, {/...}, {+/}, {+.}, etc.
• Standard options: -t/--print-commands, --tag, --joblog, -k/--keep-order, --shuf, --block
• File input: :::: file syntax and -a/--arg-file
• Job control: -X/--xargs, --halt-on-error, -p/--pipe, -D/--dry-run

🔧 Enhanced Features

• Better defaults: Default jobs = CPU count (not unlimited)
• Input flexibility: Supports piped input + ::: arguments together (GNU doesn't)
• Additional placeholders: {ext}, {v}, {^}, {t}, {T}, {d}, {r}, {md5}, {len}, {wc}, {trim}
• Simplified usage: --plus not needed (features auto-enabled)

⚠️ Simplified Behaviors

• --round-robin is implicit when --pipe is used
• --trim only does full trim (no <n|l|r|lr|rl> options)
• --halt-on-error is binary (no complex exit condition options)
• No input permutation between ::: and stdin/--arg-file

License

Copyright (c) 2016, Ariel Flesler All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

• Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

• Neither the name of the organization nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.