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nq: queue utilities

These small utilities allow creating very lightweight job queue systems which require no setup, maintenance, supervision, or any long-running processes.

nq should run on any POSIX.1-2008 compliant system which also provides a working flock(2). Tested on Linux 2.6.37, Linux 4.1, OpenBSD 5.7, FreeBSD 10.1, NetBSD 7.0.2, Mac OS X 10.3 and SmartOS joyent_20160304T005100Z.

The intended purpose is ad-hoc queuing of command lines (e.g., for building several targets of a Makefile, downloading multiple files one at a time, running benchmarks in several configurations, or simply as a glorified nohup). But as any good Unix tool, it can be abused for whatever you like.

Job order is enforced by a timestamp nq gets immediately when started. Synchronization happens on file-system level. Timer resolution is milliseconds. No sub-second file system time stamps are required. Polling is not used. Exclusive execution is maintained strictly.

Enforcing job order works like this:

  • every job has a flock(2)ed output file, ala ,TIMESTAMP.PID
  • every job starts only after all earlier flock(2)ed files are unlocked
  • Why flock(2)? Because it locks the file handle, which is shared across exec(2) with the child process (the actual job), and it will unlock when the file is closed (usually when the job terminates).

You enqueue (get it?) new jobs using nq CMDLINE.... The job ID is output (unless suppressed using -q) and nq detaches immediately, running the job in the background. STDOUT and STDERR are redirected into the log file.

nq tries hard (but does not guarantee) to ensure the log file of the currently running job has +x bit set. Thus you can use ls -F to get a quick overview of the state of your queue.

The “file extension” of the log file is actually the PID, so you can kill jobs easily. Before the job is started, it is the PID of nq, so you can cancel a queued job by killing it as well.

Due to the initial exec line in the log files, you can resubmit a job by executing it as a shell command file (i.e. running sh $jobid).

You can wait for jobs to finish using nq -w, possibly listing job IDs you want to wait for; the default is all of them. Likewise, you can test if there are jobs which need to be waited upon using -t.

By default, job IDs are per-directory, but you can set $NQDIR to put them elsewhere. Creating nq wrappers setting $NQDIR to provide different queues for different purposes is encouraged.

All these operations take worst-case quadratic time in the amount of lock files produced, so you should clean them regularly.


Build targets clean, depends, all, without occupying the terminal:

% nq make clean
% nq make depends
% nq make all
% fq
... look at output, can interrupt with C-c any time
without stopping the build ...

Simple download queue, accessible from multiple terminals:

% mkdir -p /tmp/downloads
% alias qget='NQDIR=/tmp/downloads nq wget'
% alias qwait='NQDIR=/tmp/downloads fq -q'
window1% qget http://mymirror/big1.iso
window2% qget http://mymirror/big2.iso
window3% qget http://mymirror/big3.iso
% qwait
... wait for all downloads to finish ...

As nohup replacement (The benchmark will run in background, every run gets a different output file, and the command line you ran is logged, too!):

% ssh remote
remote% nq ./run-benchmark
remote% ^D
% ssh remote
remote% fq
... see output, fq exits when job finished ...


nq will only work correctly when:

  • $NQDIR (respectively .) is writable.
  • flock(2) works in $NQDIR (respectively .).
  • gettimeofday behaves monotonic (using CLOCK_MONOTONIC would create confusing file names). Else job order can be confused and multiple tasks can run at once due to race conditions.
  • No other programs put files matching ,* into $NQDIR (respectively .).

nq helpers

Two helper programs are provided:

fq outputs the log of the currently running jobs, exiting when the jobs are done. If no job is running, the output of the last job is shown. fq -a shows the output of all jobs, fq -q only shows one line per job. fq uses inotify on Linux and falls back to polling for size change else. (fq.sh is a similar tool, not quite as robust, implemented as shell-script calling tail.)

tq wraps nq and displays the fq output in a new tmux or screen window.

(A pure shell implementation of nq is provided as nq.sh. It needs flock from util-linux, and only has a timer resolution of 1s. Lock files from nq and nq.sh should not be mixed.)


Use make all to build, make install to install relative to PREFIX (/usr/local by default). The DESTDIR convention is respected. You can also just copy the binaries into your PATH.

You can use make check to run a simple test suite, if you have Perl’s prove installed.

Comparison to at, batch, and task-spooler

  • at runs jobs at a given time. batch runs jobs “when system load levels permit”. nq and task-spooler run jobs in sequence with no regard to the system’s load average.

  • at and batch have 52 built-in queues: a-z and A-Z. Any directory can be a queue for nq. task-spooler can have different queues for different terminals.

  • You can follow the output of an nq queue tail-style with fq.

  • The syntax is different: at and batch take whole scripts from the standard input or a file; nq takes a single command as its command line arguments.

  • nq doesn’t rely on a daemon, and uses a directory to manage the queue. task-spooler automatically launches a daemon to manage a queue.

  • task-spooler can set a maximum number of simultaneous jobs.


nq is in the public domain.

To the extent possible under law, Leah Neukirchen leah@vuxu.org has waived all copyright and related or neighboring rights to this work.