A simple command-line message queue, intended as an IPC utility between shell processes, built on top of sysv message queues and shopify's sysv wrapper.

• Introduction
• Installation
• Usage

Operational work, that is heavily invested in shell scripting, will inevitably run across the need to provide queueing logic, as a means of coordination between parallel processes, whether they be disparate or forks of a common parent. Coordinating "thread" access, to shared resources, using posix supplied mechanisms, is awkward at best:

• FIFO Pipes are best used as a one-in, one-out structures, that require many hoops to support multiple consumer/producer models, that adhere to some form of message order and still provide selective blocking.
• Files require mutex locks, using tools like flock or custom logic, are "user-space" bound (as opposed purely kernel managed) and awareness has to be dedicated to state and clean.
• Local Sockets work as IPC mechanism, but are awkward to fit into a queue paradigm.
• External queues, like redis, obviously require expanding a given stack and thus increased operational complexity.

The mq tool acts as an interface to sysv message queues, which are kernel managed and available on most *nix platforms, including BSD; message order is guarenteed, reads will block on (lack of)availability and *perfomance will exceed "user space" queueing systems, for those of you that care.

*mq is not intended as a performant bus framework

Binary releases are available for Darwin/Linux AMD64 targets, with no plans to provide additional platform support; feel free review the Makefile if you wish to compile for a given platform.

$ ./build/mq -h
Usage:
  mq key [value] [flags]

Flags:
  -c, --count    Number of messages in the queue
  -d, --delete   Delete the queue
  -h, --help     help for mq

* Write to queue asdf.

$ ./build/mq asdf one
$ echo $?
#$ ipcs -qo
IPC status from <running system> as of Thu Apr 25 14:29:50 EDT 2019
T     ID     KEY        MODE       OWNER    GROUP CBYTES  QNUM
Message Queues:
q 393216 0x2a042aa4 --rw-rw---- christiancalloway    staff      3      1

* Write to queue asdf from stdin and read.

$ cat <<eof | ./build/mq asdf
> one
> two
eof
$ ./build/mq asdf
one
$ ./build/mq asdf
two
$ echo $?
0

* Block until message is available.

$ ( sleep 5 && ./build/mq asdf delay ) & gtime -f '%e' ./build/mq asdf
[1] 42090
delay
5.01
[1]+  Done                    ( sleep 5 && ./build/mq asdf delay )

* Get the message count.

$ ./build/mq asdf astring
$ ./build/mq asdf --count
1
$ ./build/mq asdf
astring
$ ./build/mq asdf --count
0

The last two examples should make clear that its up to the implemntor to manage read blocks.