wait-for-it.sh is a pure bash/ash script that will wait until response from a host and TCP port, then execute a command.

Useful when waitig for a service to start up in a containerized environment, or during deployments, ensuring that dependency services are available before proceeding.

Supports customizable timeouts, silent operation, and strict mode checks, allowing for versatile and robust startup or deployment scripts.

The script is now usable both with Bash and Ash (a fork of Kenneth Almquist's ash shell integrated to BusyBox).

https://en.wikipedia.org/wiki/Almquist_shell

Ash / BusyBox is used in distributions like Alpine Linux, DSLinux, and Linux-based router firmware such as OpenWrt, Tomato and DD-WRT. Alpine Linux is popular for building small container images for Docker or K8s.

Since it is a pure bash/ash script, it does not have any external dependencies on systems with BusyBox, like Alpine Linux, as all commands are provided by BusyBox (through symlinks).

On systems with bash, these external commands are used: basename, date, echo, expr, nc (netcat/ncat), sleep.

 WAIT-FOR.SH bash/ash script by Péter Vámos https://github.com/pvamos/wait-for

   Wait until response from a host and TCP port, then execute a command.

 Usage:
  wait-for.sh host:port [-s] [-t timeout] [-q] [-- command args]
    or
  wait-for.sh -h host -p port [-s] [-t timeout] [-q] [-- command args]

  -h HOST | -h=HOST | --host=HOST | --host HOST   Host or IP under test
  -p PORT | -p=PORT | --port=PORT | --port PORT   TCP port under test
                          Alternatively, specify the host and port as host:port

  -s | --strict               Only execute subcommand if the test succeeds
  -q | --quiet                Don't output any status messages

  -t TIMEOUT | -t=TIMEOUT | --timeout=TIMEOUT | --timeout TIMEOUT
                              Timeout in seconds, zero for no timeout

  -- COMMAND ARGS             Execute command with args after the test finishes

Copyright (c) 2024 Péter Vámos

https://github.com/pvamos [email protected] https://linkedin.com/in/pvamos

This software is based on "wait-for-it" originally created by Giles Hall (copyright (c) 2016, last updated in 2020), published and available at: https://github.com/vishnubob/wait-for-it

This version includes modifications made by Péter Vámos in 2024, published and available at: https://github.com/pvamos/wait-for

Both the original and modified software are distributed under the MIT License, which is included below.

Focused on making it more reliable, user-friendly, and versatile across different Unix-like environments.

The improvements specifically targeted error handling, precise timeout control, efficient network checks, detailed user guidance, and strict adherence to POSIX standards for broader shell compatibility. These make the script suitable for critical automation tasks in modern, containerized infrastructures.

Implemented a precise timeout mechanism using actual elapsed time calculated with date +%s. This method ensures that the script adheres strictly to the user-specified timeout regardless of network delays or system load, which could affect loop iteration speed.

The argument parsing was enhanced to handle both shorthand and long-form arguments robustly, supporting a broader range of input formats. Expanded the usage function to give detailed descriptions of all available options, improving user guidance.

Rewritten for full POSIX compliance, ensuring that the script can run not only in Bash but also in more restricted shells like the Ash / BusyBox fork of Kenneth Almquist's ash shell, which is common in lightweight environments like Docker or Kubernetes containers based on Alpine Linux.

For example, let's test to see if we can access port 80 on www.google.com, and if it is available, echo the message google is up.

$ ./wait-for.sh www.google.com:80 -- echo "google is up"
2024-04-28T11:28:32,315Z wait-for.sh www.google.com:80 - Timeout parameter not specified, defauting to 15 seconds.
2024-04-28T11:28:32,352Z wait-for.sh www.google.com:80 - Response after 0 seconds, executing command: 'echo google is up'
google is up

You can set your own timeout with the -t or --timeout= option. Setting the timeout value to 0 will disable the timeout:

$ ./wait-for.sh -t 0 www.google.com:80 -- echo "google is up"
2024-04-28T11:28:55,048Z wait-for.sh www.google.com:80 - Timeout parameter value is 0, waiting indefinitely.
2024-04-28T11:28:55,079Z wait-for.sh www.google.com:80 - Response after 0 seconds, executing command: 'echo google is up'
google is up

The subcommand will be executed regardless if the service is up or not. If you wish to execute the subcommand only if the service is up, add the --strict or -s argument. In this example, we will test port 81 on www.google.com which will fail:

./wait-for.sh www.google.com:81 --timeout=3 --strict -- echo "google is up"
2024-04-28T11:29:27,304Z wait-for.sh www.google.com:81 - Waiting for 3 seconds.
2024-04-28T11:29:30,335Z wait-for.sh www.google.com:81 - No response in 3 seconds, strict mode, refusing to execute command: 'echo google is up'

If you don't want to execute a subcommand, leave off the -- argument. This way, you can test the exit condition of wait-for-it.sh in your own scripts, and determine how to proceed:

$ ./wait-for.sh www.google.com:80
2024-04-28T11:30:39,702Z wait-for.sh www.google.com:80 - Timeout parameter not specified, defauting to 15 seconds.
2024-04-28T11:30:39,736Z wait-for.sh www.google.com:80 - Response after 0 seconds and no command specified.
$ echo $?
0
$ ./wait-for.sh www.google.com:81
2024-04-28T11:31:45,954Z wait-for.sh www.google.com:81 - Timeout parameter not specified, defauting to 15 seconds.
2024-04-28T11:32:01,134Z wait-for.sh www.google.com:81 - No response in 15 seconds and no command specified.
$ echo $?
1

The silent mode (--quiet or -q) disables all output from the script:

$ ./wait-for.sh -s -t 5 -q www.google.com:81 -- echo "google is up"
$ ./wait-for.sh -s -t 5 -q www.google.com:80 -- echo "google is up"
google is up

MIT License

Copyright (c) 2024 Péter Vámos [email protected] https://github.com/pvamos

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.