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recur

recur is a command-line tool that runs a single command repeatedly until it succeeds or no more attempts are left. It implements optional exponential backoff with configurable jitter. It lets you write the success condition in Starlark.

Installation

Prebuilt binaries

Prebuilt binaries for FreeBSD (amd64), Linux (aarch64, riscv64, x86_64), macOS (arm64, x86_64), NetBSD (amd64), OpenBSD (amd64), and Windows (amd64, x86) are attached to releases.

Go

Install Go, then run the following command:

go install github.com/dbohdan/recur@latest

Build requirements

• Go 1.19
• POSIX Make for testing

Usage

Usage: recur [-a <attempts>] [-b <backoff>] [-c <condition>] [-d <delay>] [-f]
[-j <jitter>] [-m <max-delay>] [-t <timeout>] [-v] <command> [<arg> ...]

Retry a command with exponential backoff and jitter.

Arguments:
  <command>
          Command to run

  [<arg> ...]
          Arguments to the command

Flags:
  -h, --help
          Print this help message and exit

  -V, --version
          Print version number and exit

  -a, --attempts 10
          Maximum number of attempts (negative for infinite)

  -b, --backoff 0
          Base for exponential backoff (duration)

  -c, --condition 'code == 0'
          Success condition (Starlark expression)

  -d, --delay 0
          Constant delay (duration)

  -f, --forever
          Infinite attempts

  -j, --jitter '0,0'
          Additional random delay (maximum duration or 'min,max' duration)

  -m, --max-delay 1h
          Maximum allowed sum of constant delay and exponential backoff
(duration)

  -t, --timeout -1s
          Timeout for each attempt (duration; negative for no timeout)

  -v, --verbose
          Increase verbosity (up to 3 times)

The "duration" arguments take Go duration strings; for example, 0, 100ms, 2.5s, 0.5m, or 1h. The value of -j/--jitter must be either one duration string or two joined with a comma, like 1s,2s.

Setting the delay (-d/--delay) increases the maximum delay (-m/--max-delay) to that value when the maximum delay is shorter. Use -m/--max-delay after -d/--delay if you want a shorter maximum delay.

recur exits with the last command's exit code unless the user overrides this in the condition. When the command is not found during the last attempt, recur exits with the code 255.

recur sets the environment variable RECUR_ATTEMPT for the command it runs to the current attempt number. This way the command can access the attempt counter. recur also sets RECUR_MAX_ATTEMPTS to the value of -a/--attempts.

The following command succeeds on the last attempt:

$ recur sh -c 'echo "Attempt $RECUR_ATTEMPT of $RECUR_MAX_ATTEMPTS"; exit $((RECUR_MAX_ATTEMPTS - RECUR_ATTEMPT))'
Attempt 1 of 10
Attempt 2 of 10
Attempt 3 of 10
Attempt 4 of 10
Attempt 5 of 10
Attempt 6 of 10
Attempt 7 of 10
Attempt 8 of 10
Attempt 9 of 10
Attempt 10 of 10

Conditions

recur supports a limited form of scripting. You can define the success condition using an expression in Starlark, a small scripting language derived from Python. The default condition is code == 0. It means recur will stop retrying when the exit code of the command is zero.

If you know Python, you can quickly start writing recur conditions in Starlark. The most significant differences between Starlark and Python for this purpose are:

• Starlark has no is. You must write code == None, not code is None.
• Starlark has no sets. Write code in (1, 2, 3) or code in [1, 2, 3] rather than code in {1, 2, 3}.

You can use the following variables in the condition expression:

• attempt: int — the number of the current attempt, starting at one. Combine with --forever to use the condition instead of the built-in attempt counter.
• code: int | None — the exit code of the last command. code is None when the command was not found.
• command_found: bool — whether the last command was found.
• max_attempts: int — the value of the option --attempts. --forever sets it to -1.
• time: float — the time the most recent attempt took, in seconds.
• total_time: float — the time between the start of the first attempt and the end of the most recent, again in seconds.

recur defines one custom function:

• exit(code: int | None) -> None — exit with the exit code. If code is None, exit with the exit code for a missing command (255).

This function allows you to override the default behavior of returning the last command's exit code. For example, you can make recur exit with success when the command fails.

recur --condition 'code != 0 and exit(0)' sh -c 'exit 1'
# or
recur --condition 'False if code == 0 else exit(0)' sh -c 'exit 1'

In the following example we stop early and do not retry when the command's exit code indicates incorrect usage or a problem with the installation.

recur --condition 'code == 0 or (code in (1, 2, 3, 4) and exit(code))' curl "$url"

License

MIT.

Alternatives

recur was inspired by retry-cli. I wanted something like retry-cli but without the Node.js dependency.

There are other similar tools:

• eb. Written in Rust. cargo install eb.
• retry (joshdk). Written in Go. go install github.com/joshdk/retry@master.
• retry (kadwanev). Written in Bash.
• retry (minfrin). Written in C. Packaged for Debian and Ubuntu. sudo apt install retry.
• retry (timofurrer). Written in Rust. cargo install retry-cmd.
• retry-cli. Written in JavaScript for Node.js. npx retry-cli.
• SysBox includes the command splay. Written in Go. go install github.com/skx/sysbox@latest.