Just Programmer's Manual

Signal Handling

Signals are messages sent to running programs to trigger specific behavior. For example, SIGINT is sent to all processes in the terminal foreground process group when ctrl-c is pressed.

just tries to exit when requested by a signal, but it also tries to avoid leaving behind running child processes, two goals which are somewhat in conflict.

If just exits leaving behind child processes, the user will have no recourse but to ps aux | grep for the children and manually kill them, a tedious endeavor.

Fatal Signals

SIGHUP, SIGINT, and SIGQUIT are generated when the user closes the terminal, types ctrl-c, or types ctrl-\, respectively, and are sent to all processes in the foreground process group.

SIGTERM is the default signal sent by the kill command, and is delivered only to its intended victim.

When a child process is not running, just will exit immediately on receipt of any of the above signals.

When a child process is running, just will wait until it terminates, to avoid leaving it behind.

Additionally, on receipt of SIGTERM, just will forward SIGTERM to any running children^1.41.0, since unlike other fatal signals, SIGTERM, was likely sent to just alone.

Regardless of whether a child process terminates successfully after just receives a fatal signal, just halts execution.

Continuing Execution

The [continue]^1.54.0 attribute can be used to make just continue execution even if it receives a fatal signal as long as the child process it’s running exits successfully.

With no arguments, [continue] handles SIGINT (ctrl-c) so SIGQUIT (ctrl-\) can still be used to stop execution.

With arguments, one or more signals to handle may be given explicitly, as "SIGHUP", "SIGINT", and "SIGQUIT".

In this example, if main.py catches SIGINT and exits successfully, cleanup will still run and just will exit successfully:

[continue]
test: && cleanup
  python3 main.py

cleanup:
  echo cleanup

SIGINFO

SIGINFO is sent to all processes in the foreground process group when the user types ctrl-t on BSD-derived operating systems, including macOS, but not Linux.

just responds by printing a list of all child process IDs and commands^1.41.0.

Windows

On Windows, just behaves as if it had received SIGINT when the user types ctrl-c. Other signals are unsupported.