Does enabling huponexit ensure background jobs receive SIGHUP on shell exit, or does disown override it?

Direct Answer

Enabling huponexit ensures background jobs receive SIGHUP on shell exit only if they remain in the shell's job table. The disown builtin removes a job from that table entirely, making it immune to huponexit regardless of the option's state. A migration script cannot rely on the default huponexit state (which is off in Bash 3.x–5.x) to deliver SIGHUP; explicit set -o huponexit is required, and any prior disown on the target job nullifies the effect.

How huponexit Works

When huponexit is enabled (set -o huponexit or set -huponexit), an interactive login shell sends SIGHUP to every job still tracked in its job table upon exit. This includes stopped jobs and background jobs started with & that have not been disowned. The option is disabled by default in all Bash versions 3.0 through 5.2+, so background jobs receive no SIGHUP on shell exit unless the script explicitly enables it.

How disown Overrides huponexit

disown (with or without -h) removes the specified job from the shell's job table. Once removed, the job is no longer a "shell job" and the shell has no record of it at exit. Consequently, huponexit has no target to signal. This behavior is absolute and consistent across Bash 3.x, 4.x, and 5.x. Even disown -h (which only marks the job to suppress SIGHUP from the shell) keeps the job in the table but prevents SIGHUP delivery; a plain disown removes it entirely.

Version Consistency

The interaction between huponexit and disown has not changed materially between Bash 3.0 (2004) and 5.2 (2022). Both builtins operate on the same job-table data structure. Scripts written for this mechanism will behave identically across that range, provided they run in an interactive login shell (or a shell with huponexit enabled in a context where it takes effect). Non-interactive shells ignore huponexit entirely.

Verification Steps

Confirm behavior in your target Bash version:

1. Test huponexit delivery (no disown):

   bash -c 'set -o huponexit; trap "echo SIGHUP received" HUP; sleep 10 & exit'

   The sleep process should receive SIGHUP and terminate when the shell exits.

2. Test disown suppression:

   bash -c 'set -o huponexit; sleep 10 & disown; exit'

   The sleep process should continue running after shell exit (verify with ps).

3. Inspect job table before exit:

   jobs -l

   Only jobs listed here are subject to huponexit.

Practical Recommendation for Migration Scripts

For a zero-downtime restart using exec:

• Do not disown the child process you intend to signal.
• Explicitly run set -o huponexit in the parent shell before launching the child.
• Trap SIGHUP/SIGTERM in the child to perform cleanup, then exec the new binary.
• Test the full sequence in a staging environment matching your production Bash version.

If the child must survive shell exit without SIGHUP, use disown (or start it via nohup/setsid) and do not enable huponexit.