The s6-fghack program
s6-fghack is an anti-backgrounding tool.
Interface
s6-fghack prog...
- s6-fghack opens a lot of file descriptors (all writing to a single pipe).
- Then it spawns prog... as a child.
- If something gets written on one of those descriptors, it's a bug in prog. s6-fghack then complains and exits 102.
- Unless prog... goes out of its way to close descriptors it does not know about, s6-fghack is able to detect when prog... exits. It exits with an approximation of the same exit code.
Notes
s6-fghack is what it says: a hack. Ideally, you should never have to use it. It is only useful when you want to supervise a daemon that does not provide a "stay in the foreground" option; and even then, the right thing is to report this as a bug to the daemon author and have it fixed.