preserve: run a command and replace its output only if it has changed

[des]

Summary

Introduce a new utility, preserve, which can be used to wrap any command that produces a single output file to ensure that the file is untouched unless the output has genuinely changed.

[jrtc27]

usr.bin/preserve/preserve.c

86

I would expect a vflag-guarded message here

196

This loses the fact that the child was signalled.

212

Why exit rather than return?

[des]

usr.bin/preserve/preserve.c

196

Kind of. The exit status will still be 128 + signal. Getting it 100% right is depressingly difficult.

212

Why return rather than exit?

[des]

usr.bin/preserve/preserve.c

86

Good catch. Note that this shouldn't occur though, unless we lost a race.

[des]

Incorporate feedback.

[des]

Try to deal with signals.

[guest-patmaddox]

I have wanted a utility that does exactly this. install -C does something similar, but has other stuff I don’t want. So far I just end up scripting cmp(1) to do the check.

Would it make sense to add a -o flag to cmp(1) to provide this behavior there? It would write file1 to file2 if the contents differ.

[des]

In D39327#895486, @guest-patmaddox wrote:

Would it make sense to add a -o flag to cmp(1) to provide this behavior there? It would write file1 to file2 if the contents differ.

I don't think so. It wouldn't let you do make TOOL="preserve tool" like this does.