FTR, on modern-ish glibc-powered systems (in code that actually does use libc, and does not do its very own syscall-related thing instead), you will not find a single call to open(2) issued, in my experience. That's because the library functions shadowing these syscalls were rewired to use openat(2) under the hood.

    $ strace -e trace=open cat /dev/null 
    +++ exited with 0 +++


    $ strace -e trace=openat cat /dev/null 
    openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
    openat(AT_FDCWD, "/usr/lib/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
    openat(AT_FDCWD, "/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
    openat(AT_FDCWD, "/dev/null", O_RDONLY) = 3
    +++ exited with 0 +++

If you want to catch both `open` and `openat`, the opensnoop BPF[1] program is pretty nifty, especially if you are trying to figure out file stuff across several different programs ("which #$%^-ing program keeps modifying this file", for example).

[1] I've been dipping my toes into BPF recently, and while complicated (best to simply clone the bpftools repo and work off of that) there's a lot that can be done that tools like strace won't be able to match.

Ok, but then you will still need to parse the output to get the filenames. That's ok, but since it is something that is used a lot, you'd expect a flag.

Check out strace -y and strace -yy (--decode-fds)

You still need to pull out the paths?

A sprinkling of grep/perl (awk/sed/ruby/...) is mostly good enough eg:

strace -e trace=%file cat /etc/passwd 2>&1 >/dev/null | grep ^open | grep -Po '(?<=").*(?=")'

Is your example situation really all that common?

If so, what format do you expect for the output?

If it's one filename-per-line then how do you encode filenames with embedded newlines?

How do you encode non-UTF8 characters, or is the file meant to be parsed only in binary mode?

I don't know of any generally agreed upon spec for this, so no matter what you think is right, most people are going to have to write a special-purpose parser.

In which case you might as well parse the native strace output since one is about as complex as the other.

It can use the same format as the Unix find utility. This utility has a -print0 flag to separate filenames by NUL characters instead of newlines if desired.

That is a good point.

I still don't see it as common use case.