separate stdout and stderr of system() command

Question

I can catch the output of running a system command via system() or systemlist(), but that actually gives me the result of stdout and stderr combined. As I am only interested in stdout (and may use stderr for other purposes), this isn't useful for me.

How can I separate the output of a system command between stdout and stderr?

Answer 1

Vim's system() indeed captures both standard out and standard error by default. If you just want one of those, just use the normal shell means to redirect the other to the null device.

For example, ignore stderr (2>) with Bash:

:echo system('grep root /etc/passwd /doesnotexist 2>/dev/null')

On Windows, you'd use 2>NUL instead.

Alternatively, you could (temporarily) modify the 'shellredir' setting; e.g. by dropping the 2>&1 from it to ignore stderr. (Thanks @ChristianBrabandt for the tip!)

separate stdout and stderr

If you need both (but separately), that's a bit tricky. You could:

• redirect one / both into a temp file and read that via readfile(), or via another system('cat /tmpfile')
• invoke the command twice, ignoring first stdout and then stderr
• add prefixes (via sed) to one / both (possible with fancy piping and redirections), read all in one fell swoop, and separate inside Vim

Answer 2

The separation can be done using jobs.

vim9script

job_start('grep root /etc/passwd /doesnotexist', {
  out_cb: On_stdout,
  err_cb: On_stderr,
})

def On_stdout(ch: channel, msg: string)
  # do something with msg
enddef

def On_stderr(ch: channel, msg: string)
  # do something with msg
enddef