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Tim Perry
Tim Perry

Reputation: 13216

Cross-platform method to detect whether /dev/tty is available & functional

I have a bash script from which I want to access /dev/tty, but only when it's available.

When it's not available (in my case: when running my script in GitHub Actions) then when I try to access it I get /dev/tty: No such device or address, and I'm trying to detect that in advance to avoid the error and provide fallback behaviour instead.

To do so I need a bash test that can detect cleanly this case, and which will work reliably across platforms (i.e. not using the tty command, which has issues on Mac).

I'm currently using [[ -e "/dev/tty" ]] which doesn't work - it appears to return true even on GitHub Actions, where it seems that /dev/tty exists but accessing it will fail. What should I use instead?

Upvotes: 9

Views: 4218

Answers (5)

Jack G
Jack G

Reputation: 5322

Instead of spawning a new shell process to test if /dev/tty can really be opened for writing (test -w lies, you know?), you can try to redirect stdout to /dev/tty from a subshell like so:

if (exec < /dev/tty) ; then
  # /dev/tty is available
else
  # no tty is available
fi

This is POSIX syntax and should work in any shell.

Upvotes: 1

Tim Perry
Tim Perry

Reputation: 13216

After testing lots of promising but not quite perfect suggestions (see the other answers), I think I've found my own solution that does exactly fit my needs:

if sh -c ": >/dev/tty" >/dev/null 2>/dev/null; then
    # /dev/tty is available and usable
else
    # /dev/tty is not available
fi

To explain:

: >/dev/tty does nothing (using the : bash built-in) and outputs the nothing to /dev/tty, thereby checking that it exists & it's writable, but not actually producing any visible output. If this succeeds, we're good.

If we do that at the top level without a /dev/tty, bash itself produces a noisy error in our output, complaining about /dev/tty being unusable. This can't be redirected and silenced because it comes from bash itself, not the : command.

Wrapping that with sh -c "..." >/dev/null 2>/dev/null runs the test in a bash subshell, with stdout/stderr removed, and so silences all errors & warnings while still returning the overall exit code.

Suggestions for further improvements welcome. For reference, I'm testing this with setsid <command>, which seems to be a good simulation of the TTY-less environment I'm having trouble with.

Upvotes: 9

ErikMD
ErikMD

Reputation: 14733

Beyond the other answers mentioned in this thread (and as an alternative to the other idea involving $-, which did not seem to work for you), what about this other idea mentioned in the bash manual?

if [ -z "$PS1" ]; then
    echo This shell is not interactive
else
    echo This shell is interactive
fi

Upvotes: 0

Philippe
Philippe

Reputation: 26592

Try this approach :

if  test "$(ps -p "$$" -o tty=)" = "?"; then
    echo "/dev/tty is not available."
else
    echo "/dev/tty is available."
fi

Upvotes: 1

ErikMD
ErikMD

Reputation: 14733

It seems that adapting this answer from this question on ServerFault (entitled How can I check in bash if a shell is running in interactive mode?, which is close to your question albeit not an exact duplicate) could be a solution for your use case.

So, could you try writing either:

  • [ -t 0 ] && [ -t 1 ] && echo your code
  • or [ -t 0 ] && echo your code ?

For completeness, here is one link documenting this POSIX flag -t, which is thus portable:

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html

-t file_descriptor
True if file descriptor number file_descriptor is open and is associated with a terminal.
False if file_descriptor is not a valid file descriptor number, or if file descriptor number file_descriptor is not open, or if it is open but is not associated with a terminal.

Furthermore, if you use bash (not just a POSIX-compliant shell), you might want to combine this idea with the special 255 file descriptor number: [ -t 255 ].

Source: On Unix&Linux-SE,

That 255 file descriptor is an open handle to the controlling tty and is only used when bash is run in interactive mode. […]

− In Bash, what is file descriptor 255 for, can I use it? (by @mosvy)

Upvotes: 1

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