Am I being run interactively?

In one of last month's columns, we looked at a method for determining whether a process was being run by cron. In that column, we used the ptree command, which shows a process' lineage in an indented display like that shown below, to determine whether the "ancester" of the process is the cron process. In this week's column, we'll examine an even simpler way to determine whether a process is being run interactively -- by using the tty command.

% ptree $$
335   /usr/local/sbin/sshd
  2675  /usr/local/sbin/sshd
    2677  /usr/local/sbin/sshd
      2679  -bash
        2682  ptree 2679

The tty command displays the name of the terminal that is open as standard input. In other words, it identifies the terminal device that is associated with your login session. Type the tty command at your system prompt and you will see something like this:

% tty
/dev/pts/2

The who command displays this same information along with some additional data, but displays your terminal in a different and less useful format. In particular, if you are logged in more than once on the same system, the tty command will quickly tell you which tty is associated with each login session; the who command will not.

% who
shs        pts/2        Jan 13 08:22    (10.11.10.11)
shs        pts/3        Jan 13 09:11    (10.11.10.11)

You can use the tty command in a script to easily determine whether the script has a controlling terminal and is, thus, being run interactively. When the tty command is run non-interactively, the output of the command (at least on a Solaris system) is "not a tty". To test this, you can insert a line like this in your crontab file:

10 10 * * * /usr/bin/tty > /export/home/shs/tty-log

Once every day, at 10:10 AM, a line "not a tty" will be added to your log.

A skeletal script for executing different logic depending on the output of the tty command might look like this:

---------------------------- cut here ----------------------------
#!/bin/bash

if `tty >/dev/null 2>&1` ; then
        echo "interactive"
else
        echo "non-interactive"
fi
---------------------------- cut here ----------------------------

The "if `tty >/dev/null 2>&1`" statement is one of those cute little Unix tricks that requires a little explanation. Knowing that the tty command will either yield a string such as "/dev/pts/2" or will issue the phrase "not a tty", we need to determine which response we get to determine whether or not we have a controlling terminal.

=One way we could do this would be to construct an (excuse the pun!) awkward command such as this:

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