who

Basic Usage: Displaying Logged-in Users

At its core, the who command provides a concise list of users currently active on the system. The output typically includes the username, terminal used for login, login time, and sometimes the remote host from which the user connected.

who

This simple command will output something similar to:

user1  pts/0        2023-10-27 10:30 (192.168.1.100)
user2  pts/1        2023-10-27 11:00 (192.168.1.101)

This shows user1 logged in on terminal pts/0 at 10:30 AM from IP 192.168.1.100, and similarly for user2.

Refining Output with Options

The true power of who lies in its options. Let’s look at some key ones:

who -u (Detailed User Information): This option provides a more detailed view of each user’s session, including the process ID (PID) and the time the user became idle.

who -u

Example output:

user1  pts/0        2023-10-27 10:30   0.00.00  192.168.1.100
user2  pts/1        2023-10-27 11:00   1:23:00  192.168.1.101

Notice the addition of idle time.

who -H (Header Information): This adds a header to the output for better readability, specifying the column headings.

who -H

who -a (All Information): This combines the functionalities of many other options, providing a detailed summary. It’s often equivalent to using who -bdH.

who -a

who am i (Your Own Session Information): This displays information specifically about your current login session.

who am i

Combining Options: You can combine multiple options for even more tailored output. For instance, to get a detailed output with a header, use:

who -aH

who -r (Runlevel Information): This displays the current runlevel of the system. This is particularly useful for system administrators monitoring the system state. (Note: The usefulness of this option depends on the specific system and init system used.)

who -r

Beyond the Basics: Using who with Other Commands

The output of who can be piped to other commands for further processing and analysis. For example, you can count the number of currently logged-in users using wc:

who | wc -l

This will provide a numerical count of lines in the output of who, which represents the number of users.

Similarly, you can use grep to search for specific users:

who | grep user1

This will filter the who output to show only lines containing “user1”.

By mastering the who command and its various options, you gain a tool for monitoring and managing your Linux system effectively. Its simplicity belies its power, making it an indispensable command for both beginners and experienced users.