Overview of 'who'
`who` command shows the currently active login sessions based on the information recorded in the system's `/var/run/utmp` (or `/var/log/wtmp`) file. This file stores information such as user login and logout records, system boot/shutdown times. `who` is essential for determining who is currently logged into the system, especially in a multi-user environment.
Main Roles of 'who'
`who` command is primarily used for the following purposes:
Main Use Cases
- Check Logged-in Users: Displays the names, terminals, and login times of all users currently logged into the system.
- System Monitoring: Monitors for unnecessary or abnormal logins on the server.
- Collaborative Environment: Checks if other users are logged into the system to facilitate collaboration or avoid conflicts.
- Script Automation: Used in logic to check if a specific user is logged in via the `who` command.
`who` vs `w` vs `whoami`
There are several commands related to user information, each serving different purposes.
- `who`: Displays information about 'all' users currently logged into the system.
- `w`: Shows detailed information about what each logged-in user is 'doing' (currently running processes) along with their login information. Provides more information than `who`.
- `whoami`: Displays the effective username of 'yourself' executing the current command. (i.e., the logged-in username)
Main Options for the 'who' Command
`who` command can be used without options by default, but it offers useful options to add specific information or change the output format.
1. Basic Information Output
2. Additional Information Output
3. Concise Output
Generated command:
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Description:
`who` Executes the command.
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Usage Examples
Learn how to effectively check the information of logged-in users through various usage examples of the `who` command.
Check Current Logged-in User List
whoThe most basic execution of the `who` command, showing the names, terminals, and login times of all users logged into the system.
Display Column Headers with Logged-in User Information
who -HIncludes headers for each piece of information in the output results (USER, LINE, WHEN, etc.) to enhance readability.
Check Boot Time of the System
who -bOutputs the exact date and time the system was last booted.
Check Logged-in User Count Briefly
who -qDisplays only the total number of logged-in users and the list of usernames currently logged in briefly.
Check Logged-in Users and Idle Time, PID
who -uAdditionally displays the idle time (inactive time) of each logged-in user and the process ID (PID) controlling the login session.
Check Current Run Level
who -rChecks the current 'run level' of the system (multi-user mode, single-user mode, etc.).