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User commands and logs

whoami

For convenience, many distributions and users make the current user part of the command prompt – some for every user, others only for the command prompt. However, on systems without such prompts, remembering which account you are logged into can become confusing, such as when you are using su to change accounts while administering or configuring. For such systems, the whoami command (Figure 6) can serve as a general compass. It is used without options – unless you are interested in the version number – and returns only the login name of the current account.

Figure 6: whoami answers its own question and nothing else.

who

The who command is one of the most important commands for gathering information about users and general system settings (Figure 7).

Figure 7: The who command is a versatile tool, not only for gathering information about users but also for general system statistics.

Unmodified by any options, the who command lists the users except root who are currently logged in and the date and time they logged in. Users may be listed multiple times if they have one or more virtual terminals open. However, if you add the -q or --count option, the same information is displayed in a short format that emphasizes the current number of logins. Different options also give basic system information (see Table 1).

Table 1

System Information from the who Command

Short Option	Long Option	Information
`-d`	`--dead`	Lists dead processes
`-b`	`--boot`	Displays the last time the system was booted
`-p`	`--processes`	Prints active processes
`-r`	`--runlevel`	Shows current runlevel
`-t`	`--time`	Gives the last time the system clock was changed

id

When you log in to Linux, you establish your real username and groups. However, you also can use commands like sudo and su to run as a different or effective user. With the id command, you can quickly find both real and effective usernames.

The unmodified id command (Figure 8) gives the real UID for the current account, plus the GIDs for any groups to which the account belongs. However, if you add the -a option, you get the effective UID. Similarly, the -g or --group command displays the real GID, but -G or --groups (note the spelling difference) prints all the group IDs. If you do not have multiple logins, you might not notice any difference between the real and effective user.