In: Categories » Computers and technology » Linux Commands » Lunux who command who is logged in
On systems that allow users to log in to other users’ machines or special servers, you will want to know who is logged in. You can generate such a report by using the who command:
[michael@workbox michael]$ who
The who report looks like this:
michael tty1 Jun 14 18:22
rwhite pts/9 Jun 14 18:29 (:0)
root pts/11 Jun 14 21:12 (:0)
root pts/12 Jun 14 23:38 (:0)
A Variation on who: w
The w command displays the same information that who does and a whole lot more. The details of the report include who is logged in, what their terminal is, where they are logged in from, how long they’ve been logged in, how long they’ve been idle, and their CPU utilization. The top of the report also gives you the same output as the uptime command.
Networking Tools
If you are starting your experience with Linux in an already networked environment, you may find some of these tools handy for getting around.
CAUTION
These networking tools are useful, but allowing telnet, rsh, remote X, and ftp connections to a server can be risky from a security standpoint. See Linux tutorial 9 for more information on securing a Linux server.
telnet
As was mentioned in earlier modules, Linux understands the concept of multiple users logged in to the same workstation at the same time. So in order to allow for someone to log in to another host from remote, the telnet command was created. The format of the command is as follows:
[rwhite@workbox rwhite]$ telnet gregory
where gregory is the name of another computer running either Linux or a variant of UNIX. Once you run the command, you will get a login prompt from that machine. Logging in to that machine enables you to run programs on the other host just as if you were running them on your own. If you aren’t sure where you are logged in, use the uname command (discussed earlier) to find out where you are. Due to security concerns, telnet has fallen out of favor as a method for logging in to remote machines (see Linux tutorial 15 for a better alternative). However, because of its ability to send and receive ASCII text over a network port, telnet is still very useful as a diagnostic tool. You’ll see some examples of this in later modules.
Remote Shell: rsh
As you’ve seen with telnet, you must enter a username and password in order to log in to another host. For many people who found the need to log in to other hosts often, this became a nuisance. They wanted the other host to automatically trust them so that when issued a remote login request, the system would allow them access without asking for a login and password. The double-win with a feature like that is the ability to automate tasks that require one host to invoke a program on another host. To accommodate this, the rsh command was created. In its simplest form, specify the name of the host you wish to log in to the way you would for telnet. For example:
[rwhite@workbox rwhite]$ rsh gregory
If you set up your configuration files so that gregory knows to trust you, the system will automatically give you a shell prompt. If the configuration file does not automatically trust you, you will need to enter your password again. To set up the configuration file, edit the .rhosts file (yes, there is a period in front of the filename) in your home directory. When there, create a line that looks like this:
host_to_trust your_username
If you want to be able to rsh from workbox to gregory without being prompted for a password, you would create the following file in an .rhosts file in your home directory on gregory: workbox rwhite And you’d be ready to go! Once again, rsh is covered here for completeness, but before you decide to use it for anything serious, I recommend that you explore ssh in Linux tutorial 15.
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