Command

Summary Use

arp

Command mostly used for checking existing Ethernet connectivity and IP address

Most common use: arp

This command should be used in conjunction with the ifconfig and route commands. It is mostly useful for me to check a network card and get the IP address quick. Obviously there are many more parameters, but I am trying to share the basics of server administration, not the whole book of commands.

df

Display filesystem information

Most common use: df -h

Great way to keep tabs on how much hard disk space you have on each mounted file system.

du

Display usage

Most common use, under a specific directory: du -a

Easily and quickly identify the size of files/programs in certain directories. A word of caution is that you should not run this command from the / directory. It will actually display size for every file on the entire Linux harddisk.

find

Find locations of files/directories quickly across entire filesystem

Most common use: find / -name appname -type d -xdev

(replace the word appname with the name of a file or application like gimp)

This is a very powerful command and is best used when running as root or superuser. The danger is that you will potentially look across every single file on every filesystem, so the syntax is very important. The example shown allows you to search against all directories below / for the appname found in directories but only on the existing filesystem. It may sound complex but the example shown allows you to find a program you may need within seconds!

Other uses and more complex but beneficial functions include using the -exec or execute a command.
You may also try the commands: locate or try slocate

ifconfig

Command line tool to configure or check all network cards/interfaces

Most common uses: ifconfig and also ifconfig eth0 10.1.1.1

Using the plain ifconfig command will show you the details of all the already configured network cards or interfaces. This is a great way to get a check that your network hardware is working properly. You may also benefit from this review of server configuration. Using the many other options of ifconfig such as the one listed allows you to assign a particular interface a static IP address. I only show an example and not a real world command above. Your best bet, if you want to configure your network card using this command is to first read the manual pages. You access them by typing: man ifconfig

init

Allows you to change the server bootup on a specific runlevel

Most common use: init 5

This is a useful command, when for instance a servers fails to identify video type, and ends up dropping to the non-graphical boot-up mode (also called runlevel 3).

The server runlevels rely on scripts to basically start up a server with specific processes and tools upon bootup. Runlevel 5 is the default graphical runlevel for Linux servers. But sometimes you get stuck in a different mode and need to force a level. For those rare cases, the init command is a simple way to force the mode without having to edit the inittab file.

Of course, this command does not fix the underlying problem, it just provides a fast way to change levels as needed. For a more permanent correction to the runlevel, edit your /etc/inittab file to state: id:5:initdefault:

joe or nano

Easy to use command line editors that are often included with the major Linux flavors

Most common uses:
joe filename
nano filename

A real world example for you to get a better sense on how this works:
nano /etc/dhcp3/dhcpd.conf
This allows you to edit using nano the dhcpd.conf configuration file from the command line.

Maybe you are not up to speed on vi, or never learned how to use emacs? On most Linux flavors the text editor named joe or one named nano are available. These basic but easy to use editors are useful for those who need a text editor on the command line but don't know vi or emacs. Although, I do highly recommend that you learn and use Vi and Emacs editors as well. Regardless, you will need to use a command line editor from time to time. You can also use cat and more commands to list contents of files, but this is basic stuff found under the basic linux commands listing. Try: more filename to list contents of the filename.

netstat

Summary of network connections and status of sockets

Most common uses: netstat and also netstat |head and also netstat -r

Netstat command simply displays all sockets and server connections. The top few lines are usually most helpful regarding webserver administration. Therefore if you are doing basic webserver work, you can quickly read the top lines of the netstat output by including the |head (pipe and head commands). Using the -r option gives you a very good look at the network routing addresses. This is directly linked to the route command.

nslookup

Checks the domain name and IP information of a server

Most common use: nslookup www.hostname.com

You are bound to need this command for one reason or another. When performing server installation and configuration this command gives you the existing root server IP and DNS information and can also provide details from other remote servers. Therefore, it is also a very useful security command where you can lookup DNS information regarding a particular host IP that you may see showing up on your server access logs. There is a lot more to this command and using the man pages will get you the details by typing: man nslookup

ping

Sends test packets to a specified server to check if it is responding properly

Most common use: ping 10.0.0.0 (replace the 10.0.0.0 with a true IP address)

This is an extremely useful command that is necessary to test network connectivity and response of servers. It creates a series of test packets of data that are then bounced to the server and back giving an indication whether the server is operating properly.

It is the first line of testing if a network failure occurs. If ping works but for instance FTP does not, then chances are that the server is configured correctly, but the FTP daemon or service is not. However, if even ping does not work there is a more significant server connectivity issue… like maybe the wires are not connected or the server is turned off! The outcome of this command is pretty much one of two things. Either it works, or you get the message destination host unreachable. It is a very fast way to check even remote servers.

ps

Lists all existing processes on the server

Most common uses: ps and also ps -A |more

The simple command will list every process associated with the specific user running on the server. This is helpful in case you run into problems and need to for instance kill a particular process that is stuck in memory. On the other hand, as a system administrator, I tend to use the -A with the |more option. This will list every process running on the server one screen at a time. I use it to quickly check what others are goofing with on my servers and often find that I'm the one doing the dangerous goofing!

rm

Removes/deletes directories and files

Most common use: rm -r name (replace name with your file or directory name)

The -r option forces the command to also apply to each subdirectory within the directory. For instance if you are trying to delete the entire contents of the directory x which includes directories y and z this command will do it in one quick process. That is much more useful than trying to use the rmdir command after deleting files! Instead use the rm -r command and you will save time and effort. You may already have known this but since server administrators end up spending a lot of time making and deleting I included this tip!

route

Lists the routing tables for your server

Most common use: route -v

This is pretty much the exact same output as the command netstat -r. You can suit yourself which you prefer to run. I tend to type netstat commands a lot more than just route and so it applies less to my situation, but who knows, maybe you are going to love and use route the most!

shred

Deletes a file securely by overwriting its contents

Most common use: shred -v filename (replace filename with your specific file)

The -v option is useful since it provides extra view of what exactly the shred tool is doing while you wait. On especially BIG files this could take a bit of time. The result is that your file is so thoroughly deleted it is very unlikely to ever be retrieved again. This is especially useful when trying to zap important server related files that may include confidential information like user names or hidden processes. It is also useful for deleting those hundreds of love notes you get from some of the users on your server, another bonus of being a server administrator. :)

sudo

The super-user do command that allows you to run specific commands that require root access.

Most common use: sudo command (replace command with your specific one)

This command is useful when you are logged into a server and attempt a command that requires super-user or root privileges. In most cases, you can simply run the command through sudo, without having to log in as root. In fact, this is a very beneficial way to administer your server without daily use of the root login, which is potentially dangerous.

Below is a simple example of the sudo capabilities:
sudo cd /root
This command allows you to change directories to the /root without having to login as root. Note that you must enter the root password once, when running a sudo command.

top

Displays many system statistics and details regarding active processes

Most common use: top

This is a very useful system administrator tool that basically gives you a summary view of the system including number of users, memory usage, CPU usage, and active processes. Often during the course of a day when running multiple servers, one of my Xwindows workstations just displays the top command from each of the servers as a very quick check of their status and stability.

touch

Allows you to change the timestamp on a file.

Most common use: touch filename

Using the basic touch command, as above, will simply force the current date and time upon the specified file. This is helpful, but not often used.

However, another option that I've used in the past when administering servers, is to force a specific timestamp on a set of files in a directory.

For instance, to force a specific date and time upon all files in a directory, type:
touch *

You can also force a specific date/time stamp using the -t option like this: touch -t200103041200.00 *
The command above will change all files in the current directory to take on the new date of March 4th, 2001 at noon. The syntax follows this pattern: YYYYMMDDhhmm.ss

YYYY represents the four digit year, then the two digit month, day, hour and minutes. You can even specify seconds as noted above. In any case, this is a useful way to control timestamps on any files on your server.

traceroute

Traces the existing network routing for a remote or local server

Most common use: traceroute hostname

(replace hostname with the name of your server such as reallylinux.com)

This is a very powerful network command that basically gives the exact route between your machine and a server. In some cases you can actually watch the network hops from country to country across an ocean, through data centers, etc.

This comes in handy when trying to fix a network problem, such as when someone on the network can not get access to your server while others can. This can help identify the break or error along the network line. One strong note to you is not to misuse this command! When you run the traceroute everyone of those systems you see listed also sees YOU doing the traceroute and therefore as a matter of etiquette and respect this command should be used when necessary not for entertainment purposes. A key characteristic of gainfully employed server administrators: knowing when to use commands and when not to use them!

w

An extension of the who command that displays details of all users currently on the server

Most common uses: w

This is a very important system admin tool I use commonly to track who is on the server and what processes they are running. It is obviously most useful when run as a superuser.

The default setting for the w command is to show the long list of process details. You can also run the command w -s to review a shorter process listing, which is helpful when you have a lot of users on the server doing a lot of things! Remember that this is different than the who command that can only display users not their processes.

who

Tool used to monitor who is on the system and many other server related characteristics

Most common uses: who and also who -q and also who -b

The plain command just lists the names of users currently on the server. Using the -q option allows you to quickly view just the total number of users on the system. Using the -b option reminds you how long it has been since you rebooted that stable Linux server! One of my servers had a -b of almost three years! Yes, that's really Linux!