Linux system administration commands

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halt

This command shuts down the operating system, but can only be run by the root user.

#halt

reboot

This command shuts down and restarts the operating system. It also can only be run by root.

#reboot           [will perform simple reboot]

#reboot -f        [will perform fast reboot ]

init 0

This command also shuts down the operating system, and can only be run by root.

#init 0

init 6

This command restart the operating system. It also can only be run by root.

#init 6

man

This command opens the manual page for the command or utility specified. The man utility is a very useful tool. If you are unsure how to use any command, use man to access its manual page. For example, you could enter man ls at the shell prompt to learn how to use the ls utility.

#man ls

info

The info utility also displays a help page for the indicated command or utility. The information displayed with info command will be in-depth than that displayed in the man page for the same command.

info ls

su

This command switches the current user to a new user account. For example, if you’re logged in as vickey and need to change to user account to vinita, you can enter su vinita at the shell prompt. This command is most frequently used to switch to the superuser root account. 
In fact, if you don’t supply a username, this utility assumes that you want to change to the root account. If you enter su -, then you will switch to the root user account and have all of root’s environment variables applied.
This command require password of the user you want switch.

Looking for Files

There are two basic commands used for file searches: find and locate

find

The find command searches through directories and subdirectories for a desired file. For example, if you wanted to find the directory with thegrub.conf linux boot loader file, you could use the following command, which would start the search in the top-level root (/) directory:

# find / -name grub.conf

But this search took several minutes to get it task done. Alternatively, if you know that this file is located in the /etc subdirectory tree, or /boot/grub/grub.conf you could start in that directory with the following command:

# find /etc -name grub.conf

locate

If this is all too time-consuming, RHEL 5 includes a default database of all files and directories. Searches with the locate command are almost instantaneous. And locate searches don't require the full file name. The drawback is that the locate command database is normally updated only once each day, as documented in the /etc/cron.daily/mlocate.cron script.

Getting into the Files

Now that you see how to find and get around different files, it's time to start reading, copying, and moving the files around. Most Linux configuration files are text files. Linux editors are text editors. Linux commands are designed to read text files. If in doubt, you can check the file types in the current directory with the 
file * command.

cat

The most basic command for reading files is cat. The cat filename command scrolls the text within the filename file. It also works with multiple file names; it concatenates the file names that you might list as one continuous output to your screen. You can redirect the output to the file name of your choice.

more and less

Larger files demand a command that can help you scroll through the file text at your leisure. Linux has two of these commands: 
more and less.
With the more filename command, you can scroll through the text of a file, from start to finish, one screen at a time. With the less filename command, you can scroll in both directions through the same text with the PAGE UP and PAGE DOWN keys. Both commands support vi-style searches.

head and tail

The head and tail commands are separate commands that work in essentially the same way. By default, the head filename command looks at the first 10 lines of a file; the tail filename command looks at the last 10 lines of a file. You can specify the number of lines shown with the -nxswitch. Just remember to avoid the space when specifying the number of lines; for example, the

# tail -n15 /etc/passwd

command lists the last 15 lines of the /etc/passwd file.

cp

The cp (copy) command allows you to take the contents of one file and place a copy with the same or different name in the directory of your choice. For example, the cp file1 file2 command takes the contents of file1 and saves the contents in file2. One of the dangers of cp is that it can easily overwrite files in different directories, without prompting you to make sure that's what you really wanted to do.

mv

While you can't rename a file in Linux, you can move it. The mv command essentially puts a different label on a file. For example, the mv file1 file2 command changes the name of file1 to file2. Unless you're moving the file to a different partition, everything about the file, including the inode number, remains the same.

ln

You can create a linked file. 
linked files are common with device files such as /dev/dvdwriter and /dev/par0. They're also useful for making sure that multiple users have a copy of the same file in their directories. Hard links include a copy of the file. As long as the hard link is made within the same partition, the inode numbers are identical. You could delete a hard-linked file in one directory, and it would still exist in the other directory. For example, the following command creates a hard link from the actual Samba configuration file to smb.conf in the local directory:

# ln smb.conf /etc/samba/smb.conf

On the other hand, a soft link serves as a redirect; when you open up a file created with a soft link, you're directed to the original file. If you delete the original file, the file is lost. While the soft link is still there, it has nowhere to go. The following command is an example of how you can create a soft link:

# ln -s smb.conf /etc/samba/smb.conf

sort

You can sort the contents of a file in a number of ways. By default, the sort command sorts the contents in alphabetical order depending on the first letter in each line. For example, the sort /etc/passwd command would sort all users (including those associated with specific services and such) by username.

grep and egrep

The grep command uses a search term to look through a file. It returns the full line that contains the search term. For example, grep 'vickey' /etc/passwd looks for my name in the /etc/passwd file.

The egrep command is more forgiving; it allows you to use some unusual characters in your search, including +, ?, |, (, and). While it's possible to set up grep to search for these characters with the help of the backslash, the command can be awkward to use.

wc

The wc command, short for word count, can return the number of lines, words, and characters in a file. The wc options are straightforward: for example, wc -w filename returns the number of words in that file.

sed

The sed command, short for stream editor, allows you to search for and change specified words or even text streams in a file. For example, the following command changes the first instance of the word Windows to the word Linux in each line of the file data, and writes the result to the file newdata:

# sed 's/Windows/Linux/' data > newdata

However, this may not be enough. If a line contains more than one instance of Windows, the above sed command does not change the second instance of that word. But you can make it change every appearance of Windows by adding a "global" suffix:

# sed 's/Windows/Linux/g' data > newdata

awk

The awk command, named for its developers (Aho, Weinberger, and Kernighan), is more of a database manipulation utility. It can identify lines with a keyword and read out the text from a specified column in that line. Again, using the /etc/passwd file, for example, the following command will read out the username of every user with a vickey in the comment column:

# awk '/vickey/ {print $1}' /etc/passwd

ps

It's important to know what's running on your Linux computer. The ps command has a number of critical switches. When trying to diagnose a problem, it's common to get the fullest possible list of running processes, and then look for a specific program. For example, if the Firefox Web browser were to suddenly crash, you'd want to kill any associated processes. The ps aux | grep firefox command could then help you identify the process(es) that you need to kill.

who and w

If you want to know what users are currently logged into your system, use the who command or the w command. This can help you identify the usernames of those who are logged in, their terminal connections, their times of login, and the processes that they are running.

Wildcards

Sometimes you may not know the exact name of the file or the exact search term. This is when a wildcard is handy. The basic wildcards are shown

Wildcard	Description
*	Any number of alphanumeric characters (or no characters at all). For example, the ls ab* command would return the following file names, assuming they exist in the current directory: ab, abc, abcd.
?	One single alphanumeric character. For example, the ls ab? command would return the following file names, assuming they exist in the current directory: abc, abd, abe
[ ]	A range of options. For example, the ls ab[123] command would return the following file names, assuming they exist in the current directory: ab1, ab2, ab3. Alternatively, the ls ab[X-Z] command would return the following file names, assuming they exist in the current directory: abX, abY, abZ.

env

This command displays the environment variables for the currently logged-in user.

echo

This command is used to echo a line of text on the screen. It’s frequently used to display environment variables. For example, if you wanted to see the current value of the PATH variable, you could enter 
echo $PATH

top

This command is a very useful command that displays a list of all applications and processes currently running on the system. You can sort them by CPU usage, memory usage, process ID number, and which user owns them

which

This command is used to display the full path to a shell command or utility. For example, if you wanted to know the full path to the ls command, you would enter
which ls

whoami

This command displays the username of the currently logged-in user.

netstat

This command displays the status of the network, including current connections, routing tables, etc

route

This command is used to view or manipulate the system’s routing table.

ifconfig

This command is used to manage network boards installed in the system. It can be used to display or modify your network board configuration parameters. This command can only be run by the root user.

Do practice and practice until you feel comfortable with these commands.