Linux Commands

a2enmod

Enable/disable or view modules for apache server.

acpi

Shows battery and thermal information. Try with -t for thermal.

adduser/addgroup

Add a user or group to the system
For example "adduser reza" adds user reza to the system, "addgroup admin" adds group admin to the system, "adduser reza admin" adds user reza to admin group.

Add users to the end of /etc/sudoers to give them sudo privileges. That file is so flexible for example you can add root capability to a group, etc.
See usermod.
In CentOS sudo is not installed by default and you should install it first using yum install sudo.

alias

For example alias lh="ls -l -a $HOME" makes the command "lh" become equivalent to the longer command "lh -l -a $HOME". You can use it in bash profile and assign a simple alias to a long and frequently used command.

apt

apt-cache search clone	Searches the packages indexes for the clone name
apt-cache show sun-java6-bin	see info about that package
apt-get autoremove	removes packages that were automatically installed to satisfy dependencies for some package and that are no more needed
apt-get remove the-package	removes the-package
apt-get —purge remove gnome-panel	remove the-package with all its configuration files, etc.
apt-get —purge autoremove the-package	removes the-package with all its package dependencies and config files.
apt-get update	Updates the package indexes
apt-get upgrade	Installs the newest packages

bg

Shows the last stopped command using Ctrl-Z. See fg.

cal

Display a calendar

cat

Display the contents of a file
cat file_name | grep —color some_text

chkconfig

Updates and queries runlevel information for system services.
Adding to it: chkconfig —add the_script. Notice that the_script should be chkconfig friendly. See a section in linux-how-to

chgrp

Command	Desc
chgrp user file	Makes file belong to the group user

chmod

chmod [references][operator][modes] file1 …

Reference: u (user), g (group), o (others), a (all)
Operator: +, -, =
Mode: r, w, x, X, s, t

Example:

Command	Desc
chmod ug+rw sample	Adds the read and write permissions to the user and group for that file
chmod +x ss	Anyone can execute the file ss
chmod og=r archive.sh	others and group have read access

chown

Command	Desc
chown cliff file	Makes cliff the owner of file
chown -R cliff dir	Makes cliff the owner of dir and everything in its directory tree

cp

Copy files and directories. To copy a dir with content use "cp -r source dest" or "cp -r * dest" copies everything in current directory to dest.

crontab

"crontab -l" show all the cron jobs for the current user.

date

Shows date and time

df

Display disk space information

env

Environment variables

export

• export shows some env stuff
• export CASROOT=/usr/local/CAS3.0 Defines the environment variable CASROOT
• export PATH=$PATH:$HOME/bin I am setting the PATH environment variable to the previous value of itself, followed by a colon, and a directory-name formed from the HOME variable and "/bin"

echo

echo $CASROOT Prints out the value of CASROOT, or /usr/local/CAS3.0

fg

Goes to the last stopped command using Ctrl-Z. See bg.

find

Searches for files by name, etc.

Command	Desc
find search_path -name filename	Normal usage
find . -name aaa.txt	Finds all the files named 'aaa.txt' in the current directory or any subdirectory tree
find . -name *.sql	accepts wildcards
find / -name vimrc	Find all the files named 'vimrc' anywhere on the system
find /usr/local/games -name "*xpilot*"	Find all files whose names contain the string 'xpilot' which exist within the '/usr/local/games' directory tree
find / -name 'mypage.htm'	search for any file named mypage.htm on the root and all subdirectories from the root
find /backups/ -type f -mtime +5 -exec rm {} \;	Finds are the files in that directory older than 5 days and deletes them!

If you get "Permission denied", do it with sudo.
You can combine find and grep. As an example: find . -name build.xml | grep java

ftp

Command	Desc
ftp	goes into ftp shell
open	open www.xxx.com to open the remote host using port 22
cd	change directory after login
get	get a file and copy into your local folder
put	upload a file into the host
binary or ascii	To transfer a file in binary or ascii mode. Some ftp clients go back to ascii mode when you enter an ls or dir command. To be safe, always use the binary command before getting a binary file.

gksudo

Is a front end for sudo which sucks.

grep

Is a very powerful search tool! Can search the content of files or output of another command.

Options	Desc	Example
-r	Resursive	grep -r "reza" . OR in SunOS grep searchText *.*
-d recurse	same as -r
-n	line number
—color	add color

groupmod

modify groups.

groups

Returns the user's groups. See whoami and id.

kill

kill <PID> kills a process . if it does not work then use kill -9 <PID>

last

shows information about the last logged in users, etc

ls

Option	Desc
-l	List showing date, owner,…
-a	Shows hiddens as well
-lt	List sort by time

When you do ls -l you will get:

type and permission	no of links	owner	group	size	date	name
drwxrwxrwx	4	reza	team	122	Dec 12 18:02	xys

lsmod

Show status of modules loaded into kernel. It resides in /sbin

lsof

List of open files and processes that opened them. It includes disk files, ports, etc.
For the ports do:

sudo /usr/local/bin/lsof -i :80

lspci

Lists all pci hardware devices connected

ln

ln -s /x/file.ext : makes a symbolic link in the current dir to /x/file.ext

mount

mount by force when you get mount errors: sudo mount -t ntfs-3g /dev/sdb1 /media/Network -o force
The better way is to connect the device, restart the system and everything should be good.

mv

Rename or move files.

netstat

Networking information
netstat -an show which ports are listening, etc.

nmap

This utility is for network exploration or security auditing.

nmap -p 80 localhost	tells the status of port 80
sudo nmap -sT -O myip	all open ports on myip

pargs

Examines a process and prints related information such as arguments and environment variables, etc.
For example pargs<PID> is useful with ptree and ps.

ptree

Shows processes in a tree format. It is usually used as ptree <PID>.

ps

view processes

Command	Desc
ps -ef | grep 'jboss4/bin'	Filters and shows (e)very process in a (f)ull list.

reboot

"sudo reboot" reboots the system.

rm

rm removes files. To remove directories use -fr option.

scp

Remote copy: scp the_file reza@the_ip:/home/reza/. This will ask for the password in the remote machine.

shutdown

To shutdown or restart the system.

shutdown -P now	shutdown now
shutdown -P +30	shutdown in 30 min
shutdown -P 22:10	shutdown at a specified time

sudo

Run commands as another user.
sudo -u jboss x.sh runs the file as a jboss user. When used without -u then it is root.

tar

Command	Desc
tar xvf archive.tar	Extract from the archive file
tar cvfz archive.tar.gz SOURCE	Create a gzip compressed tar archive containing everything in the directory 'SOURCE'. You can create archives with gzip,… as well.

tail

tail -f myfile.txt -n 100 : list the last 100 lines in the file myfile.txt. and stays there to follow

top

Shows a real time view of processes, memory usage, load, CPU state, etc.

uname

Useful information about kernel, version, distribution, machine, ….

usermod

modify a user account. Can be used to rename users. If you rename a user don't forget to change these: /etc/passwd, /etc/group, /etc/shadow and home directory.

usermod -a -G root reza : adds user reza to roots users.

users

Prints the users currently logged in to the system.

view

view filename = vi or vim

vi commands

Command	Desc
i	Enter text entry mode (Esc to exit)
ZZ	Write (if there were changes), then quit
:wq	Write, then quit
:q	Quit (will only work if file has not been changed)
:q!	Quit without saving changes to file
x	Delete a character (in command mode)
dd	Delete a line (in command mode)
Esc	leave insert (text entry) mode and return to command mode
yy	Copy (yank) the current line
R	Overwrite text, press <Esc> to end
6yy	Copy (yank) six lines, beginning with the current line
yw	Copy the current word
p	Put the text after the cursor position
P	Put the text before the cursor position
/text	Search forward (down) for text. Can be used when we have / in the search text! Press n or shift+n for next and previous.
?text	Search backward (up) for text
:set nu	Display line numbers
:set nonu	Hide line numbers
gg	Goto start of the file
G	Goto end of the file

which

Shows the physical location of a command for example "which ls" tell where the "ls" is located in the disk.
yum install wget
yum update

yum

yum - Yellowdog Updater Modified, is the equivalent of apt-get for RH/CentOS.

Key combinations

• Ctrl-C: Usually sends the kill signal, which terminates a process.
• Ctrl-Z: Sends the stop signal, which stops process execution immediately, without terminating. Use “fg” or “bg” after that.
• Ctrl-L: Clears the screen and works like clear command.

who

shows who is logged in.

xev

Shows a window (in X server) and sends all X server events to it. It is good for debugging keys, mouse, etc.

Other References

A reference by Perpetual PC's

File Access and Permissions

There are three types of access:

1. read (r)
2. write (w)
3. execute (x)

There are three types of files:

1. ordinary file (-)
2. directory (d)
3. link (l)

When a file listing (ls) is done, there are 10 characters that are shown on the left that indicate type and permissions of the file:

-rw-r--r-- 1 jboss jboss  2481 Feb 25 12:43 run.conf

• The first character indicates the type of file
• The next three indicate read, write, and execute permission for user, group and other.

1	2	3	4	5	6	7	8	9	10
File Type	user r	user w	user x	group r	group w	group x	others r	others w	others x