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Linux Reference Guide

Basic Troubleshooting Steps Beep Codes
Computer Ports / Connectors  New Computer Security Threats  New
CPU Types and Specifications DVD Formats  New
File Extensions File Size Conversion Table
HTTP Status Messages IBM Diagnostic Codes
Linux Reference Guide  New Maximum Dedicated Bandwidths
Memory Styles and Types Modem AT Commands, S Registers and RS-232C pins
OSI Network Model Partition Size vs. Cluster Size
RAID Levels Explained  New Regular Expressions  New
Shortcut Keys Standard DMA, I/O and IRQ Assignments
TCP and UDP Port Numbers Technology Reference Cards
Windows XP TCP/IP Utilities - Netsh  New Wireless Networking Standards

FILE AND DIRECTORY BASICS This cateogry also includes utilities that change file/directory properties and permissions
ls List files/directories in a directory, comparable to dir in windows/dos.
ls -la Shows all files (including ones that start with a period), directories, and details attributes for each file.
cd Change directory (e.g cd /usr/local/bin)
cd ~ Go to your home directory
cd - Go to the last directory you were in
cd .. Go up a directory
cat Print file contents to the screen
cat filename.txt Print the contents of filename.txt to your screen
tail Similar to cat, but only reads the end of the file
tail /var/log/messages See the last 20 (by default) lines of /var/log/messages
tail -f /var/log/messages Watch the file continuously, while it's being updated
tail -200 /var/log/messages Print the last 200 lines of the file to the screen
head Similar to tail, but only reads the top of the file
head /var/log/messages See the first 20 (by default) lines of /var/log/messages
head -200 /var/log/messages Print the first 200 lines of the file to the screen
more Llike cat, but opens the file one screen at a time rather than all at once
more /etc/userdomains Browse through the userdomains file. hit Spaceto go to the next page, q to quit
less Page through files
od View binary files and data
xxd Also view binary files and data
gv View Postscript/PDF files
xdvi View TeX DVI files
nl Number lines
touch Create an empty file
touch /home/burst/public_html/404.html Create an empty file called 404.html in the directory /home/burst/public_html/
file Attempts to guess what type of file a file is by looking at it's content.
file * Prints out a list of all files/directories in a directory
cp Copy a file
cp filename filename.bak Copies filename to filename.bak
cp -a /etc/* /root/etc/ Copies all files, retaining permissions form one directory to another.
cp -av * ../newdirectory Copies all files and directories recurrsively in the current directory INTO newdirectory
mv Move a file command
mv oldfilename newfilename Move a file or directory from oldfilename to newfilename
rm delete a file
rm filename.txt deletes filename.txt, will more than likely ask if you really want to delete it
rm -f filename.txt deletes filename.txt, will not ask for confirmation before deleting.
rm -rf tmp/ recursively deletes the directory tmp, and all files in it, including subdirectories.

changes file access permissions. The set of 3 go in this order from left to right:

0 = --- No permission
1 = --X Execute only
2 = -W- Write only
3 = -WX Write and execute
4 = R-- Read only
5 = R-X Read and execute
6 = RW- Read and write
7 = RWX Read, write and execute

chmod 000 No one can access
chmod 644 Usually for HTML pages
chmod 755 Usually for CGI scripts
chown Changes file ownership permissions
The set of 2 go in this order from left to right:
chown root myfile.txt Changes the owner of the file to root
chown root.root myfile.txt Changes the owner and group of the file to root
stat Display file attributes
grep Llooks for patterns in files
grep root /etc/passwd Shows all matches of root in /etc/passwd
grep -v root /etc/passwd Shows all lines that do not match root
ln Create's "links" between files and directories
ln -s /usr/local/apache/conf/httpd.conf /etc/httpd.conf Now you can edit /etc/httpd.conf rather than the original. changes will affect the orginal, however you can delete the link and it will not delete the original.
wc Word count
wc -l filename.txt Tells how many lines are in filename.txt
find Utility to find files and directories on your server.
find / -name "filename" Find the file called "filename" on your filesystem starting the search from the root directory "/".
locate filename Find the file name and path of which contains the string "filename". Run 'updatedb' to build index.
EDITORS Most popular editors available on UNIX platforms.
pico Friendly, easy to use file editor
pico /home/burst/public_html/index.html Edit the index page for the user's website.
vi Popular editor, tons of features, harder to use at first than pico
vi filename.txt

Edit filename.txt. All commands in vi are preceded by pressing the escape key. Each time a different command is to be entered, the escape key needs to be used. Except where indicated, vi is case sensitive. Fore more commands go to:

H --- Upper left corner (home)
M --- Middle line
L --- Lower left corner
h --- Back a character
j --- Down a line
k --- Up a line
^ --- Beginning of line
$ --- End of line
l --- Forward a character
w --- Forward one word
b --- Back one word
fc --- Find c
; --- Repeat find (find next c)

:q! --- This force quits the file without saving and exits vi
:w --- This writes the file to disk, saves it
:wq --- This saves the file to disk and exists vi
:LINENUMBER : EG :25 --- Takes you to line 25 within the file
:$ --- Takes you to the last line of the file
:0 --- Takes you to the first line of the file


Another popular editor. For more commands go to

C-\ t --- Tutorial suggested for new emacs users.
C-x C-c exit emacs

emacs filename.txt

Edit filename.txt. While you're in emacs, use the following quickies to get around:

C-x C-f --- read a file into emacs
C-x C-s --- save a file back to disk
C-x i --- insert contents of another file into this buffer
C-x C-v --- replace this file with the contents of file you want
C-x C-w --- write buffer to specified file

C-f --- move forward one character
C-b --- move backward one character
C-n --- move to next line
C-p --- move to previous line
C-a --- move to beginning of line
C-e --- move to end of line
M-f --- move forward one word
M-b --- move backword one word
C-v --- move forward one screen
M-v --- move backward one screen
M-< --- go to beginning of file
M-> --- go to end of file

NETWORK Some of the basic networking utilities.
w Shows who is currently logged in and where they are logged in from.
who This also shows who is on the server in an shell.
netstat Shows all current network connections.
netstat -an Shows all connections to the server, the source and destination ips and ports.
netstat -rn Shows routing table for all ips bound to the server.
netstat -an |grep :80 |wc -l Show how many active connections there are to apache (httpd runs on port 80)

Shows live system processes in a formatted table, memory information, uptime and other useful info.

While in top, Shift + M to sort by memory usage or Shift + P to sort by CPU usage

top -u root Show processes running by user root only.
route -n Shows routing table for all ips bound to the server.
nslookup Query your default domain name server (DNS) for an Internet name (or IP number) host_to_find.
traceroute Have a look how you messages travel to
ifconfig Display info on the network interfaces.
ifconfig -a Display into on all network interfaces on server, active or inactive..
ping Sends test packets to a specified server to check if it is responding properly
tcpdump Print all the network traffic going through the network.
arp Command mostly used for checking existing Ethernet connectivity and IP address
SYSTEM TOOLS Many of the basic system utilities used to get things done.
ps ps is short for process status, which is similar to the top command. It's used to show currently running processes and their PID.
A process ID is a unique number that identifies a process, with that you can kill or terminate a running program on your server (see kill command).
ps U username Shows processes for a certain user
ps aux Shows all system processes
ps aux --forest Shows all system processes like the above but organizes in a hierarchy that's very useful!
kill terminate a system process
kill -9 PID Immediately kill process ID
killall program_name Kill program(s) by name. For example to kill instances of httpd, do 'killall httpd'
du Shows disk usage.
du -sh Shows a summary of total disk space used in the current directory, including subdirectories.
du / -bh | more Print detailed disk usage for each subdirectory starting at the "/".
last Shows who logged in and when
last -20 Shows only the last 20 logins
last -20 -a Shows last 20 logins, with the hostname in the last field
pwd Print working directory, i.e., display the name of my current directory on the screen.
hostname Print the name of the local host. Use netconf (as root) to change the name of the machine.
whoami Print my login name.
date Print or change the operating system date and time
time Determine the amount of time that it takes for a process to complete + other info.
uptime Show the number days server has been up including system load averages.
uname -a Displays info on about your server such as kernel version.
free Memory info (in kilobytes).
lsmod Show the kernel modules currently loaded. Run as root.
dmesg | less Print kernel messages.
man topic Display the contents of the system manual pages (help) on the topic. Do 'man netstat' to find all details of netstat command including options and examples.
reboot / halt Halt or reboot the machine.
mount Mount local drive or remote file system.
mount -t auto /dev/fd0 /mnt/floppy Mount the floppy. The directory /mnt/floppy must exist.
mount -t auto /dev/cdrom /mnt/cdrom Mount the CD. The directory /mnt/cdrom must exist.
sudo The super-user do command that allows you to run specific commands that require root access.
fsck Check a disk for errors
COMPRESSION UTILITIES There are many other compression utilities but these are the default and most widely utilized.
tar Creating and Extracting .tar.gz and .tar files
tar -zxvf file.tar.gz Extracts the file
tar -xvf file.tar Extracts the file
tar -cf archive.tar contents/ Takes everything from contents/ and puts it into archive.tar
gzip -d filename.gz gzip -d filename.gz
zip Compress files
unzip Extracting .zip files shell command
compress Compress files. compress filename
uncompress Uncompress compressed files. uncompress filename.Z
bzip2 Compress files in bzip2 format
THE (DOT) FILES The good old dot files. Let's clear up some confusion here by defining each.
.bash_login Treated by bash like .bash_profileif that doesn't exist.
.bash_logout Sourced by bash login shells at exit.
.bash_profile Sourced by bash login shells after /etc/profile
.bash_history The list of commands executed previously.
.profile Treated by bash like ~/.bash_profile if that and .bash_login don't exist.
.vimrc Default "Vim" configuration file.
.emacs Read by emacs at startup
CONFIGURATION FILES Listing everything is beyond the scope of this article.
/etc This directory contains most of the basic Linux system-configuration Files.
/etc/init.d Contains the permanent copies of System V–style run-level scripts. These scripts are often linked to files in the /etc/rc?.d directories to have each service associated with a script started or stopped for the particular run level. The ? is replaced by the run-level number (0 through 6). (Slackware puts its run-level scripts in the /etc/rc.d directory.)
/etc/cron* Directories in this set contain files that define how the crond utility runs applications on a daily (cron.daily), hourly (cron.hourly), monthly (cron.monthly), or weekly (cron.weekly) schedule.
/etc/cups Contains files used to configure the CUPS printing service.
/etc/default Contains files that set default values for various utilities. For example, the file for the useradd command defines the default group number, home directory, password expiration date, shell, and skeleton directory
/etc/skel Any files contained in this directory are automatically copied to a user’s home directory when that user is added to the system.
/etc/mail Contains files used to configure your sendmail mail service.
/etc/security Contains files that set a variety of default security conditions for your computer.
/etc/sysconfig Contains important system configuration files that are created and maintained by various services (including iptables, samba, and most networking services).
/etc/passwd Holds some user account info including passwords (when not "shadowed").
/etc/shadow Contains the encrypted password information for users' accounts and optionally the password aging information.
/etc/xinetd.d Contains a set of files, each of which defines a network service that the xinetd daemon listens for on a particular port.
/etc/syslogd.conf The configuration file for the syslogd daemon. syslogd is the daemon that takes care of logging (writing to disk) messages coming from other programs to the system.
/var Contains variable data like system logging files, mail and printer spool directories, and transient and temporary files.
/var/log Log files from the system and various programs/services, especially login (/var/log/wtmp, which logs all logins and logouts into the system) and syslog (/var/log/messages, where all kernel and system program message are usually stored).
/var/log/messages System logs. The first place you should look at if your system is in trouble.
/var/log/utmp Active user sessions. This is a data file and as such it can not be viewed normally.
/var/log/wtmp Log of all users who have logged into and out of the system. The last command can be used to access a human readable form of this file.
Apache Shell Commands Some of the basic and helpful apache commands.
httpd -v Outputs the build date and version of the Apache server.
httpd -l Lists compiled in Apache modules
httpd status Only works if mod_status is enabled and shows a page of active connections
service httpd restart Restarted Apache web server
MySQL Shell Commands Some of the basic and helpful MySQL commands.
mysqladmin processlist Shows active mysql connections and queries
mysqladmin processlist |wc -l Show how many current open connections there are to mysql
mysqladmin drop database Drops/deletes the selected database
mysqladmin create database Creates a mysql database
mysql -u username -p password databasename < data.sql Restores a MySQL database from data.sql
mysqldump -u username -p password database > data.sql Backup MySQL database to data.sql
echo "show databases" | mysql -u root -p password|grep -v Database Show all databases in MySQL server.
mysqldump -u root -p password database > /tmp/database.exp Dump database including all data and structure into /tmp/database.exp tries to supply accurate and up-to-date information but it is possible that this page may contain typographical errors or technical inaccuracies. Any errors will be periodically corrected as they are made known to us. If any such errors are found, we would appreciate if you would report them here.
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