If you have two operating system eg. Windows Me, Dos 6.22, Linux. And are not able to use LILO or ntldr, then you can use any third- party bootloader software eg.XOSL.

The easiest way to create a user is by using useradd, here login name is the name which is used by the user. To do this login as root.

The easiest way to change password is by using command passwd. If you want to change password for the current login id, then use passwd and press enter. It will ask your new password. But if you are changing password of another user then use passwd login name.To do this login as root.

To switch between different login ids use Alt + F1, Alt + F2, Alt + F3, Alt + F4 by this you can log as 4 different users. And if you want ot login as superuser in between then use command su and enter password. And to exit from super user id use exit.

If you are looking for help, there are lot of documentation provided, use command man . man stands for manual.

If you want to check the amount of free disk space on your hard disk, use command df df stands for disk free. If you want to check space taken by particular directory use du /directory. Use different parameters for other options like using du -s filename will check a space taken by a file.

Linux maintains a database of all files on the system that can be quickly searched using locate xyz command, here xyz is the filename you want to find. To locate files faster than run utility updatedb. Run this utility once a week

If you have lost you root password, you can still login. When the LILO boot prompt, type linux single where you usually type linux. Now you can use passwd utility for new password. (See for utility password in List of Questions in Linux section)

You can check for disk errors by using utility fsck, FileSystem Check. Lost clusters are stored in /lost+found directory.

If you want to increase Swap Space but do not want to repartition or reinstall Linux then follow steps
by using command dd create a file of 64 MB.

dd if=/dev/zero of=/FileName bs=1024 count=65536.. By this command you create a file with file name FileName.

Initialize it
mkswap /FileName 65536 sync

Now add this space
swapon /FileName

The drawback of this method is that you are swapping this file every time you start your computer.

When the LILO boot prompt, type linux mem=xxxM,
here xxx=size of your RAM.

If you are a new user of Linux and have worked in Norton Commander of DOS then try working on Midnight Commander. It will give you the same feeling of Norton Commander. Command mc .

Assuming that you have already remove linux partition from hard disk and want to remove LILO boot, boot from DOS disk and run fdisk /mbr, this will restore your boot record.

If you use LILO boot and want to change default boot sequence between linux and dos (assuming the name given to other operating system is Dos and your default boot item is linux) use command lilo -D dos.To do this login as root.

If you want to configure your sound card use utility sndconfig.

This error comes when hard disk size is too big. Solve this problem by making / boot partition in first 8 GB of hard disk.

Copy the utility dosutils in your hard disk which you can access from Windows. These dosutils comes with mostly all distribution of linux. You can find dosutils in Linux CD. In dosutils folder you will find a batch file. Edit it .

loadlin drive:\boot.img image folder root=/dev/hdxylinux boot partition ro read only.

To mount a file system, use command mount -a [-rw] [-t vfstype]
-a Mounts all file systems in /etc/fstab.
-r mounts the file system in Read only mode.
-w mounts the file system in Read \ Write mode.
-t fstype indicate the file system type.

To check the services running, run the utility chkconfig. For gui users, use ntsysv.Eg.
chkconfig servicename off , use to switch off the service
chkconfig servicename start , use to start the service
chkconfig servicename restart , use to restart the service
chkconfig -status -all , use to check the status of all servies
.

Linux comes with its own firewall system iptables. It is a 2.4 kernel feature, which was earlier known as ipchains.Eg.
iptables -L.

If you are getting problem in ssh autnetication try, editingfile /etc/ssh/sshd_config and set PasswordAuthentication to yes.

Run following command on console: cat/etc/redhat-release

Type date command on console, system will display date used by the system.

Use vi filename to open a file in normal mode. To add text click i (insert mode) which will allow user to modify files. To exit insert mode click Esc. To exit from vi press:q and click enter to exit from editing mode.

If you want to set date in linux run date MMDDhhmm to set current system date/time. egdate 07280517Note: You need to login as root user.

scp is simplest command line tool to copy files over ssh.Format for scp:-scp filename user@destinationip:destination file eg.scp first.txt admin@111.111.11.11:/tmp

Type pwd on console to display current working directory

To create new directory use mkdir. To remove use rmdir or rm -r eg.mkdir test creates test directory and rmdir test or rm -r test removes test directory.

Command mv is used to rename a file eg.mv oldfile newfile will rename oldfile to newfile.

For cancatenation of files in linux,command cat is used which is multipurpose, let us discuss one by one:i) Displaying contents: cat filename displays contents of a file ii) Creating new file: cat >filename will create new file filename,click Ctrl-D to exit from entry mode iii)Combining text files: cat file1 file2 >file3 will combine file1 and file2 to file3 iv) Delete contents of file: cat /dev/null >filename will delete contents from filename.

Command cp is used to copy files eg.cp oldfile newfile will copy existing oldfile to newfile.

Command touch is used to create empty file.eg.touch filename creates empty file called filename.

Command chmod is been used to change access permissions.Format: chmod [options] [Mode] file, one can give options as: -f,--silent,--quiet to supress error messages,-v, --verbose for output a diagnostic for every file processed, -c, --changes like verbose but report only when a change is made and to assign permissions use following numeric modes: the octal (0-7) value is calculated by adding values for each digit read+write+execute = 4+2+1 = 7, read+execute = 4+1 = 5 and read+write = 4+2 = 6 eg. Read permission can be set as chmod 444 file, read, write and execute can be set as chmod 777 file.

Command chown is been used to change owner of a file. Format: chown [-R] newusername file -R changes permission on files that are in subdirectories of directory that you are currently in. eg. chown user1 filename.txt - This command gives permissions as owner to user user1.

tail is the command used to display last few lines or to monitor a file. Format: tail -f filename -f is the option with the help of which you will have a file continuously appending new lines which is useful while debugging log files. If user wants only 10 lines to be displayed then following command must be used tail -n 10 filename. Similarly any desired number of lines can be displayed.

Use rm -rf foldername command to remove folder and its contents recursively.rm command will remove or delete everything in the current directory without any warning so be sure before firing this command.

Type who command on console to display users currently logged in to your linux machine. who am i command displays current user id and name logged in.

Type find / -name filename -print command on console to search location of a file. This command will search files in root directory of file system. If user wants to search files in etc directory then / will be replaced by /etc in command.User has to login as root to search

Type uname -a command on console to know kernel version, linux distribution, cpu information and more . If user wants only kernel version as output then switch is replaced by uname -r

Type top command on console to list most CPU-intensive tasks on the system

Type uptime command on console, output is one line containing following information:
how long system has been running,
how many users are currently logged in and
system load averages for past 1,5 and 15 minutes.

Run command history to display last 500 commands executed previously. To display last n comannds run history n eg. history 10, to check last 10 commands.

Run command free, it gives used and free memory usage on screen along with useful information.

The dmesg command is used to write the kernel messages in Linux. It displays messages from /var/log relative to the most recent boot. Syntax dmesg.

To display a calendar, run command cal. Syntax Cal [month] [year]. eg:
cal 10 2008 - would display calendar for October 2008.

Disk Usage report the amount of disk space used by the specified files and for each subdirectory. Syntax du [options]... [file] .... A few most frequently use arguments
du -a (all files)
du -k (in KB)
du -h (Human readable format)
du -s (Displays total, not details)

To execute a program peridically and see its output, use command watch. Syntax
watch [options] command command_options. Few options:
-n: Specify an interval to run command
-d: Highlight the differences between successive updates.

Type hostname command on console to know host name of the computer they are logged into.

Type traceroute machinename_oripcommand on console to know route of a packet. One can also use tracepath machinename_orip command which is similar to traceroute but this command does not take complicated options. This command is very useful when there are network or router problems.

Using netstat command one can know status of various ports. Options available are:
eg. netstat [-a] [-n] [-v]
-a - Displays State of all sockets and routing table entries.
-n - Displays network addresses as number otherwise are displayed as symbols.
-v - Verbose to display added information for sockets and routing table
-m - Displays STREAMS statistics
-p -Display the address resolution (ARP) tables.
-i -Display state of the interfaces that are used for TCP/IP traffic
-r - Display routing tables
-d -Display state of all interfaces that are under Dynamic Host Configuration Protocol (DHCP) control
-D - Show the status of DHCP configured interfaces
-P protocol - Limit display of statistics or state of all sockets to those applicable to protocol.
-l interface - Display the state of a particular interface. interface can be any valid interface such as ie0 or le0.

kill pid command is used to kill a process. pid is the process id.
To know the process id run ps command on console, all running processes will be listed.

Type command more filename to display file on terminal one screenfull at a time. Command less can also be used for this purpose. Less is a program similar to more but allows backward movement in the file as well as forward movement. Also, less does not have to read the entire input file before starting, so with large input files it starts up faster than text editors like vi.

Commands for installing, uninstalling, upgrading, querying, listing and checking RPM packages on Linux system:
i) Install a RPM package with following command: rpm -i packagefile.rpm
ii) Uninstall RPM package with following command: rpm -e packagefile.rpm
iii) Upgrade RPM package with following command: rpm -Uvh packagefile.rpm
iv) Querying all installed package: rpm -qa
v) Querying a RPM package with following command: rpm -q packagefilename
vi) To display package information use following command: rpm -qi packagefilename
vii) To list files in installed package use follwoing command: rpm -ql packagefilename
viii) Determine which installed package a particular file belongs to with following command: rpm -qf /usr/bin/filename
ix) List files in uninstalled RPM file with following command: rpm -qpl packagefile.rpm
x) Check RPM signature package to ensure its integrity and origin with following command: rpm --checksig packagefilename

By using following command user can convert and copy files in linux dd if=report of=document conv=ebcdic, ucase . Above command converts the input file (if) report from ascii to ebcdic and while converting map alphabetic characters in uppercase and copy them to output file (of) called document. Other options available with dd command are:
i) count = n copy only n input records
ii) conv = ascii convert ebcdic to ascii
iii) conv = lcase convert alphabets to lower case
iv) conv = noerror continue processing if error encountered
v) skip = n skip n input records before starting copy
vi) bs = bytes.

Type finger username command followed by name of user account to get information about the user or type finger and press enter to see who is on the system and what are they doing.

Following command grep MemTotal /proc/meminfo will display total RAM seen by the system.

Following command grep model name /proc/cpuinfo will display CPU information present in the system.

Use following command to list mounted file systems in Linux mount | column -t .

Use command last reboot to know when your linux system got rebooted last time.

To modify a user account, use command usermod.
syntax: usermod [-c comment] [-d home_dir [ -m]] [-e expire_date] [-f inactive_time] [-g initial_group] [-G group[,...]] [-l login_name] [-p passwd] [-s shell] [-u uid [ -o]] [-L|-U] login

Edit inetd.conf file in /etc folder and comment ftp. This will not allow connection between a FTP client to another server.

Use following command to sync your hardware clock with the system time in Linux hwclock --systohc. Use hwclock --show command to read hardware clock and print the result.

To start, stop or restart the firewall in linux, run following commands:
start: service iptables start
stop: service iptables stop
restart: service iptables restart

To check the status of firewall, run command service iptables status. It will display the message with the firewall status eg. Firewall is stopped.