hi,
here i have listed some of useful commands for linux on the commandline

alias -Create an alias
awk -Find and Replace text, database sort/validate/index
break -Exit from a loop
builtin -Run a shell builtin

cal -Display a calendar
case -Conditionally perform a command
cat -Display the contents of a file
cd -Change Directory
cfdisk -Partition table manipulator for Linux
chgrp -Change group ownership
chmod -Change access permissions
chown -Change file owner and group
chroot -Run a command with a different root directory
cksum -Print CRC checksum and byte counts
clear -Clear terminal screen
cmp -Compare two files
comm -Compare two sorted files line by line
command -Run a command - ignoring shell functions
continue -Resume the next iteration of a loop
cp -Copy one or more files to another location
cron -Daemon to execute scheduled commands
crontab -Schedule a command to run at a later time
csplit -Split a file into context-determined pieces
cut -Divide a file into several parts

date -Display or change the date & time
dc -Desk Calculator
dd -Data Dump - Convert and copy a file
declare -Declare variables and give them attributes
df -Display free disk space
diff -Display the differences between two files
diff3 -Show differences among three files
dir -Briefly list directory contents
dircolors -Colour setup for `ls'
dirname -Convert a full pathname to just a path
dirs -Display list of remembered directories
du -Estimate file space usage

echo -Display message on screen
ed -A line-oriented text editor (edlin)
egrep -Search file(s) for lines that match an extended expression
eject -Eject CD-ROM
enable -Enable and disable builtin shell commands
env -Display, set, or remove environment variables
eval -Evaluate several commands/arguments
exec -Execute a command
exit -Exit the shell
expand -Convert tabs to spaces
export -Set an environment variable
expr -Evaluate expressions

factor -Print prime factors
false -Do nothing, unsuccessfully
fdformat -Low-level format a floppy disk
fdisk -Partition table manipulator for Linux
fgrep -Search file(s) for lines that match a fixed string
find -Search for files that meet a desired criteria
fmt -Reformat paragraph text
fold -Wrap text to fit a specified width.
for -Expand words, and execute commands
format -Format disks or tapes
free -Display memory usage
fsck -File system consistency check and repair
function -Define Function Macros

gawk -Find and Replace text within file(s)
getopts -Parse positional parameters
grep -Search file(s) for lines that match a given pattern
groups -Print group names a user is in
gzip -Compress or decompress named file(s)

hash -Remember the full pathname of a name argument
head -Output the first part of file(s)
history -Command History
hostname -Print or set system name

id -Print user and group id's
if -Conditionally perform a command
import -Capture an X server screen and save the image to file
info -Help info
install -Copy files and set attributes

join -Join lines on a common field

kill -Stop a process from running

less -Display output one screen at a time
let -Perform arithmetic on shell variables
ln -Make links between files
local -Create variables
locate -Find files
logname -Print current login name
logout -Exit a login shell
lpc -Line printer control program
lpr -Off line print
lprint -Print a file
lprintd -Abort a print job
lprintq -List the print queue
lprm -Remove jobs from the print queue
ls -List information about file(s)

m4 -Macro processor
man -Help manual
mkdir -Create new folder(s)
mkfifo -Make FIFOs (named pipes)
mknod -Make block or character special files
more -Display output one screen at a time
mount -Mount a file system
mtools -Manipulate MS-DOS files
mv -Move or rename files or directories

nice -Set the priority of a command or job
nl -Number lines and write files
nohup -Run a command immune to hangups

passwd -Modify a user password
paste -Merge lines of files
pathchk -Check file name portability
ping -Test a network connection
popd -Restore the previous value of the current directory
pr -Prepare files for printing
printcap -Printer capability database
printenv -Print environment variables
printf -Format and print data
ps -Process status
pushd -Save and then change the current directory
pwd -Print Working Directory

quota -Display disk usage and limits
quotacheck -Scan a file system for disk usage
quotactl -Set disk quotas

ram -ram disk device
rcp -Copy files between two machines.
read -read a line from standard input
readonly -Mark variables/functions as readonly
remsync -Synchronize remote files via email
return -Exit a shell function
rm -Remove files
rmdir -Remove folder(s)
rpm -Remote Package Manager
rsync -Remote file copy (Synchronize file trees)

screen -Terminal window manager
sdiff -Merge two files interactively
sed -Stream Editor
select -Accept keyboard input
seq -Print numeric sequences
set -Manipulate shell variables and functions
shift -Shift positional parameters
shopt -Shell Options
shutdown -Shutdown or restart linux
sleep -Delay for a specified time
sort -Sort text files
source -Run commands from a file `.'
split -Split a file into fixed-size pieces
su -Substitute user identity
sum -Print a checksum for a file
symlink -Make a new name for a file
sync -Synchronize data on disk with memory

tac -Concatenate and write files in reverse
tail -Output the last part of files
tar -Tape ARchiver
tee -Redirect output to multiple files
test -Evaluate a conditional expression
time -Measure Program Resource Use
times -User and system times
touch -Change file timestamps
top -List processes running on the system
traceroute -Trace Route to Host
trap -Run a command when a signal is set(bourne)
tr -Translate, squeeze, and/or delete characters
true -Do nothing, successfully
tsort -Topological sort
tty -Print filename of terminal on stdin
type -Describe a command

ulimit -Limit user resources
umask -Users file creation mask
umount -Unmount a device
unalias -Remove an alias
uname -Print system information
unexpand -Convert spaces to tabs
uniq -Uniquify files
units -Convert units from one scale to another
unset -Remove variable or function names
unshar -Unpack shell archive scripts
until -Execute commands (until error)
useradd -Create new user account
usermod -Modify user account
users -List users currently logged in
uuencode -Encode a binary file
uudecode -Decode a file created by uuencode

v -Verbosely list directory contents (`ls -l -b')
vdir -Verbosely list directory contents (`ls -l -b')
vi -Text Editor

watch -Execute/display a program periodically
wc -Print byte, word, and line counts
whereis -Report all known instances of a command
which -Locate a program file in the user's path.
while -Execute commands
who -Print all usernames currently logged in
whoami -Print the current user id and name (`id -un')

xargs -Execute utility, passing constructed argument list(s)
yes -Print a string until interrupted

.period -Run commands from a file
### -Comment / Remark

__________________
With,
J. Jeyaseelan
Everything Possible

Hi Jay,

These are all commands very usefull for Linux beginners...

thanx