Some of these commands are used by unix/linux administrator in his/her day-to-day activities 

cat       to concatenate files to standard output
chgrp     to change file group ownership
chmod     to change file access permissions
chown     to change file owner and group
cp        to copy files and directories
date      to print or set the system data and time
dd        to convert and copy a file
df        to report filesystem disk space usage
dmesg     to print or control the kernel message buffer
echo      to display a line of text
hostname  to show or set the system's host name
kill      to send signals to processes
ln        to make links between files
login     to begin a session on the system
ls        to list directory contents
mkdir     to make directories
mknod     to make block or character special files
more      to page through text
mount     to mount a filesystem
mv        to move/rename files
ps        to report process status
pwd       to print name of current working directory
rm        to remove files or directories
rmdir     to remove empty directories
sed       The `sed' stream editor
sh        The Bourne command shell
stty      to change and print terminal line settings
su        to change user ID
sync      to flush filesystem buffers
true      to do nothing, successfully
umount    to unmount file systems
uname     to print system information

csh     The C shell 
ed      The `ed' editor 
tar     The tar archiving  
cpio    The cpio archiving  
gzip    The GNU compression  
gunzip  The GNU uncompression  
zcat    The GNU uncompression  
netstat The network statistics  
ping    The ICMP network test

For More Commands:
An A-Z Index of the Bash command line for Linux
Shorthand at the Command Prompt
AIX, FreeBSD, HP-UX, Linux(RedHat), Solaris, Tru64 Commands
Ubuntu Commands
List of Unix Utilities

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