Important Linux Commands for interview

Linux commnads are the heart of the Linux Admins. You must know this.

Help Commands

man – Online manual
whatis – Displays manual descriptions

File and Directory Commands

ls – Lists directory contents
pwd – Prints the current directory
cd – Change directories
mv – Move files
cp – Copy files
rm – Delete files
mkdir – Create directories
rmdir – Remove directories
find – Search for files (slow)
locate – Search for files (faster)
whereis – Display binary file location
which – Display binary file location
file – Display file type
size – Display file size
stat – Display file statistics
fuser – Identify open files
touch – Update file timestamps
lsof – List open files
cksum – Calculate checksum
md5sum – Calculate md5sum
ln – Create a link
alias – Display/edit command aliases
gzip – Compress files
gunzip – Uncompress files
shred – Securely delete files
head – Display the head of a file
tail – Display the tail of a file
tee – Display and redirect output
sort – Sort input files/streams
grep – Display matching results
tree – Display directory tree
more – Display files one page at a time
less – Display files one page at a time
wc – Count words/lines/letters
cat – Display files
zcat – Display compressed files
diff – Show differences between files
strings – Display printable characters
sed – Text editing utility
awk – Text processing utility
dos2unix – Convert DOS files to Unix
unix2dos – Convert Unix files to DOS

Editors

nano – Simple text editor
vi – Advanced text editor
emacs – Ultimate text editor

Other Utilities

clear – Clear terminal screen
date – Display the date
cal – Display a calendar
watch – Monitor a command
env – Display environment variables
history – Display command history
logout – Logout of the shell
exit – Exit the shell

Users and Groups

su – Switch user
sudo – Run a program as another user
id – Display user identity
ulimit – Display user limits
who – Display who is logged in
w – Display what users are doing
users – Display active user accounts
last – Display last user logins
lastlog – Display all user’s last login
wall – Send a message to all users
whoami – Display current user id
finger – Display information about a user
chown – Change file/directory ownership
chgrp – Change file/directory group
chmod – Change file/directory permissions
umask – Display or set umask settings
passwd – Set/change password
useradd – Create user accounts
userdel – Delete user accounts
usermod – Modify user accounts
groupadd – Create group accounts
groupdel – Delete group accounts
groupmod – Modify group accounts

Process Control

ps – Display running processes
pgrep – Search for running processes
pidof – Search for PID by name
pstree – Displays process in tree view
kill – Terminate a process by PID
killall – Terminal a process by name
nice – Run a program with a modified priority
renice – Adjust a program’s priority
nohup – Run a program immune to hang-ups
& – Run a program in the background
bg – Move a job to the background
jobs – Display running jobs
fg – Move a job to the foreground

Startup and Shutdown

shutdown – Shutdown the computer
poweroff – Poweroff the computer
halt – Halt the computer
runlevel – Display the current runlevel
telinit – Change runlevel
service – Stop and stop services
sysv-rc-conf – Runlevel configuration editor
update-rc.d – Debian runlevel editor
chkconfig – Red Hat runlevel editor
rc-update – Gentoo runlevel editor
rc-status – Gentoo service monitor

Networking Commands

hostname – Display the system hostname
domainname – Display the system domain
ifconfig – Manage network interfaces
ifup – Start network interfaces
ifdown – Stop network interfaces
iwconfig – Manage wireless interfaces
iwlist – Display wireless information
ethtool – Display network card info
arp – Display the ARP cache
ping – Send ICMP echo requests
traceroute – Trace network paths
tracepath – Trace network paths
nslookup – Query DNS servers
dig – Query DNS servers
host – Query DNS servers
whois – Query the whois database
dhclient – Linux DHCP client
netstat – Display network status
route – Manage network routes
tcpdump – Capture network packets
nmap – Scan remote computers
wavemon – Monitor wireless connections
smbtree – Display SMB servers/shares
nmblookup – Look up NetBIOS information
mount – Mount file systems
showmount – Show mounted file systems
umount – Unmount file systems
ssh – SSH client
telnet – Telnet client
ftp – FTP client
ncftp – Scriptable FTP client
mail – Email client
rsync – Rsync client

Hardware Commands

lspci – List PCI devices
pcidump – List PCI devices
lsusb – List USB devices
lshw – List hardware devices
lspcmcia – List PCMCIA devices
lshal – Display all system hardware
hdparm – Configure hard drives
eject – Eject removable media

Scheduling

batch – Run processes when the CPU is free
at – Run processes at a specific time
atq – Display the at queue
atrm – Remove jobs from the at queue
crontab – Display/edit cron jobs

File System Commands

fdisk – Partition editor
parted – Partition editor
mkfs – Create file systems
fsck – Check file systems
mkswap – Create swap space
swapon – Activate swap space
swapoff – Deactivate swap space
sync – Flush disk cache

Backup Commands

tar – Archive utility
dd – File copy utility
dump – Incremental backup utility
restore – Restore dump backups
mt – Tape device utility
cpio – Archive utility

Monitoring Commands

top – Performance monitor
mpstat – Performance monitor
vmstat – Virtual memory monitor
iostat – I/O performance monitor
nfsstat – NFS performance monitor
free – Display memory usage
df – Display disk usage
du – Display disk usage
uname – Display system information
uptime – Display system uptime
lsmod – List kernel modules
modinfo – Display module information
dmesg – Display kernel messages
strace – System trace debugger
ltrace -Library trace debugger
ipcs – IPC monitor
sysctl – Configure kernel parameters

Printing Commands

lp – Print files
lpstat – Display printer status
lpq – Display print queue
lprm – Remove print jobs
cancel – Cancel print jobs
enable – Enable a printer
disable – Disable a printer

Software Commands

dpkg – Debian package manager
apt-get – Debian package utility
rpm – Red Hat package manager
yum – Red Hat package utility
emerge – Gentoo package utility
pkg_add – BSD installation utility
pkg_delete – BSD uninstallation utility
make – Compile software from source