Linux Commands

Linux is an entire family of open-source Unix operating systems, that are based on the Linux Kernel. This includes all of the most popular Linux based systems like Ubuntu, Fedora, Mint, Debian, and others. More accurately, they’re called distributions or distros.

Linux Commands

System Information

Command	Description
# arch	show architecture of machine
# cal 2022	show the timetable of 2022
# cat /proc/cpuinfo	show information CPU info
# cat /proc/interrupts	show interrupts
# cat /proc/meminfo	verify memory use
# cat /proc/swaps	show file(s) swap
# cat /proc/version	show version of the kernel
# cat /proc/net/dev	show network adpters and statistics
# cat /proc/mounts	show mounted file system(s)
# clock -w	save date changes on BIOS
# date	show system date
# date 021815002022.00	set date and time - MonthDayhoursMinutesYear.Seconds
# dmidecode -q	show hardware system components - (SMBIOS / DMI)
# hdparm -i /dev/hda	displays the characteristics of a hard-disk
# hdparm -tT /dev/sda	perform test reading on a hard-disk
# lspci -tv	display PCI devices
# lsusb -tv	show USB devices
# uname -m	show architecture of machine
# uname -r	show used kernel version

Shutdown, Restart and Logout of a System

Command	Description
# init 0	shutdown system
# logout	leaving session
# reboot	reboot
# shutdown -h now	shutdown system
# shutdown -h 16:30 &	planned shutdown of the system
# shutdown -c	cancel a planned shutdown of the system
# shutdown -r now	reboot
# telinit 0	shutdown system

Files and Directory

Command	Description
# cd /home	enter to directory '/home'
# cd ..	go back one level
# cd ../..	go back two levels
# cd	go to home directory
# cd ~user1	go to home directory
# cd -	go to previous directory
# cp file1 file2	copying a file
# cp dir/* .	copy all files of a directory within the current work directory
# cp -a /tmp/dir1 .	copy a directory within the current work directory
# cp -a dir1 dir2	copy a directory
# cp file file1	outputs the mime type of the file as text
# iconv -l	lists known encodings
# iconv -f fromEncoding -t toEncoding inputFile > outputFile	converting the coding of characters from one format to another
# find . -maxdepth 1 -name *.jpg -print -exec convert	batch resize files in the current directory and send them to a thumbnails directory (requires convert from Imagemagick)
# ln -s file1 lnk1	create a symbolic link to file or directory
# ln file1 lnk1	create a physical link to file or directory
# ls	view files of directory
# ls -F	view files of directory
# ls -l	show details of files and directory
# ls -a	show hidden files
# ls *[0-9]*	show files and directory containing numbers
# lstree	show files and directories in a tree starting from root(2)
# mkdir dir1	create a directory called 'dir1'
# mkdir dir1 dir2	create two directories simultaneously
# mkdir -p /tmp/dir1/dir2	create a directory tree
# mv dir1 new_dir	rename / move a file or directory
# pwd	show the path of work directory
# rm -f file1	delete file called 'file1'
# rm -rf dir1	remove a directory called 'dir1' and contents recursively
# rm -rf dir1 dir2	remove two directories and their contents recursively
# rmdir dir1	delete directory called 'dir1'
# touch -t 2202180000 file1	modify timestamp of a file or directory - (YYMMDDhhmm)
# tree	show files and directories in a tree starting from root

File Search

Command	Description
# find / -name file1	search file and directory into root filesystem from '/'
# find / -user user1	search files and directories belonging to 'user1'
# find /home/user1 -name \*.bin	search files with '. bin' extension within directory '/ home/user1'
# find /usr/bin -type f -atime +100	search binary files are not used in the last 100 days
# find /usr/bin -type f -mtime -10	search files created or changed within 10 days
# find / -name *.rpm -exec chmod 755 '{}' \;	search files with '.rpm' extension and modify permits
# find / -xdev -name \*.rpm	search files with '.rpm' extension ignoring removable partitions as cdrom, pen-drive, etc.…
# locate \*.ps	find files with the '.ps' extension - first run 'updatedb' command
# whereis halt	show location of a binary file, source or man
# which halt	show full path to a binary / executable

Mounting a Filesystem

Command	Description
# fuser -km /mnt/hda2	force umount when the device is busy
# mount /dev/hda2 /mnt/hda2	mount disk called hda2 - verify existence of the directory '/ mnt/hda2'
# mount /dev/fd0 /mnt/floppy	mount a floppy disk
# mount /dev/cdrom /mnt/cdrom	mount a cdrom / dvdrom
# mount /dev/hdc /mnt/cdrecorder	mount a cdrw / dvdrom
# mount /dev/hdb /mnt/cdrecorder	mount a cdrw / dvdrom
# mount -o loop file.iso /mnt/cdrom	mount a file or iso image
# mount -t vfat /dev/hda5 /mnt/hda5	mount a Windows FAT32 file system
# mount /dev/sda1 /mnt/usbdisk	mount a usb pen-drive or flash-drive
# mount -t smbfs -o username=user,password=pass //WinClient/share /mnt/share	mount a windows network share
# umount /dev/hda2	unmount disk called hda2 - exit from mount point '/ mnt/hda2' first
# umount -n /mnt/hda2	run umount without writing the file /etc/mtab - useful when the file is read-only or the hard disk is full

Disk Space

Command	Description
# df -h	show list of partitions mounted
# dpkg-query -W -f='${Installed-Size;10}t${Package}n' | sort -k1,1n	show the used space by installed deb packages, sorting by size (debian, ubuntu and alike)
# du -sh dir1	estimate space used by directory 'dir1'
# du -sk * | sort -rn	show size of the files and directories sorted by size
# ls -lSr |more	show size of the files and directories ordered by size
# rpm -q -a --qf '%10{SIZE}t%{NAME}n' | sort -k1,1n	show space used by rpm packages installed sorted by size (fedora, redhat and like)

User and Groups

Command	Description
# chage -E 2022-02-18 user1	set deadline for user password
# groupadd [group]	create a new group
# groupdel [group]	delete a group
# groupmod -n moon sun	rename a group from moon to sun
# grpck	check correct syntax and file format of '/etc/group' and groups existence
# newgrp - [group]	log into a new group to change default group of newly created files
# passwd	change password
# passwd user1	change a user password (only by root)
# pwck	check correct syntax and file format of '/etc/passwd' and users existence
# useradd -c "User Linux" -g admin -d /home/user1 -s /bin/bash user1	create a new user "user1" belongs "admin" group
# useradd user1	create a new user
# userdel -r user1	delete a user ( '-r' eliminates home directory)
# usermod -c "User FTP" -g system -d /ftp/user1 -s /bin/nologin user1	change user attributes

Permits on Files

Command	Description
# chgrp group1 file1	change group of files
# chmod ugo+rwx directory1	set permissions reading (r), write (w) and (x) access to users owner (u) group (g) and others (o)
# chmod go-rwx directory1	remove permits reading (r), write (w) and (x) access to users group (g) and others (o)
# chmod u+s /bin/file1	set SUID bit on a binary file - the user that running that file gets same privileges as owner
# chmod u-s /bin/file1	disable SUID bit on a binary file
# chmod g+s /home/public	set SGID bit on a directory - similar to SUID but for directory
# chmod g-s /home/public	disable SGID bit on a directory
# chmod o+t /home/public	set STIKY bit on a directory - allows files deletion only to legitimate owners
# chmod o-t /home/public	disable STIKY bit on a directory
# chown user1 file1	change owner of a file
# chown -R user1 directory1	change user owner of a directory and all the files and directories contained inside
# chown user1:group1 file1	change user and group ownership of a file
# find / -perm -u+s	view all files on the system with SUID configured
# ls -lh	show permits on files
# ls /tmp | pr -T5 -W$COLUMNS	divide terminal into 5 columns

Special Attributes on Files

Command	Description
# chattr +a file1	allows write opening of a file only append mode
# chattr +c file1	allows that a file is compressed / decompressed automatically by the kernel
# chattr +d file1	makes sure that the program ignores Dump the files during backup
# chattr +i file1	makes it an immutable file, which can not be removed, altered, renamed or linked
# chattr +s file1	allows a file to be deleted safely
# chattr +S file1	makes sure that if a file is modified changes are written in synchronous mode as with sync
# chattr +u file1	allows you to recover the contents of a file even if it is canceled
# lsattr	show specials attributes

Archives and Compressed Files

Command	Description
# bunzip2 file1.bz2	decompress a file called 'file1.bz2'
# bzip2 file1	compress a file called 'file1'
# gunzip file1.gz	decompress a file called 'file1.gz'
# gzip file1	compress a file called 'file1'
# gzip -9 file1	compress with maximum compression
# rar a file1.rar test_file	create an archive rar called 'file1.rar'
# rar a file1.rar file1 file2 dir1	compress 'file1', 'file2' and 'dir1' simultaneously
# rar x file1.rar	decompress rar archive
# tar -cvf archive.tar file1	create a uncompressed tarball
# tar -cvf archive.tar file1 file2 dir1	create an archive containing 'file1', 'file2' and 'dir1'
# tar -tf archive.tar	show contents of an archive
# tar -xvf archive.tar	extract a tarball
# tar -xvf archive.tar -C /tmp	extract a tarball into / tmp
# tar -cvfj archive.tar.bz2 dir1	create a tarball compressed into bzip2
# tar -xvfj archive.tar.bz2	decompress a compressed tar archive in bzip2
# tar -cvfz archive.tar.gz dir1	create a tarball compressed into gzip
# tar -xvfz archive.tar.gz	decompress a compressed tar archive in gzip
# unrar x file1.rar	decompress rar archive
# unzip file1.zip	decompress a zip archive
# zip file1.zip file1	create an archive compressed in zip
# zip -r file1.zip file1 file2 dir1	compress in zip several files and directories simultaneously

RPM Packages (Fedora, Red Hat and like)

Command	Description
# rpm -ivh [package.rpm]	install a rpm package
# rpm -ivh --nodeeps [package.rpm]	install a rpm package ignoring dependencies requests
# rpm -U [package.rpm]	upgrade a rpm package without changing configuration files
# rpm -F [package.rpm]	upgrade a rpm package only if it is already installed
# rpm -e [package]	remove a rpm package
# rpm -qa	show all rpm packages installed on the system
# rpm -qa | grep httpd	show all rpm packages with the name "httpd"
# rpm -qi [package]	obtain information on a specific package installed
# rpm -qg "System Environment/Daemons"	show rpm packages of a group software
# rpm -ql [package]	show list of files provided by a rpm package installed
# rpm -qc [package]	show list of configuration files provided by a rpm package installed
# rpm -q [package] --whatrequires	show list of dependencies required for a rpm packet
# rpm -q [package] --whatprovides	show capability provided by a rpm package
# rpm -q [package] --scripts	show scripts started during installation / removal
# rpm -q [package] --changelog	show history of revisions of a rpm package
# rpm -qf /etc/httpd/conf/httpd.conf	verify which rpm package belongs to a given file
# rpm -qp [package.rpm] -l	show list of files provided by a rpm package not yet installed
# rpm --import /media/cdrom/RPM-GPG-KEY	import public-key digital signature
# rpm --checksig [package.rpm]	verify the integrity of a rpm package
# rpm -qa gpg-pubkey	verify integrity of all rpm packages installed
# rpm -V [package]	check file size, permissions, type, owner, group, MD5 checksum and last modification
# rpm -Va	check all rpm packages installed on the system - use with caution
# rpm -Vp [package.rpm]	verify a rpm package not yet installed
# rpm -ivh /usr/src/redhat/RPMS/`arch`/[package.rpm]	install a package built from a rpm source
# rpm2cpio [package.rpm] | cpio --extract --make-directories *bin*	extract executable file from a rpm package
# rpmbuild --rebuild [package.src.rpm]	build a rpm package from a rpm source

YUM Packages Tool (Fedora, RedHat and like)

Command	Description
# yum -y install [package]	download and install a rpm package
# yum localinstall [package.rpm]	That will install an RPM, and try to resolve all the dependencies for you using your repositories.
# yum -y update	update all rpm packages installed on the system
# yum update [package]	upgrade a rpm package
# yum remove [package]	remove a rpm package
# yum list	list all packages installed on the system
# yum search [package]	find a package on rpm repository
# yum clean [package]	clean up rpm cache erasing downloaded packages
# yum clean headers	remove all files headers that the system uses to resolve dependency
# yum clean all	remove from the cache packages and headers files

DEB Packages (Debian, Ubuntu and like)

Command	Description
# dpkg -i [package.deb]	install / upgrade a deb package
# dpkg -r [package]	remove a deb package from the system
# dpkg -l	show all deb packages installed on the system
# dpkg -l | grep httpd	show all deb packages with the name "httpd"
# dpkg -s [package]	obtain information on a specific package installed on system
# dpkg -L [package]	show list of files provided by a package installed on system
# dpkg --contents [package.deb]	show list of files provided by a package not yet installed
# dpkg -S /bin/ping	verify which package belongs to a given file

APT Packages Tool (Debian, Ubuntu and like)

Command	Description
# apt-cache search [package]	returns list of packages which corresponds string "searched-packages"
# apt-cdrom install [package]	install / upgrade a deb package from cdrom
# apt-get install [package]	install / upgrade a deb package
# apt-get update	update the package list
# apt-get upgrade	upgrade all of the installed packages
# apt-get remove [package]	remove a deb package from system
# apt-get check	verify correct resolution of dependencies
# apt-get clean	clean up cache from packages downloaded

Pacman Packages Tool (Arch, Frugalware and alike)

Command	Description
# pacman -S name	Install package 'name' with dependencies
# pacman -R name	Delete package 'name' and all files of it

View File Content

Command	Description
# cat file1	view the contents of a file starting from the first row
# head -2 file1	view first two lines of a file
# less file1	similar to 'more' command but which allows backward movement in the file as well as forward movement
# more file1	view content of a file along
# tac file1	view the contents of a file starting from the last line
# tail -2 file1	view last two lines of a file
# tail -f /var/log/messages	view in real time what is added to a file

Text Manipulationt

Command	Description
# cat example.txt | awk 'NR%2==1'	remove all even lines from example.txt
# echo a b c | awk '{print $1}'	view the first column of a line
# echo a b c | awk '{print $1,$3}'	view the first and third column of a line
# cat -n file1	number row of a file
# comm -1 file1 file2	compare contents of two files by deleting only unique lines from 'file1'
# comm -2 file1 file2	compare contents of two files by deleting only unique lines from 'file2'
# comm -3 file1 file2	compare contents of two files by deleting only the lines that appear on both files
# diff file1 file2	find differences between two files
# grep Aug /var/log/messages	look up words "Aug" on file '/var/log/messages'
# grep ^Aug /var/log/messages	look up words that begin with "Aug" on file '/var/log/messages'
# grep [0-9] /var/log/messages	select from file '/var/log/messages' all lines that contain numbers
# grep Aug -R /var/log/*	search string "Aug" at directory '/var/log' and below
# paste file1 file2	merging contents of two files for columns
# paste -d '+' file1 file2	merging contents of two files for columns with '+' delimiter on the center
# sdiff file1 file2	find differences between two files and merge interactively alike "diff"
# sed 's/string1/string2/g' example.txt	replace "string1" with "string2" in example.txt
# sed '/^$/d' example.txt	remove all blank lines from example.txt
# sed '/ *#/d; /^$/d' example.txt	remove comments and blank lines from example.txt
# sed -e '1d' exampe.txt	eliminates the first line from file example.txt
# sed -n '/string1/p'	view only lines that contain the word "string1"
# sed -e 's/ *$//' example.txt	remove empty characters at the end of each row
# sed -e 's/string1//g' example.txt	remove only the word "string1" from text and leave intact all
# sed -n '1,5p' example.txt	print from 1th to 5th row of example.txt
# sed -n '5p;5q' example.txt	print row number 5 of example.txt
# sed -e 's/00*/0/g' example.txt	replace more zeros with a single zero
# sort file1 file2	sort contents of two files
# sort file1 file2 | uniq	sort contents of two files omitting lines repeated
# sort file1 file2 | uniq -u	sort contents of two files by viewing only unique line
# sort file1 file2 | uniq -d	sort contents of two files by viewing only duplicate line
# echo 'word' | tr '[:lower:]' '[:upper:]'	convert from lower case in upper case

Character Set and Format File Conversion

Command	Description
# dos2unix filedos.txt fileunix.txt	convert a text file format from MSDOS to UNIX
# recode ..HTML page.txt page.html	convert a text file to html
# recode -l | more	show all available formats conversion
# unix2dos fileunix.txt filedos.txt	convert a text file format from UNIX to MSDOS

Filesystem Analysis

Command	Description
# badblocks -v /dev/hda1	check bad blocks on disk hda1
# dosfsck /dev/hda1	repair / check integrity of dos filesystems on disk hda1
# e2fsck /dev/hda1	repair / check integrity of ext2 filesystem on disk hda1
# e2fsck -j /dev/hda1	repair / check integrity of ext3 filesystem on disk hda1
# fsck /dev/hda1	repair / check integrity of linux filesystem on disk hda1
# fsck.ext2 /dev/hda1	repair / check integrity of ext2 filesystem on disk hda1
# fsck.ext3 /dev/hda1	repair / check integrity of ext3 filesystem on disk hda1
# fsck.vfat /dev/hda1	repair / check integrity of fat filesystem on disk hda1
# fsck.msdos /dev/hda1	repair / check integrity of dos filesystem on disk hda1

Format a Filesystem

Command	Description
# fdformat -n /dev/fd0	format a floppy disk
# mke2fs /dev/hda1	create a filesystem type linux ext2 on hda1 partition
# mke2fs -j /dev/hda1	create a filesystem type linux ext3 (journal) on hda1 partition
# mkfs /dev/hda1	create a filesystem type linux on hda1 partition
# mkfs -t vfat 32 -F /dev/hda1	create a FAT32 filesystem
# mkswap /dev/hda3	create a swap filesystem

Filesystem SWAP

Command	Description
# mkswap /dev/hda3	create a swap filesystem
# swapon /dev/hda3	activating a new swap partition
# swapon /dev/hda2 /dev/hdb3	activate two swap partitions

Backup

Command	Description
# find /var/log -name '*.log' | tar cv --files-from=- | bzip2 > log.tar.bz2	find all files with '.log' extention and make an bzip archive
# find /home/user1 -name '*.txt' | xargs cp -av --target-directory=/home/backup/ --parents	find and copy all files with '.txt' extention from a directory to another
# dd bs=1M if=/dev/hda | gzip | ssh [email protected]_addr 'dd of=hda.gz'	make a backup of a local hard disk on remote host via ssh
# dd if=/dev/sda of=/tmp/file1	backup content of the harddrive to a file
# dd if=/dev/hda of=/dev/fd0 bs=512 count=1	make a copy of MBR (Master Boot Record) to floppy
# dd if=/dev/fd0 of=/dev/hda bs=512 count=1	restore MBR from backup copy saved to floppy
# dump -0aj -f /tmp/home0.bak /home	make a full backup of directory '/home'
# dump -1aj -f /tmp/home0.bak /home	make a incremental backup of directory '/home'
# restore -if /tmp/home0.bak	restoring a backup interactively
# rsync -rogpav --delete /home /tmp	synchronization between directories
# rsync -rogpav -e ssh --delete /home ip_address:/tmp	rsync via SSH tunnel
# rsync -az -e ssh --delete ip_addr:/home/public /home/local	synchronize a local directory with a remote directory via ssh and compression
# rsync -az -e ssh --delete /home/local ip_addr:/home/public	synchronize a remote directory with a local directory via ssh and compression
# tar -Puf backup.tar /home/user	make a incremental backup of directory '/home/user'
# ( cd /tmp/local/ && tar c . ) | ssh -C [email protected]_addr 'cd /home/share/ && tar x -p'	copy content of a directory on remote directory via ssh
# ( tar c /home ) | ssh -C [email protected]_addr 'cd /home/backup-home && tar x -p'	copy a local directory on remote directory via ssh
# tar cf - . | (cd /tmp/backup ; tar xf - )	local copy preserving permits and links from a directory to another

CDROM

Command	Description
# cd-paranoia -B	rip audio tracks from a CD to wav files
# cd-paranoia --	rip first three audio tracks from a CD to wav files
# cdrecord -v gracetime=2 dev=/dev/cdrom -eject blank=fast -force	clean a rewritable cdrom
# cdrecord -v dev=/dev/cdrom cd.iso	burn an ISO image
# gzip -dc cd_iso.gz | cdrecord dev=/dev/cdrom -	burn a compressed ISO image
# cdrecord --scanbus	scan bus to identify the channel scsi
# dd if=/dev/hdc | md5sum	perform an md5sum on a device, like a CD
# mkisofs /dev/cdrom > cd.iso	create an iso image of cdrom on disk
# mkisofs /dev/cdrom | gzip > cd_iso.gz	create a compressed iso image of cdrom on disk
# mkisofs -J -allow-leading-dots -R -V	create an iso image of a directory
# mount -o loop cd.iso /mnt/iso	mount an ISO image

Networking (LAN / WiFi)

Command	Description
# dhclient eth0	active interface 'eth0' in dhcp mode
# ethtool eth0	show network statistics of eth0
# host www.example.com	lookup hostname to resolve name to ip address and viceversa
# hostname	show hostname of system
# ifconfig eth0	show configuration of an ethernet network card
# ifconfig eth0 192.168.1.1 netmask 255.255.255.0	configure IP Address
# ifconfig eth0 promisc	configure 'eth0' in promiscuous mode to gather packets (sniffing)
# ifdown eth0	disable an interface 'eth0'
# ifup eth0	activate an interface 'eth0'
# ip link show	show link status of all network interfaces
# iwconfig eth1	show wireless networks
# iwlist scan	wifi scanning to display the wireless connections available
# mii-tool eth0	show link status of 'eth0'
# netstat -tup	show all active network connections and their PID
# netstat -tupl	show all network services listening on the system and their PID
# netstat -rn	show routing table alike "route -n"
# nslookup www.example.com	lookup hostname to resolve name to ip address and viceversa
# route -n	show routing table
# route add -net 0/0 gw IP_Gateway	configure default gateway
# route add -net 192.168.0.0 netmask 255.255.0.0 gw 192.168.1.1	configure static route to reach network '192.168.0.0/16'
# route del 0/0 gw IP_gateway	remove static route
# echo "1" > /proc/sys/net/ipv4/ip_forward	activate ip routing
# tcpdump tcp port 80	show all HTTP traffic
# whois www.example.com	lookup on Whois database

Microsoft Windows Networks (Samba)

Command	Description
# mount -t smbfs -o username=user,password=pass //WinClient/share /mnt/share	mount a windows network share
# nbtscan ip_addr	netbios name resolution
# nmblookup -A ip_addr	netbios name resolution
# smbclient -L ip_addr/hostname	show remote shares of a windows host
# smbget -Rr smb://ip_addr/share	like wget can download files from a host windows via smb

IPTABLES (Firewall)

Command	Description
# iptables -t filter -L	show all chains of filtering table
# iptables -t nat -L	show all chains of nat table
# iptables -t filter -F	clear all rules from filtering table
# iptables -t nat -F	clear all rules from table nat
# iptables -t filter -X	delete any chains created by user
# iptables -t filter -A INPUT -p tcp --dport telnet -j ACCEPT	allow telnet connections to input
# iptables -t filter -A OUTPUT -p tcp --dport http -j DROP	block HTTP connections to output
# iptables -t filter -A FORWARD -p tcp --dport pop3 -j ACCEPT	allow POP3 connections to forward chain
# iptables -t filter -A INPUT -j LOG --log-prefix	Logging on input chain
# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE	configure a PAT (Port Address Traslation) on eth0 masking outbound packets
# iptables -t nat -A PREROUTING -d 192.168.0.1 -p tcp -m tcp --dport 22 -j DNAT --to-destination 10.0.0.2:22	redirect packets addressed to a host to another host

Monitoring adn Debugging

Command	Description
# free -m	displays status of RAM in megabytes
# kill -9 process_id	force closure of the process and finish it
# kill -1 process_id	force a process to reload configuration
# last reboot	show history reboot
# lsmod	display kernel loaded
# lsof -p process_id	display a list of files opened by processes
# lsof /home/user1	displays a list of open files in a given path system
# ps -eafw	displays linux tasks
# ps -e -o pid,args --forest	displays linux tasks in a hierarchical mode
# pstree	Shows a tree system processes
# smartctl -A /dev/hda	monitoring reliability of a hard-disk through SMART
# smartctl -i /dev/hda	check if SMART is active on a hard-disk
# strace -c ls >/dev/null	display system calls made and received by a process
# strace -f -e open ls >/dev/null	display library calls
# tail /var/log/dmesg	show events inherent to the process of booting kernel
# tail /var/log/messages	show system events
# top	display linux tasks using most cpu
# watch -n1 'cat /proc/interrupts'	display interrupts in real-time

Other Useful Commands

Command	Description
# alias hh='history'	set an alias for a command - hh = history
# apropos ...keyword	display a list of commands that pertain to keywords of a program , useful when you know what your program does, but you don't know the name of the command
# chsh	change shell command
# chsh --list-shells	nice command to know if you have to remote into another box
# gpg -c file1	encrypt a file with GNU Privacy Guard
# gpg file1.gpg	decrypt a file with GNU Privacy Guard
# ldd /usr/bin/ssh	show shared libraries required by ssh program
# man ping	display the on-line manual pages for example on ping command - use '-k' option to find any related commands
# mkbootdisk --device /dev/fd0 `uname -r`	create a boot floppy
# wget -r www.example.com	download an entire web site
# wget -c www.example.com/file.iso	download a file with the ability to stop the download and resume later
# echo 'wget -c www.example.com/files.iso' | at 09:00	start a download at any given time
# whatis ...keyword	displays description of what a program does
# who -a	show who is logged on, and print: time of last system boot, dead processes, system login processes, active processes spawned by init, current runlevel, last system clock change