Linux Commands Cheat Sheet with Examples

Last updated Sep 5, 2022

COMMAND	DESCRIPTION	More details	Similar request (optional)
System information
`arch`	show architecture of the machine	Example
`uname -m`	show architecture of the machine	Example
`uname -r`	show kernel version used	Example
`dmidecode -q`	show hardware system components - (SMBIOS / DMI)	Example
`hdparm -i /dev/hda`	displays the characteristics of a hard-disk	Example
`hdparm -tT /dev/sda`	perform t est reading on a hard-disk	Example
`cat /proc/cpuinfo`	shows information CPU info	Example
`cat /proc/interrupts`	show interrupts	Example
`cat /proc/meminfo`	verify memory use	Example
`cat /proc/swaps`	show file(s) swap	Example
`cat /proc/version`	show version of the kernel	Example
`cat /proc/net/dev`	show network adpters and statistics	Example
`cat /proc/mounts`	show mounted file system(s)	Example
`lspci -tv`	display PCI devices	Example
`lsusb -tv`	show USB devices	Example
`date`	show system date	Example
`cal 2007`	show the timetable of 2007	Example
`date 041217002007.00`	set date and time - MonthDayhoursMinutesYear.Seconds	Example
`clock -w`	save changes of date on BIOS	Example
Shutdown, Restart of a system and Logout
`shutdown -h now`	shutdown system	Example
`init 0`	shutdown system	Example
`shutdown -r hours:minutes &`	planned shutdown ofthe system	Example
`shutdown -c`	cancel a planned shutdown of the system	Example
`shutdown -r now`	reboot	Example
`reboot`	reboot	Example
`logout`	leaving session	Example
Files and Directory
`cd /home`	enter to directory '/ home'	Example
`cd ..`	go back one level	Example
`cd ../..`	go back two levels	Example
`cd`	go to home directory	Example
`cd ~utente`	go to home directory	Example
`cd -`	go to previous directory	Example
`pwd`	show the path of work directory	Example
`ls`	view files of directory	Example
`ls -F`	view files of directory	Example
`ls -l`	show details of files and directory	Example
`ls -a`	show hidden files	Example
`ls [0-9]`	show files and directory containing numbers	Example
`tree`	show files and directories in a tree starting from root	Example
`lstree`	show files and directories in a tree starting from root	Example
`mkdir dir1`	create a directory called 'dir1'	Example
`mkdir dir1 dir2`	create two directories simultaneously	Example
`mkdir -p /tmp/dir1/dir2`	create a directory tree	Example
`rm -f file1`	delete file called 'file1'	Example
`rmdir dir1`	delete directory called 'dir1'	Example
`rm -rf dir1`	remove a directory called 'dir1' and contents recursively	Example
`rm -rf dir1 dir2`	remove two directories and their contents recursively	Example
`mv dir1 new_dir`	rename / move a file or directory	Example
`cp file1 file2`	copying a file	Example
`cp dir/* .`	copy all files of a directory within the current work directory	Example
`cp -a /tmp/dir1 .`	copy a directory within the current work directory	Example
`cp -a dir1 dir2`	copy a directory	Example
`ln -s file1 lnk1`	create a symbolic link to file or directory	Example
`ln file1 lnk1`	create a physical link to file or directory	Example
`touch -t 0712250000 fileditest`	modify timestamp of a fileor directory - (YYMMDDhhmm)	Example
File search
`find / -name file1`	search file and directory into root filesystem from '/'	Example
`find / -user user1`	search files and directories belonging to 'user1'	Example
`find /home/user1 -name *.bin`	search files with '. bin' extension within directory '/ home/user1'	Example
`find /usr/bin -type f atime +100`	search bynary files are not used in the last 100 days	Example
`find /usr/bin -type f mtime 10`	search files created or changed within 10 days	Example
`find / -name *.rpm exec chmod 755 {} ;`	search files with '.rpm' extension and modify permits	Example
`find / -xdev name *.rpm`	search files with '.rpm'extension ignoring removable partitions as cdrom, pen-drive, etc.…	Example
`locate *.ps`	find files with the '.ps' extension - first run 'updatedb' command	Example
`whereis halt`	show location of a binary file, source or man	Example
`which halt`	show full path to a binary / executable	Example
Mounting a Filesystem
`mount /dev/hda2 /mnt/hda2`	mount disk called hda2 - verify existence of the directory '/ mnt/hda2'	Example
`umount /dev/hda2`	unmount disk called hda2 - exit from mount point '/ mnt/hda2' first	Example
`fuser -km /mnt/hda2`	force umount when the device is busy	Example
`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	Example
`mount /dev/fd0 /mnt/floppy`	mount a floppy disk	Example
`mount /dev/cdrom /mnt/cdrom`	mount a cdrom / dvdrom	Example
`mount /dev/hdc /mnt/cdrecorder`	mount a cdrw / dvdrom	Example
`mount /dev/hdb /mnt/cdrecorder`	mount a cdrw / dvdrom	Example
`mount -o loop file.iso /mnt/cdrom`	mount a file or iso image	Example
`mount -t vfat /dev/hda5 /mnt/hda5`	mount a Windows FAT32 file system	Example
`mount /dev/sda1 /mnt/usbdisk`	mount a usb pen-drive or flashdrive	Example
`mount -t smbfs o username=user,password=pass //winclient/share /mnt/share`	mount a windows network share	Example
Disk Space
`df -h`	show list of partitions mounted	Example
`ls -lSr \|more`	show size of the files and directories ordered by size	Example
`du -sh dir1`	estimate space used by directory 'dir1'	Example
`du -sk * \| sort rn`	show size of the files and directories sorted by size	Example
`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)	Example
`dpkg-query W f='${InstalledSize;10}t${Package}n' \| sort -k1,1n`	show space used by deb packages installed sorted by size (ubuntu, debian and like)	Example
Users and Groups
`groupadd group_name`	create a new group	Example
`groupdel group_name`	delete a group	Example
`groupmod -n new_group_name old_group_name`	rename a group	Example
`useradd -c Nome Cognome" g admin d /home/user1 s /bin/bash user1`"	create a new user belongs admin" group"	Example
`useradd user1`	create a new user	Example
`userdel -r user1`	delete a user ( '-r' eliminates home directory)	Example
`usermod -c User FTP" g system d /ftp/user1 s /bin/nologin user1`"	change user attributes	Example
`passwd`	change password	Example
`passwd user1`	change a user password (only by root)	Example
`chage -E 20051231 user1`	set deadline for user password	Example
`pwck`	check correct syntax and file format of '/etc/pasws d' and users existence	Example
`grpck`	check correct syntax and file format of '/etc/group' and groups existence	Example
`newgrp group_name`	log in to a new group to change default group of newly created files	Example
Permits on File - use +" to set permissions and "-" to remove"
`ls -lh`	show permits	Example
`ls /tmp \| pr -T5 W$COLUMNS`	divide terminal into 5 columns	Example
`chmod ugo+rwx directory1`	set permissions reading (r), write (w) and (x)access to users owner (u) group (g) and others (o)	Example
`chmod go-rwx directory1`	remove permits reading (r), write (w) and (x) access to users group (g) and others (or	Example
`chown user1 file1`	change owner of a file	Example
`chown user1 -R directory1`	change user owner of a directory and allthe files and directories contained inside	Example
`chgrp gruppo1 file1`	change group of files	Example
`chown user1:gruppo1 file1`	change user and group ownership of a file	Example
`find / -perm u+s`	view all files on the system with SUID configured	Example
`chmod u+s /bin/file_eseguibile`	set SUID bit on a binary file - the user that running that file gets same privileges as owner	Example
`chmod u-s /bin/file_binario`	disable SUID bit on a binary file	Example
`chmod g+s /home/public`	set SGID bit on a directory - similar to SUID but for directory	Example
`chmod g-s /home/public`	disable SGID bit on a directory	Example
`chmod o+t /home/comune`	set STIKY bit on a directory - allows files deletion only to legitimate owners	Example
`chmod o-t /home/comune`	disable STIKY bit on a directory	Example
Special Attributes on file - use +" to set permissions and "-" to remove"
`chattr +a file1`	allows write opening of a file only append mode	Example
`chattr +c file1`	allows that a file is compressed / decompressed automatically by the kernel	Example
`chattr +d file1`	makes sure that the program ignores Dump the files during backup	Example
`chattr +i file1`	makes it an immutable file, which cannot be removed, altered, renamed or linked	Example
`chattr +s file1`	allows a file to be deleted safely	Example
`chattr +S file1`	makes sure that if a file is modified changes are written in synchronous mode as with sync	Example
`chattr +u file1`	allows you to recover the contents of a file even if it is canceled	Example
`lsattr`	show specials attributes	Example
Archives and compressed files
`bunzip2 file1.bz2`	decompress a file called 'file1.bz2'	Example
`bzip2 file1`	compress a file called 'file1'	Example
`gunzip file1.gz`	decompress a file called 'file1.gz'	Example
`gzip file1`	compress a file called 'file1'	Example
`gzip -9 file1`	compress with maximum compression	Example
`rar a file1.rar test_file`	create an archive rar called'file1.rar'	Example
`rar a file1.rar file1 file2 dir1`	compress 'file1', 'file2' and 'dir1' simultaneously	Example
`rar x file1.rar`	decompress rar archive	Example
`unrar x file1.rar`	decompress rar archive	Example
`tar -cvf archive.tar file1`	create a uncompressed tarball	Example
`tar -cvf archive.tar file1 file2 dir1`	create an archive containing 'file1', 'file2' and 'dir1'	Example
`tar -tf archive.tar`	show contents of an archive	Example
`tar -xvf archive.tar`	extract a tarball	Example
`tar -xvf archive.tar C /tmp`	extract a tarball into / tmp	Example
`tar -cvfj archive.tar.bz2 dir1`	create a tarball compressed into bzip2	Example
`tar -xvfj archive.tar.bz2`	decompress a compressed tar archive in bzip2	Example
`tar -cvfz archive.tar.gz dir1`	create a tarball compressed into gzip	Example
`tar -xvfz archive.tar.gz`	decompress a compressed tar archive in gzip	Example
`zip file1.zip file1`	create an archive compressed in zip	Example
`zip -r file1.zip file1 file2 dir1`	compress in zip several files and directories simultaneously	Example
`unzip file1.zip`	decompress a zip archive	Example
RPM Packages - Fedora, Red Hat and like
`rpm -ivh package.rpm`	install a rpm package	Example
`rpm -ivh nodeeps package.rpm`	install a rpm package ignoring dependencies requests	Example
`rpm -U package.rpm`	upgrade a rpm package without changing configuration files	Example
`rpm -F package.rpm`	upgrade a rpm package only if it is already installed	Example
`rpm -e package_name.rpm`	remove a rpm package	Example
`rpm -qa`	show all rpm packagesinstalled on the system	Example
`rpm -qa \| grep httpd`	show all rpm packageswith the name httpd""	Example
`rpm -qi package_name`	obtain information on a specific package installed	Example
`rpm -qg System Environment/Daemons"`"	show rpm packages of a groupsoftware	Example
`rpm -ql package_name`	show list of files provided by a rpm package installed	Example
`rpm -qc package_name`	show list of configuration files provided by a rpm package installed	Example
`rpm -q package_name whatrequires`	show list of dependencies required fora rpm packet	Example
`rpm -q package_name whatprovides`	show capability provided by a rpm package	Example
`rpm -q package_name scripts`	show scripts started during installation / removal	Example
`rpm -q package_name changelog`	show history of revisions of a rpm package	Example
`rpm -qf /etc/httpd/conf/httpd.conf`	verify which rpm package belongs to a given fiel	Example
`rpm -qp package.rpm -l`	show list of files provided by a rpm package not yet installed	Example
`rpm -import /media/cdrom/RPMGPGKEY`	import public-key digital signature	Example
`rpm -checksig package.rpm`	verify the integrityof a rpm package	Example
`rpm -qa gpgpubkey`	verify integrity of all rpm packages installed	Example
`rpm -V package_name`	check file size, permissions, type, owner, group, MD5 checksum and last modification	Example
`rpm -Va`	check all rpm packages installed on the system - use with caution	Example
`rpm -Vp package.rpm`	verify a rpm package not yet installed	Example
`rpm2cpio package.rpm \| cpio -extract -make-directories bin`	extract executable file from a rpm package	Example
rpm -ivh /usr/src/redhat/RPMS/`arch`/package.rpm	install a package built from a rpm source	Example
`rpmbuild -rebuild package_name.src.rpm`	build a rpm package from a rpm source	Example
YUM packages updater - Fedora, RedHat and like
`yum install package_name`	download and install a rpm package	Example
`yum update`	update all rpm packages installed on the system	Example
`yum update package_name`	upgrade a rpm package	Example
`yum remove package_name`	remove a rpm package	Example
`yum list`	list all packages installed on the system	Example
`yum search package_name`	find a package on rpm repository	Example
`yum clean packages`	clean up rpm cache erasing downloaded packages	Example
`yum clean headers`	remove all files headers that the system uses to resolve dependency	Example
`yum clean all`	remove from the cache packages and headers files	Example
DEB packages - Debian, Ubuntu and like
`dpkg -i package.deb`	install / upgrade a deb package	Example
`dpkg -r package_name`	remove a deb package from the system	Example
`dpkg -l`	show all deb packages installed on the system	Example
`dpkg -l \| grep httpd`	show all rpm packageswith the name httpd""	Example
`dpkg -s package_name`	obtain information on a specific package installed on system	Example
`dpkg -L package_name`	show list of files provided by a package installed on system	Example
`dpkg -contents package.deb`	show list of files provided by a package not yet installed	Example
`dpkg -S /bin/ping`	verify which package belongs to a given file	Example
APT packages updater - Debian, Ubuntu e like
`apt-get install package_name`	install / upgrade a deb package	Example
`apt-cdrom install package_name`	install / upgrade a deb package from cdrom	Example
`apt-get update`	update all deb packages installed on system	Example
`apt-get remove package_name`	remove a deb package from system	Example
`apt-get check`	verify correct resolution of dependencies	Example
`apt-get clean`	clean up cache from packages downloaded	Example
View file content
`cat file1`	view the contents of a file starting from the first row	Example
`tac file1`	view the contents of a file starting from the last line	Example
`more file1`	view content of a file along	Example
`less file1`	similar to 'more' command but which allows backward movement in the file as well as forwardmovement	Example
`head -2 file1`	view first two lines of a file	Example
`tail -2 file1`	view last two lines of a file	Example
`tail -f /var/log/messages`	view in real time what is added to a file	Example
Text Manipulation
`cat file_test \| [operation: sed, grep, awk, grep, etc] > result.txt`	syntax to elaborate the text of a file, and write result to a new file	Example
`cat file_originale \| [operazione: sed, grep, awk, grep, etc] >> result.txt`	syntax to elaborate the text of a file and append result onto existing file	Example
`grep Aug /var/log/messages`	look up words Aug" in the file onto 'messages'"	Example
`grep ^Aug /var/log/messages`	look up words that begin with Aug" onto file 'messages'"	Example
`grep [0-9] /var/log/messages`	select from file 'messages' all lines that contain numbers	Example
`grep Aug -R /var/log/*`	search string Aug" at directory '/var/log' and below"	Example
`sed 's/stringa1/stringa2/g' example.txt`	replace string1" with "string2" onto example.txt"	Example
`sed '/^$/d' example.txt`	remove all blank lines from example.txt	Example
`sed '/ *#/d; /^$/d' example.txt`	remove comments andblank lines from example.txt	Example
`echo 'esempio' \| tr '[:lower:]' '[:upper:]'`	convert from lower case in upper case	Example
`sed -e '1d' result.txt`	eliminates the first line from file example.txt	Example
`sed -n '/stringa1/p'`	view only lines that contain the word string1""	Example
`sed -e 's/ *$//' example.txt`	remove empty characters at the end of each row	Example
`sed -e 's/stringa1//g' example.txt`	remove only the word string1" from text and leave intact all"	Example
`sed -n '1,5p;5q' example.txt`	view from 1th to 5th row	Example
`sed -n '5p;5q' example.txt`	view row number 5	Example
`sed -e 's/00*/0/g' example.txt`	replace more zeros with a single zero	Example
`cat -n file1`	number row of a file	Example
`cat example.txt \| awk 'NR%2==1'`	remove all lines even from examplet.xt	Example
`echo a b c \| awk '{print $1}'`	view the first column of line	Example
`echo a b c \| awk '{print $1,$3}'`	view the first and third column of line	Example
`paste file1 file2`	merging contents of two files for columns	Example
`paste -d '+' file1 file2`	merging contents of two files for columns with '+' delimiter on the center	Example
`sort file1 file2`	sort contents of two files	Example
`sort file1 file2 \| uniq`	sort contents of two files omitting lines repeated	Example
`sort file1 file2 \| uniq -u`	sort contents of two files by viewing only unique line	Example
`sort file1 file2 \| uniq -d`	sort contents of two files by viewing only duplicate line	Example
`comm -1 file1 file2`	compare contents of two files by deleting only unique lines from 'file1'	Example
`comm -2 file1 file2`	compare contents of two files by deleting only unique lines from 'file2'	Example
`comm -3 file1 file2`	compare contents of two files by deleting only the lines that appear on both files	Example
Character set and Format file conversion
`dos2unix filedos.txt fileunix.txt`	convert a text file format from MSDOS to UNIX	Example
`unix2dos fileunix.txt filedos.txt`	convert a text file format from UNIX to MSDOS	Example
`recode ..HTML < page.txt > page.html`	convert a text file to html	Example
`recode -l \| more`	show all available formats conversion	Example
Filesystem Analysis
`badblocks -v /dev/hda1`	check bad blocks onto disk hda1	Example
`fsck /dev/hda1`	repair / check integrity of linux filesystem on disk hda1	Example
`fsck.ext2 /dev/hda1`	repair / check integrity of ext2 filesystem on disk hda1	Example
`e2fsck /dev/hda1`	repair / check integrity of ext2 filesystem on disk hda1	Example
`e2fsck -j /dev/hda1`	repair / check integrity of ext3 filesystem on disk hda1	Example
`fsck.ext3 /dev/hda1`	repair / check integrity of ext3 filesystem on disk hda1	Example
`fsck.vfat /dev/hda1`	repair / check integrity of fat filesystem on disk hda1	Example
`fsck.msdos /dev/hda1`	repair / check integrity of dos filesystem on disk hda1	Example
`dosfsck /dev/hda1`	repair / check integrity of dos filesystems on disk hda1	Example
Format a Filesystem
`mkfs /dev/hda1`	create a filesystem type linux on hda1 partition	Example
`mke2fs /dev/hda1`	create a filesystem type linux ext2 on hda1 partition	Example
`mke2fs -j /dev/hda1`	create a filesystem type linux ext3 (journal) on hda1 partition	Example
`mkfs -t vfat 32 F /dev/hda1`	create a FAT32 filesystem	Example
`fdformat -n /dev/fd0`	format a floppy disk	Example
`mkswap /dev/hda3`	create a swap filesystem	Example
SWAP filesystem
`mkswap /dev/hda3`	create a swap filesystem	Example
`swapon /dev/hda3`	activating a new swap partition	Example
`swapon /dev/hda2 /dev/hdb3`	activate two swap partitions	Example
Backup
`dump -0aj f /tmp/home0.bak /home`	make a full backup of directory '/home'	Example
`dump -1aj f /tmp/home0.bak /home`	make a incremental backup of directory '/home'	Example
`restore -if /tmp/home0.bak`	restoring a backup interactively	Example
`rsync -rogpav delete /home /tmp`	synchronization between directories	Example
`rsync -rogpav e ssh delete /home ip_address:/tmp`	rsync via SSH tunnel	Example
`rsync -az e ssh delete ip_addr:/home/public /home/local`	synchronize a local directory with a remote directory via ssh and compression	Example
`rsync -az -e ssh -delete /home/local ip_addr:/home/public`	synchronize a remote directory with a local directory via ssh and compression	Example
`dd bs=1M if=/dev/hda \| gzip \| ssh user@ip_addr 'dd of=hda.gz'`	make a backup of a local hard disk on remote host via sh	Example
`tar -Puf backup.tar /home/user`	make a incremental backup of directory '/home/user'	Example
`( cd /tmp/local/ && tar c . ) \| ssh -C user@ip_addr 'cd /home/share/ && tar x -p'`	copy content of a directory on remote directory via ssh	Example
`( tar c /home ) \| ssh -C user@ip_addr 'cd /home/backup-home && tar x p'`	copy a local directory on remote directory via ssh	Example
`tar cf - . \| (cd /tmp/backup ; tar xf )`	local copy preserving permits and links from a directoryto another	Example
`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	Example
`find /var/log -name '*.log' \| tar cv -filesfrom= \| bzip2 > log.tar.bz2`	find all files with '.log' extention and make an bzip archive	Example
`dd if=/dev/hda of=/dev/fd0 bs=512 count=1`	make a copy of MBR (Master Boot Record) to floppy	Example
`dd if=/dev/fd0 of=/dev/hda bs=512 count=1`	restore MBR from backup copy saved to floppy	Example
CDROM
`cdrecord -v gracetime=2 dev=/dev/cdrom -eject blank=fast -force`	clean a rewritable cdrom	Example
`mkisofs /dev/cdrom > cd.iso`	create an iso image of cdrom on disk	Example
`mkisofs /dev/cdrom \| gzip > cd_iso.gz`	create a compressed iso image of cdrom on disk	Example
`mkisofs -J -allowleadingdots -R -V Label CD" -iso-level 4 -o ./cd.iso data_cd`"	create an iso image of a directory	Example
`cdrecord -v dev=/dev/cdrom cd.iso`	burn an ISO image	Example
`gzip -dc cd_iso.gz \| cdrecord dev=/dev/cdrom`	burn a compressed ISO image	Example
`mount -o loop cd.iso /mnt/iso`	mount an ISO image	Example
`cd-paranoia B`	rip audio tracks from a CD to wav files	Example
`cd-paranoia 3"`"	rip first three audio tracks froma CD to wav files	Example
`cdrecord -scanbus`	scan bus to identify the channel scsi	Example
Networking - LAN and WiFi
`ifconfig eth0`	show configuration ofan ethernet network card	Example
`ifup eth0`	activate an interface 'eth0'	Example
`ifdown eth0`	disable an interface 'eth0'	Example
`ifconfig eth0 192.168.1.1 netmask 255.255.255.0`	configure IP Address	Example
`ifconfig eth0 promisc`	configure 'eth0' inpromiscuous mode to gather packets (sniffing)	Example
`dhclient eth0`	active interface 'eth0' in dhcp mode	Example
`route -n`	show routing table	Example
`route add -net 0/0 gw IP_Gateway`	configura default gateway	Example
`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'	Example
`route del 0/0 gw IP_gateway`	remove static route	Example
`echo 1" > /proc/sys/net/ipv4/ip_forward`"	activate ip routing	Example
`hostname`	show hostname	Example
`host linuxtutorials.net`	lookup hostname to resolve name to ip addressand viceversa	Example
`ip link show`	show link status of all interfaces	Example
`mii-tool eth0`	show link status of 'eth0'	Example
`ethtool eth0`	show statistics of network card 'eth0'	Example
`netstat -tup`	show all active network connections and their PID	Example
`netstat -tupl`	show all network services listening on the system and their PID	Example
`tcpdump tcp port 80`	show all HTTP traffic	Example
`iwlist scan`	show wireless networks	Example
`iwconfig eth1`	show configuration of a wireless network card	Example
Microsoft Windows networks - SAMBA
`nbtscan ip_addr`	netbios name resolution	Example
`nmblookup -A ip_addr`	netbios name resolution	Example
`smbclient -L ip_addr/hostname`	show remote shares of a windows host	Example
`smbget -Rr smb://ip_addr/share`	like wget can download files from a host windows via smb	Example
`mount -t smbfs o username=user,password=pass //winclient/share /mnt/share`	mount a windows network share	Example
IPTABLES - firewall
`iptables -t filter L`	show all chains of filtering table	Example
`iptables -t nat L`	show all chains of nat table	Example
`iptables -t filter F`	clear all rules from filtering table	Example
`iptables -t nat F`	clear all rules from table nat	Example
`iptables -t filter X`	delete any chains created by user	Example
`iptables -t filter -A INPUT -p tcp -dport telnet -j ACCEPT`	allow telnet connections to input	Example
`iptables -t filter -A OUTPUT -p tcp -dport http -j DROP`	block HTTP connections to output	Example
`iptables -t filter -A FORWARD -p tcp -dport pop3 -j ACCEPT`	allow POP3 connections to forward chain	Example
`iptables -t filter -A INPUT -j LOG -logprefix DROP INPUT"`"	logging sulla chain di input Logging on chain input	Example
`iptables -t nat A POSTROUTING o eth0 j MASQUERADE`	configure a PAT (Port Address Traslation) on eth0 masking outbound packets	Example
`iptables -t nat A PREROUTING d 192.168.0.1 p tcp -m tcp -dport 22 -j DNAT -todestination 10.0.0.2:22`	redirect packets addressed to a host to another host	Example
Monitoring and debugging
`top`	display linux tasks using most cpu	Example
`ps -eafw`	displays linux tasks	Example
`ps -e o pid,args forest`	displays linux tasks in a hierarchical mode	Example
`pstree`	mostra un albero dei processi sistema Shows a tree system processes	Example
`kill -9 ID_Processo`	force closure of the process and finish it	Example
`kill -1 ID_Processo`	force a process to reload configuration	Example
`lsof -p $$`	display a list of files opened by processes	Example
`lsof /home/user1`	displays a list of open files in a given path system	Example
`strace -c ls >/dev/null`	display system calls made and received by a process	Example
`strace -f e open ls >/dev/null`	display library calls	Example
`watch -n1 'cat /proc/interrupts'`	display interrupts in real-time	Example
`last reboot`	show history reboot	Example
`lsmod`	display kernel loaded	Example
`free -m`	displays status of RAM in megabytes	Example
`smartctl -A /dev/hda`	monitoring reliability of a hard-disk through SMART	Example
`smartctl -i /dev/hda`	check if SMART is active on a hard-disk	Example
`tail /var/log/dmesg`	show events inherent to the process of booting kernel	Example
`tail /var/log/messages`	show system events	Example
Other useful commands
`apropos …keyword`	display a list of commands that pertainto keywords of a program , useful when youknow what your program does, but you don't know the name of the command	Example
`man ping`	display the on-line manual pages for example on ping command	Example
`whatis …keyword`	displays description of what a program does	Example
mkbootdisk -device /dev/fd0 `uname r`	create a boot floppy	Example
`gpg -c file1`	encrypt a file with GNU Privacy Guard	Example
`gpg file1.gpg`	decrypt a file with GNU Privacy Guard	Example
`wget -r www.example.com`	download an entire web site	Example
`wget -c www.example.com/file.iso`	download a file with the ability to stop the download and resume later	Example
`echo 'wget -c www.example.com/files.iso' \| at 09:00`	start a download at any given time	Example
`ldd ssh`	show shared libraries required by ssh program	Example
`alias hh='history'`	set an alias for a command - hh = history	Example
`$ ip a`	Show current IP address		Check the Current IP Address