Advance Linux Commands
System information
Command Description
# arch show architecture of machine(1) [man]
# cal 2007 show the timetable of 2007 [man]
# cat /proc/cpuinfo show information CPU info [man]
# cat /proc/interrupts show interrupts [man]
# cat /proc/meminfo verify memory use [man]
# cat /proc/swaps show file(s) swap [man]
# cat /proc/version show version of the kernel [man]
# cat /proc/net/dev show network adpters and statistics [man]
# cat /proc/mounts show mounted file system(s) [man]
# clock -w save date changes on BIOS [man]
# date show system date [man]
# date 041217002007.00 set date and time - MonthDayhoursMinutesYear.Seconds [man]
# dmidecode -q show hardware system components - (SMBIOS / DMI) [man]
# hdparm -i /dev/hda displays the characteristics of a hard-disk [man]
# hdparm -tT /dev/sda perform test reading on a hard-disk [man]
# lspci -tv display PCI devices [man]
# lsusb -tv show USB devices [man]
# uname -m show architecture of machine(2) [man]
# uname -r show used kernel version [man]
Shutdown, Restart and Logout of a system
Command Description
# init 0 shutdown system(2) [man]
# logout leaving session [man]
# reboot reboot(2) [man]
# shutdown -h now shutdown system(1) [man]
# shutdown -h 16:30 & planned shutdown of the system at 16:30 [man]
# shutdown -c cancel a planned shutdown of the system [man]
# shutdown -r now reboot(1) [man]
# telinit 0 shutdown system(3) [man]
Files and Directory
Command Description
# cd /home enter to directory '/ home' [man]
# cd .. go back one level [man]
# cd ../.. go back two levels [man]
# cd go to home directory [man]
# cd ~user1 go to home directory [man]
# cd - go to previous directory [man]
# cp file1 file2 copying a file [man]
# cp dir/* . copy all files of a directory within the current work directory [man]
# cp -a /tmp/dir1 . copy a directory within the current work directory [man]
# cp -a dir1 dir2 copy a directory [man]
# cp file file1 outputs the mime type of the file as text [man]
# iconv -l lists known encodings [man]
# iconv -f fromEncoding -t toEncoding inputFile > outputFile converting the coding of characters from one format to another [man]
# 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) [man]
# ln -s file1 lnk1 create a symbolic link to file or directory [man]
# ln file1 lnk1 create a physical link to file or directory [man]
# ls view files of directory [man]
# ls -F view files of directory [man]
# ls -l show details of files and directory [man]
# ls -a show hidden files [man]
# ls *[0-9]* show files and directory containing numbers [man]
# lstree show files and directories in a tree starting from root(2) [man]
# mkdir dir1 create a directory called 'dir1' [man]
# mkdir dir1 dir2 create two directories simultaneously [man]
# mkdir -p /tmp/dir1/dir2 create a directory tree [man]
# mv dir1 new_dir rename / move a file or directory [man]
# pwd show the path of work directory [man]
# rm -f file1 delete file called 'file1' [man]
# rm -rf dir1 remove a directory called 'dir1' and contents recursively [man]
# rm -rf dir1 dir2 remove two directories and their contents recursively [man]
# rmdir dir1 delete directory called 'dir1' [man]
# touch -t 0712250000 file1 modify timestamp of a file or directory - (YYMMDDhhmm) [man]
# tree show files and directories in a tree starting from root(1) [man]
File search
Command Description
# find / -name file1 search file and directory into root filesystem from '/' [man]
# find / -user user1 search files and directories belonging to 'user1' [man]
# find /home/user1 -name \*.bin search files with '. bin' extension within directory '/ home/user1' [man]
# find /usr/bin -type f -atime +100 search binary files are not used in the last 100 days [man]
# find /usr/bin -type f -mtime -10 search files created or changed within 10 days [man]
# find / -name *.rpm -exec chmod 755 '{}' \; search files with '.rpm' extension and modify permits [man]
# find / -xdev -name \*.rpm search files with '.rpm' extension ignoring removable partitions as cdrom, pen-drive, etc.… [man]
# locate \*.ps find files with the '.ps' extension - first run 'updatedb' command [man]
# whereis halt show location of a binary file, source or man [man]
# which halt show full path to a binary / executable [man]
Mounting a Filesystem
Command Description
# fuser -km /mnt/hda2 force umount when the device is busy [man]
# mount /dev/hda2 /mnt/hda2 mount disk called hda2 - verify existence of the directory '/ mnt/hda2' [man]
# mount /dev/fd0 /mnt/floppy mount a floppy disk [man]
# mount /dev/cdrom /mnt/cdrom mount a cdrom / dvdrom [man]
# mount /dev/hdc /mnt/cdrecorder mount a cdrw / dvdrom [man]
# mount /dev/hdb /mnt/cdrecorder mount a cdrw / dvdrom [man]
# mount -o loop file.iso /mnt/cdrom mount a file or iso image [man]
# mount -t vfat /dev/hda5 /mnt/hda5 mount a Windows FAT32 file system [man]
# mount /dev/sda1 /mnt/usbdisk mount a usb pen-drive or flash-drive [man]
# mount -t smbfs -o username=user,password=pass //WinClient/share /mnt/share mount a windows network share [man]
# umount /dev/hda2 unmount disk called hda2 - exit from mount point '/ mnt/hda2' first [man]
# 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 [man]
Disk Space
Command Description
# df -h show list of partitions mounted [man]
# 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) [man]
# du -sh dir1 estimate space used by directory 'dir1' [man]
# du -sk * | sort -rn show size of the files and directories sorted by size [man]
# ls -lSr |more show size of the files and directories ordered by size [man]
# rpm -q -a --qf '%10{SIZE}t%{NAME}n' | sort -k1,1n show the used space by rpm packages installed sorted by size (fedora, redhat and alike) [man]
Users and Groups
Command Description
# chage -E 2005-12-31 user1 set deadline for user password [man]
# groupadd [group] create a new group [man]
# groupdel [group] delete a group [man]
# groupmod -n moon sun rename a group from moon to sun [man]
# grpck check correct syntax and file format of '/etc/group' and groups existence [man]
# newgrp - [group] log into a new group to change default group of newly created files [man]
# passwd change password [man]
# passwd user1 change a user password (only by root) [man]
# pwck check correct syntax and file format of '/etc/passwd' and users existence [man]
# useradd -c "User Linux" -g admin -d /home/user1 -s /bin/bash user1 create a new user "user1" belongs "admin" group [man]
# useradd user1 create a new user [man]
# userdel -r user1 delete a user ( '-r' eliminates home directory) [man]
# usermod -c "User FTP" -g system -d /ftp/user1 -s /bin/nologin user1 change user attributes as description, group and other [man]
Permits on Files
Command Description
# chgrp group1 file1 change group of files [man]
# chmod ugo+rwx directory1 set permissions reading (r), write (w) and (x) access to users owner (u) group (g) and others (o) [man]
# chmod go-rwx directory1 remove permits reading (r), write (w) and (x) access to users group (g) and others (or [man]
# chmod u+s /bin/file1 set SUID bit on a binary file - the user that running that file gets same privileges as owner [man]
# chmod u-s /bin/file1 disable SUID bit on a binary file [man]
# chmod g+s /home/public set SGID bit on a directory - similar to SUID but for directory [man]
# chmod g-s /home/public disable SGID bit on a directory [man]
# chmod o+t /home/public set STIKY bit on a directory - allows files deletion only to legitimate owners [man]
# chmod o-t /home/public disable STIKY bit on a directory [man]
# chown user1 file1 change owner of a file [man]
# chown -R user1 directory1 change user owner of a directory and all the files and directories contained inside [man]
# chown user1:group1 file1 change user and group ownership of a file [man]
# find / -perm -u+s view all files on the system with SUID configured [man]
# ls -lh show permits on files [man]
# ls /tmp | pr -T5 -W$COLUMNS divide terminal into 5 columns [man]
Special Attributes on files
Command Description
# chattr +a file1 allows write opening of a file only append mode [man]
# chattr +c file1 allows that a file is compressed / decompressed automatically by the kernel [man]
# chattr +d file1 makes sure that the program ignores Dump the files during backup [man]
# chattr +i file1 makes it an immutable file, which can not be removed, altered, renamed or linked [man]
# chattr +s file1 allows a file to be deleted safely [man]
# chattr +S file1 makes sure that if a file is modified changes are written in synchronous mode as with sync [man]
# chattr +u file1 allows you to recover the contents of a file even if it is canceled [man]
# lsattr show specials attributes [man]
Archives and compressed files
Command Description
# bunzip2 file1.bz2 decompress a file called 'file1.bz2' [man]
# bzip2 file1 compress a file called 'file1' [man]
# gunzip file1.gz decompress a file called 'file1.gz' [man]
# gzip file1 compress a file called 'file1' [man]
# gzip -9 file1 compress with maximum compression [man]
# rar a file1.rar test_file create an archive rar called 'file1.rar' [man]
# rar a file1.rar file1 file2 dir1 compress 'file1', 'file2' and 'dir1' simultaneously [man]
# rar x file1.rar decompress rar archive [man]
# tar -cvf archive.tar file1 create a uncompressed tarball [man]
# tar -cvf archive.tar file1 file2 dir1 create an archive containing 'file1', 'file2' and 'dir1' [man]
# tar -tf archive.tar show contents of an archive [man]
# tar -xvf archive.tar extract a tarball [man]
# tar -xvf archive.tar -C /tmp extract a tarball into / tmp [man]
# tar -cvfj archive.tar.bz2 dir1 create a tarball compressed into bzip2 [man]
# tar -xvfj archive.tar.bz2 decompress a compressed tar archive in bzip2 [man]
# tar -cvfz archive.tar.gz dir1 create a tarball compressed into gzip [man]
# tar -xvfz archive.tar.gz decompress a compressed tar archive in gzip [man]
# unrar x file1.rar decompress rar archive [man]
# unzip file1.zip decompress a zip archive [man]
# zip file1.zip file1 create an archive compressed in zip [man]
# zip -r file1.zip file1 file2 dir1 compress in zip several files and directories simultaneously [man]
RPM Packages ( Fedora, Red Hat and like)
Command Description
# rpm -ivh [package.rpm] install a rpm package [man]
# rpm -ivh --nodeeps [package.rpm] install a rpm package ignoring dependencies requests [man]
# rpm -U [package.rpm] upgrade a rpm package without changing configuration files [man]
# rpm -F [package.rpm] upgrade a rpm package only if it is already installed [man]
# rpm -e [package] remove a rpm package [man]
# rpm -qa show all rpm packages installed on the system [man]
# rpm -qa | grep httpd show all rpm packages with the name "httpd" [man]
# rpm -qi [package] obtain information on a specific package installed [man]
# rpm -qg "System Environment/Daemons" show rpm packages of a group software [man]
# rpm -ql [package] show list of files provided by a rpm package installed [man]
# rpm -qc [package] show list of configuration files provided by a rpm package installed [man]
# rpm -q [package] --whatrequires show list of dependencies required for a rpm packet [man]
# rpm -q [package] --whatprovides show capability provided by a rpm package [man]
# rpm -q [package] --scripts show scripts started during installation / removal [man]
# rpm -q [package] --changelog show history of revisions of a rpm package [man]
# rpm -qf /etc/httpd/conf/httpd.conf verify which rpm package belongs to a given file [man]
# rpm -qp [package.rpm] -l show list of files provided by a rpm package not yet installed [man]
# rpm --import /media/cdrom/RPM-GPG-KEY import public-key digital signature [man]
# rpm --checksig [package.rpm] verify the integrity of a rpm package [man]
# rpm -qa gpg-pubkey verify integrity of all rpm packages installed [man]
# rpm -V [package] check file size, permissions, type, owner, group, MD5 checksum and last modification [man]
# rpm -Va check all rpm packages installed on the system - use with caution [man]
# rpm -Vp [package.rpm] verify a rpm package not yet installed [man]
# rpm -ivh /usr/src/redhat/RPMS/`arch`/[package.rpm] install a package built from a rpm source [man]
# rpm2cpio [package.rpm] | cpio --extract --make-directories *bin* extract executable file from a rpm package [man]
# rpmbuild --rebuild [package.src.rpm] build a rpm package from a rpm source [man]
YUM packages tool (Fedora, RedHat and alike)
Command Description
# yum -y install [package] download and install a rpm package [man]
# yum localinstall [package.rpm] That will install an RPM, and try to resolve all the dependencies for you using your repositories. [man]
# yum -y update update all rpm packages installed on the system [man]
# yum update [package] upgrade a rpm package [man]
# yum remove [package] remove a rpm package [man]
# yum list list all packages installed on the system [man]
# yum search [package] find a package on rpm repository [man]
# yum clean [package] clean up rpm cache erasing downloaded packages [man]
# yum clean headers remove all files headers that the system uses to resolve dependency [man]
# yum clean all remove from the cache packages and headers files [man]
DEB packages (Debian, Ubuntu and like)
Command Description
# dpkg -i [package.deb] install / upgrade a deb package [man]
# dpkg -r [package] remove a deb package from the system [man]
# dpkg -l show all deb packages installed on the system [man]
# dpkg -l | grep httpd show all deb packages with the name "httpd" [man]
# dpkg -s [package] obtain information on a specific package installed on system [man]
# dpkg -L [package] show list of files provided by a package installed on system [man]
# dpkg --contents [package.deb] show list of files provided by a package not yet installed [man]
# dpkg -S /bin/ping verify which package belongs to a given file [man]
APT packages tool (Debian, Ubuntu and alike)
Command Description
# apt-cache search [package] returns list of packages which corresponds string "searched-packages" [man]
# apt-cdrom install [package] install / upgrade a deb package from cdrom [man]
# apt-get install [package] install / upgrade a deb package [man]
# apt-get update update the package list [man]
# apt-get upgrade upgrade all of the installed packages [man]
# apt-get remove [package] remove a deb package from system [man]
# apt-get check verify correct resolution of dependencies [man]
# apt-get clean clean up cache from packages downloaded [man]
Pacman packages tool (Arch, Frugalware and alike)
Command Description
# pacman -S name Install package 'name' with dependencies [man]
# pacman -R name Delete package 'name' and all files of it [man]
View file content
Command Description
# cat file1 view the contents of a file starting from the first row [man]
# head -2 file1 view first two lines of a file [man]
# less file1 similar to 'more' command but which allows backward movement in the file as well as forward movement [man]
# more file1 view content of a file along [man]
# tac file1 view the contents of a file starting from the last line [man]
# tail -2 file1 view last two lines of a file [man]
# tail -f /var/log/messages view in real time what is added to a file [man]
Text Manipulation
Command Description
# cat example.txt | awk 'NR%2==1' remove all even lines from example.txt [man]
# echo a b c | awk '{print $1}' view the first column of a line [man]
# echo a b c | awk '{print $1,$3}' view the first and third column of a line [man]
# cat -n file1 number row of a file [man]
# comm -1 file1 file2 compare contents of two files by deleting only unique lines from 'file1' [man]
# comm -2 file1 file2 compare contents of two files by deleting only unique lines from 'file2' [man]
# comm -3 file1 file2 compare contents of two files by deleting only the lines that appear on both files [man]
# diff file1 file2 find differences between two files [man]
# grep Aug /var/log/messages look up words "Aug" on file '/var/log/messages' [man]
# grep ^Aug /var/log/messages look up words that begin with "Aug" on file '/var/log/messages' [man]
# grep [0-9] /var/log/messages select from file '/var/log/messages' all lines that contain numbers [man]
# grep Aug -R /var/log/* search string "Aug" at directory '/var/log' and below [man]
# paste file1 file2 merging contents of two files for columns [man]
# paste -d '+' file1 file2 merging contents of two files for columns with '+' delimiter on the center [man]
# sdiff file1 file2 find differences between two files and merge interactively alike "diff" [man]
# sed 's/string1/string2/g' example.txt replace "string1" with "string2" in example.txt [man]
# sed '/^$/d' example.txt remove all blank lines from example.txt [man]
# sed '/ *#/d; /^$/d' example.txt remove comments and blank lines from example.txt [man]
# sed -e '1d' exampe.txt eliminates the first line from file example.txt [man]
# sed -n '/string1/p' view only lines that contain the word "string1" [man]
# sed -e 's/ *$//' example.txt remove empty characters at the end of each row [man]
# sed -e 's/string1//g' example.txt remove only the word "string1" from text and leave intact all [man]
# sed -n '1,5p' example.txt print from 1th to 5th row of example.txt [man]
# sed -n '5p;5q' example.txt print row number 5 of example.txt [man]
# sed -e 's/00*/0/g' example.txt replace more zeros with a single zero [man]
# sort file1 file2 sort contents of two files [man]
# sort file1 file2 | uniq sort contents of two files omitting lines repeated [man]
# sort file1 file2 | uniq -u sort contents of two files by viewing only unique line [man]
# sort file1 file2 | uniq -d sort contents of two files by viewing only duplicate line [man]
# echo 'word' | tr '[:lower:]' '[:upper:]' convert from lower case in upper case [man]
Character set and Format file conversion
Command Description
# dos2unix filedos.txt fileunix.txt convert a text file format from MSDOS to UNIX [man]
# recode ..HTML < page.txt > page.html convert a text file to html [man]
# recode -l | more show all available formats conversion [man]
# unix2dos fileunix.txt filedos.txt convert a text file format from UNIX to MSDOS [man]
Filesystem Analysis
Command Description
# badblocks -v /dev/hda1 check bad blocks on disk hda1 [man]
# dosfsck /dev/hda1 repair / check integrity of dos filesystems on disk hda1 [man]
# e2fsck /dev/hda1 repair / check integrity of ext2 filesystem on disk hda1 [man]
# e2fsck -j /dev/hda1 repair / check integrity of ext3 filesystem on disk hda1 [man]
# fsck /dev/hda1 repair / check integrity of linux filesystem on disk hda1 [man]
# fsck.ext2 /dev/hda1 repair / check integrity of ext2 filesystem on disk hda1 [man]
# fsck.ext3 /dev/hda1 repair / check integrity of ext3 filesystem on disk hda1 [man]
# fsck.vfat /dev/hda1 repair / check integrity of fat filesystem on disk hda1 [man]
# fsck.msdos /dev/hda1 repair / check integrity of dos filesystem on disk hda1 [man]
Format a Filesystem
Command Description
# fdformat -n /dev/fd0 format a floppy disk [man]
# mke2fs /dev/hda1 create a filesystem type linux ext2 on hda1 partition [man]
# mke2fs -j /dev/hda1 create a filesystem type linux ext3 (journal) on hda1 partition [man]
# mkfs /dev/hda1 create a filesystem type linux on hda1 partition [man]
# mkfs -t vfat 32 -F /dev/hda1 create a FAT32 filesystem [man]
# mkswap /dev/hda3 create a swap filesystem [man]
Filesystem SWAP
Command Description
# mkswap /dev/hda3 create a swap filesystem [man]
# swapon /dev/hda3 activating a new swap partition [man]
# swapon /dev/hda2 /dev/hdb3 activate two swap partitions [man]
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 [man]
# 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 [man]
# 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 ssh [man]
# dd if=/dev/sda of=/tmp/file1 backup content of the harddrive to a file [man]
# dd if=/dev/hda of=/dev/fd0 bs=512 count=1 make a copy of MBR (Master Boot Record) to floppy [man]
# dd if=/dev/fd0 of=/dev/hda bs=512 count=1 restore MBR from backup copy saved to floppy [man]
# dump -0aj -f /tmp/home0.bak /home make a full backup of directory '/home' [man]
# dump -1aj -f /tmp/home0.bak /home make a incremental backup of directory '/home' [man]
# restore -if /tmp/home0.bak restoring a backup interactively [man]
# rsync -rogpav --delete /home /tmp synchronization between directories [man]
# rsync -rogpav -e ssh --delete /home ip_address:/tmp rsync via SSH tunnel [man]
# rsync -az -e ssh --delete ip_addr:/home/public /home/local synchronize a local directory with a remote directory via ssh and compression [man]
# rsync -az -e ssh --delete /home/local ip_addr:/home/public synchronize a remote directory with a local directory via ssh and compression [man]
# tar -Puf backup.tar /home/user make a incremental backup of directory '/home/user' [man]
# ( 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 [man]
# ( tar c /home ) | ssh -C user@ip_addr 'cd /home/backup-home && tar x -p' copy a local directory on remote directory via ssh [man]
# tar cf - . | (cd /tmp/backup ; tar xf - ) local copy preserving permits and links from a directory to another [man]
CDROM
Command Description
# cd-paranoia -B rip audio tracks from a CD to wav files [man]
# cd-paranoia -- rip first three audio tracks from a CD to wav files [man]
# cdrecord -v gracetime=2 dev=/dev/cdrom -eject blank=fast -force clean a rewritable cdrom [man]
# cdrecord -v dev=/dev/cdrom cd.iso burn an ISO image [man]
# gzip -dc cd_iso.gz | cdrecord dev=/dev/cdrom - burn a compressed ISO image [man]
# cdrecord --scanbus scan bus to identify the channel scsi [man]
# dd if=/dev/hdc | md5sum perform an md5sum on a device, like a CD [man]
# mkisofs /dev/cdrom > cd.iso create an iso image of cdrom on disk [man]
# mkisofs /dev/cdrom | gzip > cd_iso.gz create a compressed iso image of cdrom on disk [man]
# mkisofs -J -allow-leading-dots -R -V create an iso image of a directory [man]
# mount -o loop cd.iso /mnt/iso mount an ISO image [man]
Networking (LAN / WiFi)
Command Description
# dhclient eth0 active interface 'eth0' in dhcp mode [man]
# ethtool eth0 show network statistics of eth0 [man]
# host www.example.com lookup hostname to resolve name to ip address and viceversa [man]
# hostname show hostname of system [man]
# ifconfig eth0 show configuration of an ethernet network card [man]
# ifconfig eth0 192.168.1.1 netmask 255.255.255.0 configure IP Address [man]
# ifconfig eth0 promisc configure 'eth0' in promiscuous mode to gather packets (sniffing) [man]
# ifdown eth0 disable an interface 'eth0' [man]
# ifup eth0 activate an interface 'eth0' [man]
# ip link show show link status of all network interfaces [man]
# iwconfig eth1 show wireless networks [man]
# iwlist scan wifi scanning to display the wireless connections available [man]
# mii-tool eth0 show link status of 'eth0' [man]
# netstat -tup show all active network connections and their PID [man]
# netstat -tupl show all network services listening on the system and their PID [man]
# netstat -rn show routing table alike "route -n" [man]
# nslookup www.example.com lookup hostname to resolve name to ip address and viceversa [man]
# route -n show routing table [man]
# route add -net 0/0 gw IP_Gateway configure default gateway [man]
# 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' [man]
# route del 0/0 gw IP_gateway remove static route [man]
# echo "1" > /proc/sys/net/ipv4/ip_forward activate ip routing temporarily [man]
# tcpdump tcp port 80 show all HTTP traffic [man]
# whois www.example.com lookup on Whois database [man]
Microsoft Windows networks (samba)
Command Description
# mount -t smbfs -o username=user,password=pass //WinClient/share /mnt/share mount a windows network share [man]
# nbtscan ip_addr netbios name resolution [man]
# nmblookup -A ip_addr netbios name resolution [man]
# smbclient -L ip_addr/hostname show remote shares of a windows host [man]
# smbget -Rr smb://ip_addr/share like wget can download files from a host windows via smb [man]
IPTABLES (firewall)
Command Description
# iptables -t filter -L show all chains of filtering table [man]
# iptables -t nat -L show all chains of nat table [man]
# iptables -t filter -F clear all rules from filtering table [man]
# iptables -t nat -F clear all rules from table nat [man]
# iptables -t filter -X delete any chains created by user [man]
# iptables -t filter -A INPUT -p tcp --dport telnet -j ACCEPT allow telnet connections to input [man]
# iptables -t filter -A OUTPUT -p tcp --dport http -j DROP block HTTP connections to output [man]
# iptables -t filter -A FORWARD -p tcp --dport pop3 -j ACCEPT allow POP3 connections to forward chain [man]
# iptables -t filter -A INPUT -j LOG --log-prefix Logging on input chain [man]
# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE configure a PAT (Port Address Traslation) on eth0 masking outbound packets [man]
# 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 [man]
Monitoring and debugging
Command Description
# free -m displays status of RAM in megabytes [man]
# kill -9 process_id force closure of the process and finish it [man]
# kill -1 process_id force a process to reload configuration [man]
# last reboot show history reboot [man]
# lsmod display kernel loaded [man]
# lsof -p process_id display a list of files opened by processes [man]
# lsof /home/user1 displays a list of open files in a given path system [man]
# ps -eafw displays linux tasks [man]
# ps -e -o pid,args --forest displays linux tasks in a hierarchical mode [man]
# pstree Shows a tree system processes [man]
# smartctl -A /dev/hda monitoring reliability of a hard-disk through SMART [man]
# smartctl -i /dev/hda check if SMART is active on a hard-disk [man]
# strace -c ls >/dev/null display system calls made and received by a process [man]
# strace -f -e open ls >/dev/null display library calls [man]
# tail /var/log/dmesg show events inherent to the process of booting kernel [man]
# tail /var/log/messages show system events [man]
# top display linux tasks using most cpu [man]
# watch -n1 'cat /proc/interrupts' display interrupts in real-time [man]
Others useful commands
Command Description
# alias hh='history' set an alias for a command - hh = history [man]
# 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 [man]
# chsh change shell command [man]
# chsh --list-shells nice command to know if you have to remote into another box [man]
# gpg -c file1 encrypt a file with GNU Privacy Guard [man]
# gpg file1.gpg decrypt a file with GNU Privacy Guard [man]
# ldd /usr/bin/ssh show shared libraries required by ssh program [man]
# man ping display the on-line manual pages for example on ping command - use '-k' option to find any related commands [man]
# mkbootdisk --device /dev/fd0 `uname -r` create a boot floppy [man]
# wget -r www.example.com download an entire web site [man]
# wget -c www.example.com/file.iso download a file with the ability to stop the download and resume later [man]
# echo 'wget -c www.example.com/files.iso' | at 09:00 start a download at any given time [man]
# whatis ...keyword displays description of what a program does [man]
# 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 [man]
