System information

Command

Description

# arch

show architecture of machine(1)

# cal 2007

show the timetable of 2007

# 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 041217002007.00

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(2)

# uname -r

show used kernel version

Shutdown, Restart and Logout of a system

Command

Description

# init 0

shutdown system(2)

# logout

leaving session

# reboot

reboot(2)

# shutdown -h now

shutdown system(1)

# shutdown -h 16:30 &

planned shutdown of the system at 16:30

# shutdown -c

cancel a planned shutdown of the system

# shutdown -r now

reboot(1)

# telinit 0

shutdown system(3)

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 0712250000 file1

modify timestamp of a file or directory - (YYMMDDhhmm)

# tree

show files and directories in a tree starting from root(1)

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 the used space by rpm packages installed sorted by size (fedora, redhat and alike)

Users and Groups

Command

Description

# chage -E 2005-12-31 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 as description, group and other

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 (or

# 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 alike)

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 alike)

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 Manipulation

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 user@ip_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 user@ip_addr 'cd /home/share/ && tar x -p'

copy content of a directory on remote directory via ssh

# ( tar c /home ) | ssh -C user@ip_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 temporarily

# 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 and 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

mostra un albero dei processi sistema 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

Others 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