Filesystem
cd [-] [directory] ·Change directory
- :Change to the previous directory you were in
chmod [opts] ·Change permissions
-R :Change permissions recursively
chown [-R] [.group] ·Change ownership
-R :Change ownership recursively
cp [opts] ·Copy files and directories
-i :Interactive mode. Prompt before overwriting
-p :Preserve file permissions and ownership
-R :Copy directories recursively
df [opts] [device name] ·Print filesystem usage info
-a :Show all filesystems.
-h :Human readable format. Quantify byte information.
-i :Show inode usage info.
du [opts] [pattern] ·Show space usage on files and dirs
-c :Produce a grand total for all arguments.
-h :Human readable format. Quantify byte information.
-s :Summarize. Only show a total for each argument.
-S :Do not include size of subdirectories.
find [-name ‘pattern'] ·Search for a file.
Ex: find /home –name ‘index.html’ –perms –644 -ls
gzip [opts] ·Compress a file or files
-1..9 : Set compression level. 9=highest, 1=lowest.
-d :Decompress file. Same as the gunzip command.
-l :List the statistics for a compressed file.
ln [opts] ·Create a sym/hard link
-s :Create a symbolic link between files. (alias name)
-f :Force creation, even if the link file exists.
ls [opts] [pattern] ·List file and directory entries
-a :List all files including . and ..
-d :List directories themselves, not their contents.
-l :Long list. Shows permissions and modified time.
-r :Recusively list files in directories.
-S :Sort output by file size.
-u :Sort by the last access time.
-X :Sort by filename extension.
-1 :Print output files one per line.
--time=atime :Show last access timestamp for file.
mkdir [opts] ·Make a new directory
-m mode: Set the initial permissions to mode.
-p :Create parent directories if they don’t exist.
mv [-i] ·Move/Rename a file
-i :Interactive move. Prompt before moving files.
rm [opts] ·Remove a file
-f :Force removal. Don’t ask if it’s ok to remove.
-i :Interactive remove. Prompt before each file.
-r :Recusively delete directories an their contents.
tar [opts] [tarfile] [pattern] ·Create an archive
c :Create mode. Create a tar archive.
x :Extract mode. Untar archive contents.
t :List mode. List the contents of the archive.
f :Specify a tarfile to use.
v :Verbose mode. Show files being added or untared.
z :Compress. Filter input/output through gzip.
touch [opts] ·Update the timestamp on a file
-a :Only change the access time on the file.
-t :Specify a timestamp to use instead of current time

Informational
cat [opts] [filepattern] ·Print file contents on STOUT
-E :Display a $ at the end of each line.
-T :Show tabs as ^I.
-v :Show non-printing characters.
date [opts] ·Print or set the system date and time
--date=STRING :display time described by STRING.
--set=STRING :set time described by STRING.
dmesg [opts] ·Print or control the kernel ring buffer
-c :Clear the contents of the ring buffer.
file [opts] [filepattern] ·Determine the file type
-z :Try to look inside compressed files.
finger [opts] [userpattern] ·Show info about system users
-m :Match the exact username specified.
free [opts] ·Display free and used memory in the system
-b :Display the information in bytes.
hexdump [opts] ·Show all the characters of a file
-c :Display the input offset in hexidecimal
last [opts] [username] ·Show last system logins for users
-num :Show last num of sessions.
-a :Display the hostname in the last column.
-d :Translates IP numbers to their hostname.
-f :Use file as last log.
less [opts] [filepattern] ·View a file a page at a time
-i :Do case insensitive searching.
-S :Don’t wrap long lines.
man [opts] [section] ·View online manual pages.
-a :View all available manual pages for name.
-k string :Search for the specified string.
md5sum [opts] [filepattern] ·Show the uniqueness of files
ps [opts] ·Show what processes are running on the system
a :Select all processes on a terminal.
u :Display user oriented format. More columns.
x :Select processes without a controlling TTY.
w :Show an extra line of process entry per w.
Ex: ps auxwww =Displays all process information on system.
quota [opts] [user] ·Display disk usage and limits
-v :Display filesystems where no quota is set.
time [opts] [command] ·Show resource usage for a command
top [opts] ·Display top CPU processes every X seconds
-d sec :Set the delay to sec seconds before refreshing.
umask [opts] [mode] ·Set the default file permissions
-S :Show current symbolic umask.
uname [opts] ·Show OS and system information
-a :Show everything
uptime ·Show system uptime and load
w [opts] [user] ·Show who is logged in/what they are doing
whereis [command] ·Locate the related files for a command
which [command] ·Show full path to the specified command
who [opts] [args] ·Show who is logged in

Text Filtering / Mutative
awk [opts] [exp] ·pattern scanning and processing language
-Ffs :Set the field separator for commands.
Ex: cat access_log | awk {‘print $1’} (prints hostnames)
Do a ‘man awk’ for more information and examples.
comm [opts] [file1] [file2] ·Compare two sorted files
-1 :Suppress lines unique to left file.
-2 :Suppress lines unique to right file.
-3 :Supress lines unique to both files.
csplit [opts] [file] [pattern] ·Split a file on context
-f prefix :Use prefix instead of xx in output filenames.
-n digits :Use digits number of digits instead of 2.
-z :Remove empty output files.
Ex: csplit mailspoolfile "/^From /" {*}
cut [opts] [filepattern] ·Remove sections from each line
-c range :Output only the characters in range
Ex: cut –c 1-80 file (truncate lines at 80 characters)
diff [opts] [file1] [file2] ·Differentiate two files
Ex: diff program-old.c program.c > program.patch
echo [opts] [string] ·Print a line of text
-e :Enable interpretation of backslashed sequences.
-n :Don’t automatically insert a newline character.
grep [opts] [pattern] [file] ·Print lines matching pattern
-B num :Print num lines of leading context on matches.
-C num :Print num lines of trailing context on matches.
-E :Interpret pattern as an extended regular expression
-i :Do case insensitve matching.
-l :Just print the files that match the pattern.
-r :Read all files under each directory recursively.
-v :Print the lines that don’t match pattern.
head [opts] [file] ·Print the first part of a file
-n num :Print the first num lines instead of the first 10.
nl [opts] [file] ·Number the lines of a file
patch [opts] <[patchfile] ·Patch a file using a diff file
sed [expression] [file] ·Stream editor
Ex: cat file | sed ‘s/frompattern/topattern/’ > output
sort [opts] [file] ·Sort lines of text files
-n :Compare according to string numerical value.
-r :Reverse the result of comparisons.
split [opts] [file] ·Split a file into pieces
-l num :Put num lines per output file.
tail [opts] [file] ·Print the last lines of a file
-f :Output appended data as the file grows.
-n num :Print last num lines of a file instead of last 10
tr [opts] [set2] ·Translate char. from set1 to set2
Ex: cat index.html | tr A-Z a-z > index-new.html
uniq [opts] [input] [output] ·Remove duplicate lines
-c :Prefix lines with number of occurances.
-d :Only print duplicated lines.
-u :Only print unique lines.
wc [opts] [file] ·Print the number of lines in files, etc.
-m :Print the character count.
-l :Print the line count.
-w :Print the word count.
-L :Print the length of the longest line.

Network
ifconfig [devicename] [action] [options]
ipchains [opts] ·Manip. ipchains firewall(kernel 2.2+)
iptables [opts] ·Manip. iptables firewall(kernel 2.4+)
mail [opts] [address] ·Send mail from the command line
-s subject :Specify the subject as subject.
-c list :Send carbon copy to list of users.
-b list :Send blind carbon copy to list of users.
Ex: echo “Meet me at noon.” | mail –s “Reminder” –c \
bob@company.com,suzy@company.com jack@company.com
netstat [opts] ·Print network connections and info
-a :Show both listening and non-listening sockets.
-n :Do not attempt to resolve IP addresses.
-t :Only show tcp socket connection table.
ping [opts] [host] ·Send ICMP packets to network hosts
-c count :Send count number of packets and then quit.
-i sec :Wait sec seconds between sending packets.
route [opts] [target] ·Show/Manipulate IP routing table
-n :Show numerical addresses instead of hostnames.
scp [opts] [[host:]fromfile] [[host:]to] · Secure copy
-C :Compresses the data that is sent over the session.
-r :Recursively copy directories.
ssh [opts] [[user@]host] [command] · Secure shell/login
-C :Compresses the data that is sent over the session.
sniffit [opts] ·Record TCP network traffic
-i :Interactive mode. Shows all traffic.
-l :Set the maximum sniff length to length.
-p :Specify the port number on the target host.
-s :Set the source IP from which packets come.
-t :Set the IP to which packets are going.
tcpdump [opts] [expression] ·Dump traffic on a network
telnet [opts] [host] [port] ·Open TCP socket to a host
-n : Opens file for recording trace information.
-x :Turns on encryption of the data stream if possible.
traceroute [opts] [host] ·Show the route packets take
-n :Don’t do DNS lookups of the IP addresses.
wget [opts] [URL] ·Make a HTTP request from the shell
-r :Recursive get the URL and all it’s links.
-k :Convert the non-relative links to relative ones.
whois [opts] ·Query a whois database
Ex: whois domain.com
whois domain.com@whois.networksolutions.com
whois 127.0.0.1@arin.net
Bash Shell
> ·Send STDOUT to a file. overwrite/create a file
Ex: ls –l > list-of-files.txt
>newfile
>> ·Send STDOUT to a file, appending to te end of the file
Ex: ps aux > pslog.txt
date >> pslog.txt
| ·Send the STDOUT from a command to the STDIN of another
Ex: cat listofnames | sort
cat access_log | awk {‘print $1’} | sort | uniq
2> ·Send STDERR to a file, overwriting the filename
Ex: startx 2> .errorlog
alias ·Create a command alias in the shell
Ex: alias ls=’ls –la –-color=auto’
cd [-] [directory] ·Change the current working directory
- :Change to the previous directory you were in.
clear ·Clear the terminal display
env [opts] [command] ·Run command in modified environment
export [opts] [variable] ·Export an environment variable
Ex: export TERM USERNAME PS1 MAILSPOOL
export TERM=vt100
for ·Execute sequence of commands for a list of items
Ex: for i in *.mp3 ; do mpg123 $i ; done
for n in 1 2 3 4 ; do mkdir $n ; done
history ·Show the command history up til now
nice [opts] [command] ·Set the OS process priority
Ex: nice 19 gzip access_log (lowest priority on Linux)
Ex: nice –20 kswapd (real time priority on Linux)
pwd ·Print out the current working directory
renice [opts] ·Change priority of a running process
-p PID: Specify a process id to renice
Ex: $ ps auxw | grep gzip
6319 ? S 0:20 gzip bigfile.txt
$ renice 19 –p 6319
reset ·Initializes the terminal as if you just logged in
set ·Set a shell option or variable (run ‘help set’)
sleep ·Pause for specified period before continuing
Ex: ps aux ; sleep 3600 ; ps aux
umask ·Set the default file permissions
Ex: umask 022 (files will be created 644 by default.) while ·A loop that runs commands while a condition is true
Ex: while (true) ; do ps auxw ; sleep 1m ; done > pslog
xargs [opts] [command] ·Execute a command for each arg
-n number :How many arguments to give each command run.
-p :Prompt the user before each command is run.
Admin
adduser [opts] ·Add a user to the local system
-d : Set the home directory for the user to dir.
-g : Set the primary group for the user to group.
-G : Set additional groups for the user.
-s : Set the default shell for the user to shell.
chfn ·Change the finger information for a user
chsh ·Change the shell used for the user
edquota [opts] ·Edit a user’s or group’s quota
-g : Edit the group quota instead of user quota.
kill [-signal] ·Terminate a process/Send it a signal
-HUP,-1 : Signal usually makes process to reread config.
-9 :Send a SIGKILL, process must die.
-l :Print a list of signal names and numbers.
killall [-signal] [name]·Kill processes by name
-e :Require an exact name of a process.
-i :Interactively ask for confirmation before killing.
ldd [opts] [program] ·Show a programs library dependencies
mount [opts] [mountpoint] ·Mount a file system
-o : Specify options for mounting. Listed below.
loop - Mount a disk file such as a CD-ROM image or floppy image.
remount – Remount the filesystem with new options
ro, rw - Mount filesystem in readonly or read-write mode.
user - Allow normal users to mount this filesystem.
-r :Mount the filesystem read-only. Same as ‘–o ro’
-t :Specify the type of filesystem to mount.
ext2, ext3 – Native Linux partition types.
vfat – Windows 9x 32-bit partition type.
iso9660 – CD-ROM filesystem.
nfs – Network remote filesystem.
passwd [opts] [username]·Change user’s system password
-l :Lock the password for the account.
-u :Unlock the password for the account.
-S :Show the status of the password for the account.
su [-] [username] ·Switch users or login as the superuser
- :Make shell a login shell
-c : Run command as username.
umount [opts] [path/device]·Unmount a mounted filesystem
-f :Force unmounting (in case of unreachable NFS system).
†Most commands accept the use of –v, -h or –help for displaying verbose information and help information.
‡Commands and options displayed in red can only be used by the superuser (root).