UNIX / Linux Command Summary

access() Used to check the accessibility of files int Access(pathname, access_mode) Char* pathname; int access-mode; The access modes are. 04 read 02 write 01 execute (search) 00 checks existence of a file & operator execute a command as a background process. banner prints the specified string in large letters. Each argument may be upto 10 characters long. break is used to break out of a loop. It does not exit from the program. Cal Produces a calender of the current month as standard output. The month (1-12) and year (1-9999) must be specified in full numeric format. Cal [[ month] year] Calendar Displays contents of the calendar file case operator The case operator is used to validate multiple conditions. Case $string in Pattern 1) Command list;; Command list;; Pattern 3) Command list;; easc cat (for concatenate) command is used to display the contents of a file. Used without arguments it takes input from standard input <Dtrl d> is used to terminate input. cat [filename(s)] cat > [filename] Data can be appended to a file using >>  Some of the available options are : Cat [-options] filename(S) -s silent about files that cannot be accessed -v enables display of non printinging characters (except tabs, new lines, form- feeds) -t when used with –v, it causes tabs to be printed as ^I’s -e when used with –v, it causes $ to be printed at the end of each line The –t and –e options are ignored if the –v options is not specified. cd Used to change directories chgrp Changes the group that owns a file. Chgrp [grou –id] [filename] chmod Allows file permissions to be changed for each user. File permissions can be changed only by the owner (s). Chmod [+/-][rwx] [ugo] [filename] chown Used to change the owner of a file. The command takes a file(s) as source files and the login id of another user as the target. Chown [user-id] [filename] cmp The cmp command compares two files (text or binary) byte-by-byte and displays the first occurrence where the files differ. Cmp [filename1] [filename2] -1 gives a long listing comm. The comm command compares two sorted files and displays the instances that are common. The display is separated into 3 columns. Comm. filename1 filename2 first displays what occurs in first files but not in the second second displays what occurs in second file but not in first third displays what is common in both files continue statement The rest of the commands in the loop are ignored. It moves out of the loop and moves on the next cycle. cp The cp (copy) command is used to copy a file. Cp [filename1] [filename2] cpio(copy input/output) Utility program used to take backups. Cpio operates in three modes: -o output -i input -p pass creat() the system call creates a new file or prepares to rewrite an existing file. The file pointer is set to the beginning of file. #include<sys/tyes.h> #include<sys/stat.h> int creat(path, mode) char *path; int mode; cut used to cut out parts of a file. It takes filenames as command line arguments or input from standard input. The command can cut columns as well as fields in a file. It however does not delete the selected parts of the file. Cut [-ef] [column/fie,d] filename Cut-d “:” –f1,2,3 filename Where –d indicates a delimiter specified within “:” df used to find the number of free blocks available for all the mounted file systems. #/etc/df [filesystem] diff the diff command compares text files. It gives an index of all the lines that differ in the two files along with the line numbers. It also displays what needs to be changed. Diff filename1 filename2 echo The echo command echoes arguments on the command line. echo [arguments] env Displays the permanent environment variables associated with a user’s login id exit command Used to stop the execution of a shell script. expr command Expr (command) command is used for numeric computation. The operators + (add), -(subtract), *(multiplu), /(divide), (remainder) are allowed. Calculation are performed in order of normal numeric precedence. find The find command searches through directories for files that match the specified criteria. It can take full pathnames and relative pathnames on the command line. To display the output on screen the –print option must be specified for operator The for operator may be used in looping constructs where there is repetitive execution of a section of the shell program. For var in vall val2 val3 val4; Do commnds; done fsck Used to check the file system and repair damaged files. The command takes a device name as an argument # /etc/fsck /dev/file-system-to-be-checked. grave operator Used to store the standard the output of a command in an enviroment variable. (‘) grep The grep (global regular expression and print) command can be used as a filter to search for strings in files. The pattern may be either a fixed character string or a regular expression. Grep “string” filename(s) HOME User’s home directory if operator The if operator allows conditional operator If expression; then commands; fi if … then…else… fi $ if; then commands efile; then commands fi kill used to stop background processes In used to link files. A duplicate of a file is created with another name  LOGNAME displays user’s login name ls Lists the files in the current directory Some of the available options are: -l gives a long listing -a displays all file{including hidden files lp used to print data on the line printer. Lp [options] filename(s) mesg The mesg command controls messages received on a terminal. -n does not allow messages to be displayed on screen -y allows messages to be displayed on screen mkdir used to create directories more The more command is used to dispay data one screenful at a time. More [filename] mv Mv (move) moves a file from one directory to another or simply changes filenames. The command takes filename and pathnames as source names and a filename or exiting directory as target names. mv [target-file] news The news command allows a user to read news items published by the system administrator. ni Displays the contents of a file with line numbers passwd Changes the password paste The paste command joins lines from two files and displays the output. It can take a number of filenames as command line arguments. paste file1 file2 PATH The directories that the system searches to find commands pg Used to display data one page (screenful) at a time. The command can take a number of filenames as arguments. Pg [option] [filename] [filename2]….. pipe Operator (1) takes the output of one commands as input of another command. ps Gives information about all the active processes. PS1 The system prompt pwd (print working directory) displays the current directory. rm The rm (remove) command is used to delete files from a directory. A number of files may be deleted simultaneously. A file(s) once deleted cannot be retrieved. rm [filename 1] [filename 2]… sift command Using shift $1becomes the source string and other arguments are shifted. $2 is shifted to $1,$3to $2 and so on. Sleep The sleep command is used to suspend the execution of a shell script for the specified time. This is usually in seconds. sort Sort is a utility program that can be used to sort text files in numeric or alphabetical order Sort [filename] split Used to split large file into smaller files Split-n filename Split can take a second filename on the command line. su Used to switch to superuser or any other user. sync Used to copy data in buffers to files system0 Used to run a UNIX command from within a C program tail The tail command may be used to view the end of a file. Tail [filename] tar Used to save and restore files to tapes or other removable media. Tar [function[modifier]] [filename(s)] tee output that is being redirected to a file can also be viewed on standard output. test command It compares strings and numeric values. The test command has two forms : test command itself If test ${variable} = value then Do commands else do commands File The test commands also uses special operators [ ]. These are operators following the of are interpreted by the shell as different from wildcard characters. Of [ -f ${variable} ] Then Do commands Elif [ -d ${variable} ] then do commands else do commands fi many different tests are possible for files. Comparing numbers, character strings, values of environment variables. time Used to display the execution time of a program or a command. Time is reported in seconds. Time filename values tr The tr command is used to translate characters. tr [-option] [string1 [string2]] tty Displays the terminal pathname umask Used to specify default permissions while creating files. uniq The uniq command is used to display the uniq(ue) lines in a sorted file. Sort filename uniq until The operator executes the commands within a loop as long as the test condition is false. wall Used to send a message to all users logged in. # /etc/wall message wait the command halts the execution of a script until all child processes, executed as background processes, are completed. wc The wc command can be used to count the number of lines, words and characters in a fine. wc [filename(s)] The available options are: wc –[options] [filename] -1 -w -c while operator the while operator repeatedly performs an operation until the test condition proves false. $ while Ø do commands Ø done who displays information about all the users currently logged onto the system. The user name, terminal number and the date and time that each user logged onto the system. The syntax of the who command is who [options] write The write command allows inter-user communication. A user can send messages by addressing the other user’s terminal or login id. write user-name [terminal number]

Dynamic DNS No-IP

If you have a dynamic IP from your internet service provider it gets annoying when they change your IP and you do not know it, therefore you are not able to connect to your computer remotely anymore. Well Dynamic DNS services fix that problem by assigning a domain to your IP address whenever it changes. This howto will cover getting No-IP, a Dynamic DNS client working.

First if you do not have a No-IP account, create one by going to no-ip.com and registering.

It will send an email to you so you can verify that you want to create account. Click the verification link in your email and it will send you back to the No-IP site.

Log in to the site with the account you just made and click Add under Hosts / Redirects.

Write whatever you want for the subdomain and choose a domain for it to be under. I chose howtonoip for my subdomain and chose to put it under their no-ip.org domain. Scroll down to the bottom and click Create Host.

Now you will need to install the client. Open up your terminal and run “sudo apt-get install no-ip”.

Once installed run “sudo no-ip -C” and select your Internet interface from the list. Then enter your login information. If you added only one host to your no-ip account it will automatically use that. If not it will ask which one(s) to use. Next enter the update interval (in minutes). It will ask you if you want to run something at a successful update. I did not so I selected no.

Your configuration file was just created. Now whenever your IP changes this client software will automatically update the No-IP servers with your new IP. No longer will your ip change and you be locked out of your computer!

Setup Your Computer to be a Router

If you have two network cards or some other means of connecting to the internet and a network card in your ubuntu computer, it can be a very powerfull router. You can set up basic NAT, do port forwarding, set up a proxy, and even do packet prioritization so your downloads dont interfere with gaming! This howto will cover setting up Webmin which will be used to configure masquerading, DHCP, and DNS servers. It also lets you configure port forwarding. QoS (packet prioritization) will be covered in a later guide. This guide is made using kubuntu to show that it will work with any version of ubuntu.

Webmin
Webmin is a web based computer management tool. It is similar to the web interface that you get from routers you buy at stores, but allows you to control most of the computer.
To install webmin, you must first go to its website at www.webmin.com and download the latest version in the top right. Download the tar.gz version into your user folder.

Once the download is complete extract it to a folder for example \usr\webmin. I extracted it into my user folder in this tutorial because this is just a live CD so it doesn’t really matter. You may delete the tar.gz file after you extract it as it is no longer needed.

Before you install it you will want the package “libnet-ssleay-perl”. So open the terminal and run “sudo apt-get install libnet-ssleay-perl”. Now navigate to the directory the folder is (For me it would be “cd /home/ubuntu/webmin-1.280″) and then run the command “sudo sh setup.sh”.

Config file directory [/etc/webmin]:
# Leave as default, or change as you wish

Log file directory [/var/webmin]:
# Leave as default, or change as you wish

Full path to perl (default /usr/bin/perl):
# Leave as default, or change as you wish

Web server port (default 10000):
# Leave as default or change it to what ever port you want.

Login name (default admin):
# Leave as default, or change as you wish

Login password:
# Choose a password, it will not display anything while you type

Password again:
# Self explanatory

Use SSL (y/n):
# Of course ‘y’

Start Webmin at boot time (y/n):
# Once agian… ‘y’

It will not finnish up the installation and you will be able to login by using localhost:port or routerip:port like http://localhost:10000

Masquerading
Masquerading is also known as NAT. To enable it, login to Webmin, go to the networking section, and click on firewall. It will ask you to configure it now. press “Do network address translation on external interface:” and choose the external interface (the one that is connected to the internet) and click on “Enable firewall at boot time?” then click on “Setup Firewall”.

Now you must add rules to your firewall to allow traffic coming from the internal network, loopback, and traffic related to an outgoing connection (so that you may recieve a response to your requests).
To allow traffic from the internal network, under “Incoming packets (INPUT)” press add rule. Give the rule a comment (name) like internal network and press the button by the “Accept” in green writing. Then down to “Incoming interface” put equals and put your internal interface, in the picture below it is eth0. Scroll down and press create.

Now do the same but put lo as incoming interface.

Now you must create another rule for existing connection. Put a comment and press accept. For incoming interface put equals and put your external interface (eth1 for me). Scroll down to “Connection States” and put equals and select both “Existing connection (ESTABLISHED)” and “Related to existing (RELATED)” and press create.

Now you must drop external connections that do not meet any of the other rules, so set the default action of “Incoming packets (INPUT)” to drop and press the button.

Scroll down to the bottom and press apply configuration.

Now go to the System section and click “Bootup and Shutdown”. Click “Create a new bootup and shutdown action”. Make the name “webmin-ipt” without quotes, make the description whatever you please, and for bootup commands, put “/bin/echo 1 > /proc/sys/net/ipv4/ip_forward” and make a new line and put “/sbin/iptables-restore /etc/webmin/firewall/iptables.save” then press create. Find the bootup command you just created in the list and click it then press Start Now.

Configure DHCP
In webmin, go to servers and press DHCP Server and click Module Config.
Replace (without quotes)
DHCP server config file with “/etc/dhcp3/dhcpd.conf”
DHCP server executable with “/usr/sbin/dhcpd3″
Command to start DHCP server with “/etc/init.d/dhcp3-server start”
Command to apply configuration with “/etc/init.d/dhcp3-server restart”
Command to stop DHCP server with “/etc/init.d/dhcp3-server stop”
Path to DHCP server PID file with “/var/run/dhcp3-server/dhcpd.pid”
DHCP server lease file with “/var/lib/dhcp3/dhcpd.leases”
and press save.

Now press “Add a new subnet” and put a description of this subnet. For network address, make it the same as the internal address but with 0 at the end, for example my internal NIC is 192.168.1.1 so I will put 192.168.1.0 for network address. For address ranges put the range of addrseses for your DHCP server to assign. For netmask put the same as your subnet, most are 255.255.255.0 and press create.

Once created, press the new icon that was just created with the network address you choose under it and scroll down to the bottom and click “Edit Client Options”. Fill in Subnet mask with your subnet, default routers and dns servers with your internal IP and broadcast address with the internal ip but 255 at the end (for example my internal ip is 192.168.1.1 so I will put 192.168.1.255 for broadcast) and press save then press start server.

Your computer will now be a DHCP server, router, and DNS server. To do port forwarding, go to the firewall settings in webmin and go to the Network Address Translation (nat) IPTable. From there you can add rules to Packets before routing (PREROUTING).