The Unix OS

Made up of three parts kernel, shell and programs.

The kernel

The kernel of UNIX is the hub of the operating system: it allocates time and memory to programs and handles the filestore and communications in response to system calls.

As an illustration of the way that the shell and the kernel work together, suppose a user types rm myfile (which has the effect of removing the file myfile). The shell searches the filestore for the file containing the program rm, and then requests the kernel, through system calls, to execute the program rm and passes the file name as a parameter. When the process rm myfile has finished running, the shell then returns the UNIX prompt % to the user, indicating that it is waiting for further commands.

The shell

The shell acts as an interface between the user and the kernel. When a user logs in, the login program checks the username and password, and then starts another program called the shell. A shell is a command line interpreter (CLI). It interprets the commands the user types in and arranges for them to be carried out. The commands are themselves programs: when they terminate, the shell gives the user another prompt (% on our systems).

Files and processes

Everything in UNIX is either a file or a process.

A process is an executing program identified by a unique PID (process identifier).

A file is a collection of data. They are created by users using text editors, running compilers etc.

Some common commands

• ls : list of files (can also be used as ls {dir_name})
• -a : files that are normally hidden
• mkdir : To make a subdirectory called unixstuff in your current working directory type
• cp path1 path2 : to copy file specified by path1 to the destination specified by path2
• mv path1 path2 : to move/rename
• clear : clear screen
• cat : to display contents
• less : same as above but one page at a time
• head : display first 10 lines (can also be used as head -5 filename)
• tail : display the last 10 lines
• grep string filename : It searches files for specified words or patterns. First clear the screen
• The grep command is case sensitive; it distinguishes between Science and science.
• To search for a phrase or pattern, you must enclose it in single quotes (the apostrophe symbol)
• -v display those lines that do NOT match
• -n precede each matching line with the line number
• -c print only the total count of matched lines
• -i To ignore upper/lower case distinctions
• eg : grep -ivc science science.txt
• wc : word count
• -w for words
• -l for lines
• \> : Redirects the output (stdout). for eg : cat > eg
• \>> : append
• | : pipe
• sort / who / *,? wildcard
• Online manuals :
• man wc
• whatis wc
• apropos copy

The directory structure

PathDescription/This is the root directory which should contain only the directories needed at the top level of the file structure/binThis is where the executable files are located. These files are available to all users/devThese are device drivers/etcSupervisor directory commands, configuration files, disk configuration files, valid user lists, groups, ethernet, hosts, where to send critical messages/libContains shared library files and sometimes other kernel-related files/bootContains files for booting the system/homeContains the home directory for users and other accounts/mntUsed to mount other temporary file systems, such as cdrom and floppy for the CD-ROM drive and floppy diskette drive, respectively/procContains all processes marked as a file by process number or other information that is dynamic to the system/tmpHolds temporary files used between system boots/usrUsed for miscellaneous purposes, and can be used by many users. Includes administrative commands, shared files, library files, and others/varTypically contains variable-length files such as log and print files and any other type of file that may contain a variable amount of data/sbinContains binary (executable) files, usually for system administration. For example, fdisk and ifconfig utlities/kernelContains kernel files

File system security

Each file (and directory) has associated access rights, which may be found by typing ls -l. Also, ls -lg gives additional information as to which group owns the file (beng95 in the following example):

~$ ls -lg ee51ab
-rwxrw-r-- 1 ee51ab beng95 2450 Sept29 11:52 file1

In the left-hand column is a 10 symbol string consisting of the symbols d, r, w, x, -, and, occasionally, s or S. If d is present, it will be at the left hand end of the string, and indicates a directory: otherwise — will be the starting symbol of the string.

The 9 remaining symbols indicate the permissions, or access rights, and are taken as three groups of 3.

• The left group of 3 gives the file permissions for the user that owns the file (or directory) (ee51ab in the above example);
• the middle group gives the permissions for the group of people to whom the file (or directory) belongs (eebeng95 in the above example);
• the rightmost group gives the permissions for all others.

The symbols r, w, etc., have slightly different meanings depending on whether they refer to a simple file or to a directory.

Access rights on files

• r (or -), indicates read permission (or otherwise), that is, the presence or absence of permission to read and copy the file
• w (or -), indicates write permission (or otherwise), that is, the permission (or otherwise) to change a file
• x (or -), indicates execution permission (or otherwise), that is, the permission to execute a file, where appropriate

Access rights on directories

• r allows users to list files in the directory;
• w means that users may delete files from the directory or move files into it;
• x means the right to access files in the directory. This implies that you may read files in the directory provided you have read permission on the individual files.

So, in order to read a file, you must have execute permission on the directory containing that file, and hence on any directory containing that directory as a subdirectory, and so on, up the tree.

Changing permissions

For example, to remove read write and execute permissions on the file biglist for the group and others, type

SymbolDescriptionuuserggroupootheraallrreadwwrite (and delete)xexecute (and access directory)+add permission-take away permission

~$ chmod go-rwx biglist

This will leave the other permissions unaffected.

To give read and write permissions on the file biglist to all,

~$ chmod a+rw biglist

Octal permissions

r = 4 | w = 2 | x = 1

~$ chmod 755 MyDir

The root user

Both su and sudo are used to run commands with root permissions. The root user is basically equivalent to the administrator user on Windows – the root user has maximum permissions and can do anything to the system. Normal users on Linux run with reduced permissions – for example, they can’t install software or write to system directories.

To do something that requires these permissions, you’ll have to acquire them with su or sudo.

su vs sudo

The su command switches to the super user – or root user – when you execute it with no additional options. You’ll have to enter the root account’s password. This isn’t all the su command does, though – you can use it to switch to any user account. If you execute the su bob command, you’ll be prompted to enter Bob’s password and the shell will switch to Bob’s user account.

Once you’re done running commands in the root shell, you should type exit to leave the root shell and go back to limited-privileges mode.

Sudo runs a single command with root privileges. When you execute sudo command, the system prompts you for your current user account’s password before running command as the root user. By default, Ubuntu remembers the password for fifteen minutes and won’t ask for a password again until the fifteen minutes are up.

Processes and Jobs

A process is an executing program identified by a unique PID (process identifier). To see information about your processes, with their associated PID and status, type

~$ ps

A process may be in the foreground, in the background, or be suspended. In general the shell does not return the UNIX prompt until the current process has finished executing.

Some processes take a long time to run and hold up the terminal. Backgrounding a long process has the effect that the UNIX prompt is returned immediately, and other tasks can be carried out while the original process continues executing.

When a process is running, backgrounded or suspended, it will be entered onto a list along with a job number. To examine this list, type

~$ jobs

An example of a job list could be

• [1] Suspended sleep 1000
• [2] Running netscape
• [3] Running matlab

UNIX variables

Variables are a way of passing information from the shell to programs when you run them. Programs look “in the environment” for particular variables and if they are found will use the values stored. Some are set by the system, others by you, yet others by the shell, or any program that loads another program.

Standard UNIX variables are split into two categories, environment variables and shell variables. In broad terms, shell variables apply only to the current instance of the shell and are used to set short-term working conditions; environment variables have a farther reaching significance, and those set at login are valid for the duration of the session. By convention, environment variables have UPPER CASE and shell variables have lower case names.

Env variables

~$ echo $OSTYPE

ENVIRONMENT variables are set using the setenv command, displayed using the printenv or env commands, and unset using the unsetenv command.

To show all values of these variables, type

~$ printenv | less

Changing visibility of a process

To send a process to background add an ampersand & at the end of the command, for example:

~$ emacs & # This will start emacs in background, NOTE: This does not redirect the output

To keep a process running when even when the terminal is closed use & disown at the end of a command:

~$ emacs & disown # This will start emacs in the bg and keep it runnning even when the terminal is closed

To read more about &, disown and nohup, you can head here.

Further exploration

kill, sleep, file, diff, find, history, setting up env variables etc.

Random stuff

nc localhost 1123     # to connect to port 1123
nc -l 1234            # to listen to port 1234
chmod                 # change file permissions
chown                 # change file owner
chgrp                 # change file group
ping -c4 8.8.8.8      # to send 4 packets to google's server at 8.8.8.8
sudo lsof -i          # to list network sockets

DNS == Domain Name System DNS contains records , few of which map domain names to their IP addresses.

host google.com           # This command can be used to look the DNS records
dig google.com            # This command is same as host but host provides data in human
                          # readable form whereas dig provides data in script readable form
ip addr show              # to bring out the interfaces on your computer
ifconfig | less           # does the same as above
ip route show default     # shows the address of the default gateways
                          # default gateway is the router through which your machine
                          # is connected to the rest of the internet
netstat -nr               # slightly similar to above
tcpdump -n port portName  # brings out details about the connections
uname -r                  #  to check for kernel version