• The UNIX operating system was developed by Ken Thomson and Dennis Ritchie in 1969. It was released in 1970. They rewrote the UNIX using C language in 1972.
• In 1991, Linus Torvalds developed the Linux kernel for the free operating system.

• Comparison of shells
• Working in shell
• Learning basic Linux commands
• Our first script—Hello World
• Compiler and interpreter—difference in process
• When not to use scripts
• Various directories
• Working more effectively with shell—basic commands
• Working with permissions

Initially, the UNIX OS used a shell programcalledBourne Shell. Then eventually, many more shell programs were developed for different flavors of UNIX. The following is brief information about different shells:

A brief comparison of various shells is presented in the following table:

What we see here is that, generally, the syntax of all these shells is 95% similar. In this book, we are going to follow Bash shell programming.

• Reading text and parsing the entered command
• Evaluating metacharacters such as wildcards, special characters, or history characters
• Process io-redirection, pipes, and background processing
• Signal handling
• Initializing programs for execution

1. Open the Linux terminal and type in:

   $ echo $SHELL/bin/bash

2. The preceding output in terminal says that the current shell is/bin/bash such asBASH shell:

   $ bash --versionGNU bash, version 2.05.0(1)-release (i386-redhat-linux-gnu)Copyright 2000 Free Software Foundation, Inc.

• Wrong file name—Hello World.txt
• Correct file name—Hello_World.txt orHelloWorld.txt

1. Create a newhello.sh file as follows:

   #!/bin/bash# This is comment lineecho "Hello World"lsdate

2. Save the newly created file.

• Any line starting with#, will be treated as a comment line. An exception to this would be the first line with#!/bin/bash
• Theecho command will printHello World on the screen
• Thels command will display directory content on the console
• Thedate command will show the current date and time

• Technique one:

  $ bash hello.sh

• Technique two:

  $ chmod +x hello.sh

$ ./hello.sh

Hello Worldhello.shSun Jan 18 22:53:06 IST 2015

#!/bin/bash# This is the first Bash shell# Scriptname : Hello1.sh# Written by:  Ganesh Naikecho "Hello $LOGNAME, Have a nice day !"echo "Your are working in directory `pwd`."echo "You are working on a machine called `uname -n`."echo "List of files in your directory is."ls      # List files in the present working directoryecho  "Bye for now $LOGNAME. The time is `date +%T`!"

Hello student, Have a nice day !.Your are working in directory /home/student/work.You are working on a machine called ubuntu.List of files in your directory is.hello1.sh  hello.shBye for now student. The time is 22:59:03!

• Compilation: Using a compiler-based language such as C, C++, Java, and other similar languages
• Interpreter: Using interpreter-based languages such as Bash Shell scripting.

• Scripts are easy to write
• Scripts are quick to start and easy for debugging
• They save the time of development.
• Tasks of administration are automated
• No additional setup or tools are required for developing or testing Shell script

• Every line in Shell script creates a new process in the operating system. When we execute the compiled program such as C program, it runs as a single process for the complete program.
• Since every command creates a new process, Shell scripts are slow as compared to compiled programs.
• Shell scripts are not suitable if heavy math operations are involved.
• There are Problems with cross-platform portability.
• We cannot use Shell scripts in the following situations when:
  • Extensive file operations are required
  • We need data structures, such as linked lists or trees
  • We need to generate or manipulate graphics or GUIs
  • We need direct access to system hardware
  • We need a port or socket I/O
  • We need to use libraries or interface with legacy code
  • Proprietary, closed source applications are used (Shell scripts put the source code right out in the open for the entire world to see)

• /bin/: This contains commands used by a regular user.
• /boot/: The files required for the operating system startup are stored here.
• /cdrom/: When CD-ROM is mounted, the CD-ROM files are accessible here.
• /dev/: The device driver filesare stored in this folder. These device driver files will point to hardware-related programs running in kernel.
• /etc/: This folder containsconfiguration files and startup scripts.
• /home/: This folder containsa home folder of all users except the administrator.
• /lib/: The library files arestored in this folder.
• /media/: External media such as a USB pen drive is mounted in this folder.
• /opt/: The optionalpackages are installed in this folder.
• /proc/: This contains files which give information about kernel and every process running in OS.
• /root/: This is theadministrators home folder.
• /sbin/: This contains commands used by the administrator or root user.
• /usr/: This contains secondary programs, libraries, and documentation about user-related programs.
• /var/: This containsvariable data such as http, tftp, and similar other.
• /sys/: This dynamically creates thesys files

• Enter the following command. It will show the various types of manual pages displayed by theman command:

  $ man man

  From the following table, you can get an idea about various types ofman pages for the same command:

  Section number	Subject area
  1	User commands
  2	System calls
  3	Library calls
  4	Special files
  5	File formats
  6	Games
  7	Miscellaneous
  8	System admin
  9	Kernel routines

• We can enter theman command to display corresponding manual pages as follows:

  $ man 1 command$ man 5 command

• Suppose we need to know more about the passwd command, which is used for changing the currentpassword of a user, you can type the command as follows:

  $ man command  man -k passwd   // show all pages with keyword  man –K passwd  // will search all manual pages for pattern$ man passwd

  This will show information about thepasswd command:

  $ man 5 passwd

  The preceding command will give information about the filepasswd, which is stored in/etc /passwd.

• We can get brief information about the command as follows:

  $ whatis passwd

  Output:

  passwd (1ssl)        - compute password hashespasswd (1)           - change user passwordpasswd (5)           - the password file

• Every command we type in the terminal has an executable binary program file associated with it. We can check the location of a binary file as follows:

  $ which passwd/usr/bin/passwd

  The preceding line tells us that the binary file of thepasswd command is located in the/usr/bin/passwd folder.

• We can get complete information about the binary file location as well as manual page location of any command by following:

  $ whereis passwd

  The output will be as follows:

  passwd: /usr/bin/passwd /etc/passwd /usr/bin/X11/passwd /usr/share/man/man1/passwd.1.gz /usr/share/man/man1/passwd.1ssl.gz /usr/share/man/man5/passwd.5.gz

• Change the user login and effective user name:

  $ whoami

  This command displays the user name of the logged in user:

  $ su

  Thesu command (switch user) will make the user as the administrator; but, you should know the administrators, password. Thesudo command (superuser do) will run the command with administrator's privileges. It is necessary that the user should have been added in thesudoers list.

  # who am i

  This command will show the effective user who is working at that moment.

  # exit

• Many a times, you might need to create new commands from existing commands. Sometimes, existing commands have complex options to remember. In such cases, we can create new commands as follows:

  $ alias ll='ls –l'$ alias copy='cp –rf'

  To list all declared aliases, use the following command:

  $ alias

  To remove an alias, use the following command:

  $ unalias copy

• We can check about theoperating system details such as UNIX/Linux or the distribution that is installed by the following command:

  $ uname

  Output:

  Linux

  This will display the basic OS information (UNIX name)

• Linux kernel version information will be displayed by the following:

  $ uname –r

  Output:

  3.13.0-32-generic

• To get all the information about a Linux machine, use the following command:

  $ uname –a

  Output:

  Linux ubuntu 3.13.0-32-generic #57~precise1-Ubuntu SMP Tue Jul 15 03:50:54 UTC 2014 i686 i686 i386 GNU/Linux

• The following commands will give you more information about the distribution of Linux:

  $ cat /proc/version   // detailed info about distribution$ cat /etc/*release# lsb_release -a   // will tell distribution info for Ubuntu

  The commandcat is used for reading files and displayed on the standard output.

• Sometimes, we need to copy a file or directory in many places. In such situations, instead ofcopying the original file or directory again and again, we can create soft links. In Windows, a similar feature is called as creating a shortcut.

  $ ln  -s  file file_link

• To learn about the type of file, you can use the command file. In Linux, various types of files exist. Some examples are as follows:
  • Regular file (-)
  • Directory (d)
  • Soft link (l)
  • Character device driver (c)
  • Block device driver (b)
  • Pipe file (p)
  • Socket file (s)
• We can get information about a file using the following command:

  $ file   fil_name  // show type of file

• Printing some text on the screen for showing results to the user or to ask details is an essential activity.
  • The following command will create a new file calledfile_name using thecat command:

    $ cat > file_nameline 1line 2line 3< Cntrl + D will save the file    >

  But this is very rarely used, as many powerful editors are already existing, such as vi or gedit.

  • The followingcommand will printHello World on the console. Theecho command is very useful for Shell script writers:

    $ echo "Hello World"

  • The following command will copy the stringHello World to thehello.c file:

    $ echo "Hello World" >   hello.c

    The commandecho with> overwrites the content of the file. If content already exists in the file, it will be deleted and new content will be added in the file. In a situation, when we need to append the text to the file, then we can use theecho command as follows:

    $ echo  "Hello World" >> hello.c  will append the text

  • The following command will display the content of the file on screen:

    $ cat hello.c

• Read permission: The usercan read or check the content of the file
• Write permission: The user canedit or modify the file
• Execute permission: The user can execute the file

The following are the commands for changing the file permissions:

To check the file permission, give the following command:

$ ll file_name

The details of file permissions are as seen in the following image:

In the preceding diagram, as we can see, permissions are grouped in owner-user and group and other users' permissions. Permissions are of three types such as read, write, and execute permissions. As per the requirement, we may need to change permissions of the various files.

We can change the file ordirectory permissions by the following two ways:

$ chmod ugo+rwx file_name

$ chmod +rwx file_name

$ chmod +rwx file_name

We will see how Linux decides the default permissions of the newly created file or folder:

$ umask0002

The meaning of the preceding output is that, if we create a new directory, then from the permissions of+rwx, the permission0002 will be subtracted. This means that for a newly created directory, the permissions will be775 orrwx rwx r-x. For a newly created file, the file permissions will berw- rw- r--. By default, for any newly created text file, the execute bit will never be set. Therefore, the newly created text file and directory will have different permissions even though the umask is same.

Another very interesting functionality is thesetuid feature. If thesetuid bit is set for a script, then the script will always run with the owner's privileges irrespective of which user is running the script. If the administrator wants to run script written by him by other users, then he can set this bit.

Consider either of the following situations:

$ chmod u+s file_name$ chmod 4777 file

The file permissions after any of the preceding two commands will bedrwsrwxrwx.

Similar tosetuid, the setgid functionality gives the user the ability to run scripts with group owner's privileges, even if it is executed by any other user.

$ chmod g+s filename

Alternatively, you can use the following command:

$ chmod 2777 filename

File permissions after any of the preceding two commands will bedrwxrwsrwtx.

Sticky bit is a very interesting functionality. Let's say, in the administration department there are 10 users. If one folder has been set with sticky bit, then all other users can copy files to that folder. Allusers can read the files, but only the owner of the respective file can edit or delete the file. Other user can only read but not edit or modify the files if the sticky bit is set.

$ chmod +t filename

Alternatively, you can use the following command:

$ chmod 1777

File permissions after any of the preceding two commands will bedrwxrwxrwt.