• Shell Script is series of command written in plain text file.
  • To find all available shells in your system type following command:
    $cat /etc/shells
  • To find your current shell type following command
    $ echo $SHELL
  • Example:
    $vi first.sh
    #!/bin/sh
    # Comment
    clear
    echo “Knowledge is Power”
    $chmod 755 first.sh
    $./first.sh
  • RAM memory is divided into small locations, and each location had unique number called memory location/address, which is used to hold our data. Programmer can give a unique name to this memory location/address called memory variable or variable.
  • Show system variables:
    $set
  • Define a user defined variable:
    variable_name=value;
  • Rules for naming variable name:
    • Variable name must begin with Alphanumeric character or underscore character (_), followed by one or more Alphanumeric character.
    • Don't put spaces on either side of the equal sign when assigning value to variable.
    • Variables are case-sensitive.
    • You can define NULL variable as follows (NULL variable is variable which has no value at the time of definition)
      $ vech=
      $ vech=""
    • Do not use ?,* etc, to name variable names.
  • Use echo command to display text or value of variable. Display text or variables value on screen.
    Syntax: echo [options] [string, variables...]
    Options:

    -n

    Do not output the trailing new line.

    -e

    Enable interpretation of the following backslash escaped characters in the strings:

    \a: alert (bell)

    \b: backspace

    \c: suppress trailing new line

    \n: new line

    \r: carriage return

    \t: horizontal tab

    \\: backslash
    ex: $ echo -e "An apple a day keeps away \a\t\tdoctor\n"
  • Arithmetic operations
    Syntax: op1 math-operator op2
    ex.: $ echo `expr 10 \* 3`         /* expr expression is enclosed by `(back quote)signs,  \ is the escape character */
  • Three types of quotes:

    “

    Double Quotes

    "Double Quotes" - Anything enclose in double quotes removed meaning of that characters (except \ and $).

    ‘

    Single quotes

    'Single quotes' - Enclosed in single quotes remains unchanged.

    `

    Back quote

    `Back quote` - To execute command
  • The value of exit status is zero/nonzero means the results of executing shell command(s) is successful/not successful. Use $? get the value of exit status.
  • Use read to get input (data from user) from keyboard and store (data) to variable.
    Syntax: read variable1, variable2,...variableN
    #!/bin/bash
    # Script to read your name from keyboard
    echo "Your first name please:"
    read fname
    echo "Hello $fname, Lets be friend!"
  • Run more commands on one command line
    Syntax: command1; command2
  • Shell built in variables:

    $?

    Exit status.

    $#

    Number of command line arguments. Useful to test no. of command line args in shell script.

    $*, $@

    All arguments to shell.

    $-

    Option supplied to shell.

    $$

    PID of shell.

    $!

    PID of last started background process (started with &).

    $i

    i-th command line argument. Ex: $myshell foo bar, $0 is myshell, $1 is foo, $2 is bar.
  • You can't assign the new value to command line arguments.
  • Redirection of standard output/input
    • > redirector symbol
      Syntax: Linux-command > filename, to output Linux-commands result (output of command or shell script) to file.
    • >> redirector symbol
      Syntax: Linux-command >> filename, to output Linux-commands result (output of command or shell script) to END of file.
    • < redirector symbol
      Syntax: Linux-command < filename, to take input to Linux-command from file instead of key-board.
  • A pipe is a way to connect the output of one program to the input of another program without any temporary file.
  • A pipe is nothing but a temporary storage place where the output of one command is stored and then passed as the input for second command. Pipes are used to run more than two commands ( Multiple commands) from same command line.
    Syntax: command1 | command2
  • If a Linux command accepts its input from the standard input and produces its output on standard output is known as a filter.
    Ex: $sort < sname | uniq > u_name, uniq is filter which takes its input from sort command and passes this lines as input to uniq.
  • A process is program (command given by user) to perform specific Job. In Linux when you start process, it gives a number to process (called PID or process-id), PID starts from 0 to 65535.
  • An instance of running command is called process and the number printed by shell is called process-id (PID), this PID can be used to refer specific running process.
  • Commonly used commands with process:

    For this purpose

    Use this command

    Example

    To stop processes by name i.e. to kill process

    killall {Process-name}

    $ killall httpd

    To stop all process except your shell

    kill 0

    $kill 0

    To display a tree of processes

    pstree

    $pstree
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