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Shell Scripting Basics

Basic CLI Commands

chmod (user) (group) (other) <file/directory>

Linux permission scheme

0 = no permission
1 = x
2 = w
3 = x+w (1+2)
4 = r
5 = r+x (4+1)
6 = r+w (4+2)
7 = r+w+x (4+2+1)

Simplified

r = 4
w = 2
x = 1

Shell Basics

There are few types of shells available, the variant branches from the two main.

Bourne Shells

  1. Bourne Shell
  2. Korn Shell
  3. Borne Again Shell
  4. POSIX Shell

C Shells

  1. C Shell
  2. TENEX/TOPS C Shell
  3. Z Shell

Scripting

A shell script starts with #!/bin/sh which indicates the execution path of the script.

Hello world

#!/bin/sh

# Author: Leton
# My first shell script
echo "Hello World!"

Variables

You don't have to declare variables in order to use it. You have to assign like var="value" and no like var = "value". Because if you give space between variable name and "=" shell will think variable name as a command instead of an identifier.

Variable assignment

MY_SHELL="zsh" Do not use space before and after = sign.

Storing output of a commmand

HOST_NAME=$(hostname) or

HOST_NAME=`hostname`

Accessing the value of a variable

echo $HOST_NAME

Tests

File operators

-d <file>       # returns true if file is a directory
-e <file>       # returns true if file exists
-f <file>       # returns true if file exists and is a regular file
-r <file>       # returns true if file is readable by you
-w <file>       # returns true if file is writable
-x <file>       # returns true if file is executable

String operators

-z <string>             # true if string is empty
-n <string>             # true if
<string>                # true if not empty
<string> = <string>     # true if equal
<string> != <string>    # true if not equal

Arithmetic operators

arg1 -eq arg2       # true if arg1 is equal to arg2
arg1 -ne arg2       # true if arg1 is not equal to arg2
arg1 -lt arg2       # true if arg1 is less than arg2
arg1 -le arg2       # true if arg1 is less than or equal to arg2
arg1 -gt arg2       # true if arg1 is greather than arg2
arg1 -ge arg2       # true if arg1 is greather than or equal to arg2

The IF statement

if [ condition ]
then
    command
elif [ condition ]
then
    command
else
    command
fi

Commands

readonly

readonly makes a variable unassignable later and unsetable as well.

#!/bin/sh

# makes it read only variable
readonly Name="You name it"
# doesn't work
Name="Something else"
# doesn't work either
unset Name

"." (dot) operator

Normally when we run shell script, it runs in a different instance. In order to receive environment changes back to in our shell we must source the script. We can do this by two methods . ./my_script.sh or source ./my_script.sh.

Wildcards

Loops

Positional Parameters

$0...$9 are called positional parameters. The $0 is the script itself.