bash

Handful of bash & Linux commands to get started

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What is shell

Shell is a user program or it’s environment provided for user interaction.It’s an command language interpreter that executes commands read from the standard input device or from a file.

What is Shell scripting ?

Shell script is a script like dos’s bat file that executes series of commands when the file is executed in the shell. It comes with a set of build-in variables to act as a placeholder during script execution

Shell Built in Variables Define
$# Number of command line arguments.
$? Exit Status
$* string that contains all arguments to shell
$@ Same as above, except when quoted.
$- Option supplied to shell
$$ PID of shell
$! PID of last started background process (started with &)

How to write

You can make use of any text editor with nano you can do sudo nano script1 to create a script file and write the script there. Like

#!/bin/bash
# My first shell script, this is a comment
# filename:helloworld

clear # command to clear the the screen

echo "Hello World" # echo command prints whatever comes after it

# cat can also be used to print multplie lines as 
cat << END
line 1
line 2
line 3
END

Where are comments

Comments are descriptive non executable lines written to understand/define the operation, the below example explains it well

#!/bin/bash
# This line is a comment.
echo "A comment will follow this line" # Comment here.
echo "This # is not a comment."
echo 'This # is not a comment.'
echo This \# also is not a comment.
echo This ia a  # proper comment.
 
echo ${PATH#*:}       # Parameter substitution is not a comment, it will print the whole path
echo $(( 2#111011 ))  # Binary conversion is not a comment, it will convert binary to decimal

I wrote the script but not able to execute it

That’s because you don’t have premissions to execute it, after writing shell script one need to set execute permission for your script. You may read about chmod in the previous sections.

chmod +x helloworld
#or
chmod 755 helloworld
or
chmod 777 helloworld

to execute it type ./helloworld

what is #!/bin/bash

Linux comes with different kinds of shells as

… [ToDo] someting more here

How can i define variable

Variables can be defined without a keyword, atlhough it must start from letters and are case sensetive as follows Do make sure that there should be no spaces between the variable and the value

#!/bin/bash
 
x=10            #NOT x = 10 no spaces
X=20            #variables are case sensitive

How can i define a null

Extending the same example above you can define a null by not providing any valuees to it as

#!/bin/bash
 
x=10            #NOT x = 10 no spaces
X=20            #variables are case sensitive
$y=             #NULL variable

How to pass parameter to the shell

You can pass the the parameter in the command line argument or can read it from the standard input

Passing command line arguments

The $ is used to hold the command line arguments passed to it jist like in dos it was %. So the first argument will be given the placeholder $1 and the second will get $0 and it goes so on. As per limitation it goes from 0-9 that means you can pass 9 parameters to your script as $1...$9 where the first parameter $0 will always refers to the script file itself. $* or $@ can also be used to concartinate all the parameters passed to it and $# will have the count of parameters passed to the script.

#!/bin/bash
echo "Argument Count $#"
echo "$0 is script name"
echo "$1  - 1st arg"
echo "$2  - 2nd argument"
echo "All args -  $* or $@"

Refer to the list of build in variables for more info.

Reading from the Standard input

You can use the read to read a line from standard input and use it as a variable once the script runs as

#!/bin/bash
# read name from standard input after the script execution 

echo "Please type your Name: "
read myname
echo "Hello $myname, you are awesome."

Here the variable myname is defined as a placeholder and once the script runs it prints the message and waits for the keyboard input after that it uses $ to display the name as $myname

How to redirect the output to a file or something

It follows the same rule of the dos commands Command | Description —-|—- >| To output Linux-commands result to file. >>| To output Linux-commands result to END of file. <| To take input to Linux-command from file instead of keyboard.

See the below example to clear out your mind

#!/bin/bash
clear # clear screen
echo "ls > flist will put the output from ls to a file called flist"
ls > flist
 
echo "ls -la >> flist will append the output to the end of the file flist"
ls -la >> flist
 
echo "cat < flist will input all the text in the file flist into the cat command "
cat < flist

Conditional branching with if-fi

coming soon

if fi

Mathematical Operators ``` bash #!/bin/bash

if test $1 -gt 0 then echo “$1 > 0” fi

if test $1 -ge 0 then echo “$1 >= 0” fi

if test $1 -eq 0 then echo “$1 == 0” fi

if test $1 -ne 0 then echo “$1 != 0” fi

if test $1 -lt 0 then echo “$1 < 0” fi

if test $1 -le 0 then echo “$1 <= 0” fi

> Logical Operators
``` bash
if test $1 -lt 0
then
        echo "$1 < 0"
fi
 
if test $1 -le 0
then
        echo "$1 <= 0"
fi

String Operators ``` bash string_null=”” string1=”string1”

if [ $string_null -n ] then echo “not null string” else echo “null string” fi

if [ $string_null -z ] then echo “null string” else echo “not null string” fi

if [ “$string_null” == “$string1” ] then echo “strings equal” else echo “strings not equal” fi

if [ “$string_null” != “$string1” ] then echo “strings not equal” else echo “strings equal” fi


> Test for files and directories
``` bash
#!/bin/bash
 
if test -s $1
then
        echo "$1 not empty file"
fi
 
if test -f $1
then
        echo "$1 normal file. Not a directory"
fi
if test -e $1
then 
    echo "$1 exists"
fi 
 
if test -d $1
then
        echo "$1 is directory and not a file"
fi
 
if test -r $1
then
        echo "$1 is read-only file"
fi
 
if test -x $1
then
        echo "$1 is executable"
fi

if..else..fi

If given condition is true then command1 is executed otherwise command2 is executed. ``` bash #!/bin/sh #

Script to see whether argument is positive or negative

# if [ $# -eq 0 ] then echo “$0 : You must give/supply one integers” exit 1 fi

if test $1 -gt 0 then echo “$1 number is positive” else echo “$1 number is negative” fi

### if..elif..else..fi
> Multilevel if-then-else
``` bash
#
#!/bin/sh
# Script to test if..elif...else
#
if [ $1 -gt 0 ]; then
  echo "$1 is positive"
elif [ $1 -lt 0 ]
then
  echo "$1 is negative"
elif [ $1 -eq 0 ]
then
  echo "$1 is zero"
else
  echo "Opps! $1 is not number, give number"
fi

Loops

for loops

while loops

case

functions