ShellScripting.md

Shell Scripting

Notes

• To Create a blank file touch filename.sh
• To Execute an shell script bash filename.sh or ./filename.sh
• To change the permission chmod +x filename.ext
• The first line of the file that is SheBang should always be #!/usr/bin/env bash or #!/usr/bin/bash
• To execute any command from inside shell script, we can use echo $(command)

Variables

• Must begin with letter or a underscore
• Case Sensitive

First_Name=Piyush
Last_Name=Agarwal

echo Hi $First_Name $Last_Name

• Variables like Name = Piyush is not valid as space character is also taken as part of variable

Parameters

• The parameters can be referred as ${0}, ${1} ... upto ${255}
• Its better to assign the parameters to variables to give some meaning to them

./filename.sh Piyush

# ${0} -> refers to filename
# ${1} -> refers to Piyush

Conditionals

• Its syntax is

if [ condition ]
then
    ...statement...
    ...statement...
elif [ condition ]
then
    ...statement...
    ...statement...
else
    ...statement...
    ...statement...
fi

• String comparison is done using single = equals sign

Loops

WHILE

• The syntax is like

#!/usr/bin/bash
COUNT=0
while [ $COUNT -lt 10 ]
do
    echo $COUNT
    ((COUNT++))
done
echo "Outside loop at count = 10" $COUNT

FOR

• The syntax is like

NAMES=$@ # $ refferring to all parameters from ${1} to ${255}

for name in $NAMES
do
    echo $name
done
echo "Outside piyush"

Environment Variables

#!/usr/bin/env bash

echo The Path is: $PATH
echo Computer Name: $HOSTNAME
echo UserName: $USER
echo Home is: $HOME
echo Editor is $EDITOR
echo Terminal: $TERM # type of command line terminal being used

if [ -z $EDITOR ]
then
    echo "Editor is not set"
else
    echo value of editor: $EDITOR
fi

Function

function funcName(){
    ..statements..
}
funcName

funcName(){
    local varName=$1 # hello
    local varName2=$2  # piyush
    ..statements..
}
funcName hello piyush

• local is not used before variable name, it will be considered global by default
• There is no easy way of fetching some return value from a function

Pipe

• Redirects the output of one command as to input of another command

#!/usr/bin/bash

FILES=`ls -l | sort -r | head -3`

for FILE in $FILES
do
    echo $FILE
done

Reading Files

• IFS is the internal field seperator
• -r do not allow backslash to escape

#!/usr/bin/bash
while IFS='' read -r LINE
do
    echo "Line $COUNT: $LINE"
    ((COUNT++))
done < "$1"

exit 0

• To check if a file is edited we can do so using checksum command that is cksum which generates a unique command for a file based on the state of file

cksum filename
#output
# uniqueValue size filename

Sleep

• -z returns True if the variable does not exist
• For delay, we can use sleep command as sleep 2 in seconds

#!/usr/bin/bash

DELAY=$1

if [ -z $DELAY ]
then 
    sleep $DELAY
fi

Watching a Process

• Watch other process and quit when they terminate

#!/usr/bin/bash

STATUS=0

if [ -z $1 ]
then
    echo "Please supply a process PID"
    exit 1 # not run successfully
fi

echo "Watching PID $1"
while [ $STATUS -eq 0 ]
do
    ps $1 > /dev/null
    STATUS=$? # if ps command is not successful means program stopped
done

echo "Process $1 has terminated"
exit 0

Input from User

• To read inputs from user

#!/usr/bin/env bash

# -p make sures user input comes on same line as message
read -p "What is your first name: " Name
echo Your Name is : $Name

• To validate the data

#!/usr/bin/bash

VALID=0

while [ $VALID -eq 0 ]
do
    read -p "Please enter your name and age: " Name Age
    if [[ ( -z $Name) || ( -z Age) ]]
    then
        echo "Not enough parameters passed"
        continue
    # check for a regular expression
    elif [[ ! $Name =~ ^[A-Za-z]+$ ]]
    then
        echo "Non alpha characters detected [$Name]"
        continue
    elif [[ ! $Age =~ ^[0-9]+$ ]]
    then
        echo "Non numeric characters are detected [$Age]"
        continue
    fi
    VALID=1
done
echo "Name = $Name and Age = $Age"

exit 0