Shell Scripting - My Learner's guide

A Program designed to run command in Unix shell.

Bash(Bourne Again Shell) shell is a dialect of a shell syntax called bourne shell. Other example : C shell , zsh shell

• chmod +x simplescript.sh
  • make file executable
• ./simplescript.sh
  • to run file executable
• which bash
  • will show where bash in your system i.e. /bin/bash for linux
• #! /bin/bash
  • this sould be the first line in any shell script file
  • #! Sharp and bang

ECHO

echo "Hello Mr kumar"

VARIABLE

• should use Uppercase by convention
• use numbers, letters, underscores

NAME="Arpit"
 echo "my name is $NAME"
 echo "my name is ${NAME}"

USER INPUT

• -p for prompt
• -r to read line

 read -p "Enter your name: " NAME
 echo "Hello $NAME, its good to see you"

CONDITIONAL STATEMENT

• IF

 if [ "$NAME" == "Sobhit" ]
 then
   echo "you are admin"
 fi

• IF-ELSE

 if [ "$NAME" == "Sobhit" ]
 then
   echo "you are admin"
 else 
   echo "you are not admin"
 fi

• ELSE-IF (elif)

if [ "$NAME" == "Sobhit" ]
then
  echo "you are admin"
elif [ "$NAME" == "Arpit" ]  
then
  echo "you are staff"
else 
  echo "you are not admin"
fi

COMPARISION

• val1 -eq val2
  • Returns true if the values are equal
• val1 -ne val2
  • Returns true if the values are not equal
• val1 -gt val2
  • Returns true if val1 is greater than val2
• val1 -ge val2
  • Returns true if val1 is greater than or equal to val2
• val1 -lt val2
  • Returns true if val1 is less than val2
• val1 -le val2
  • Returns true if val1 is less than or equal to val2

NUM1=10
NUM2=7
if [ "$NUM1" -gt "$NUM2" ]
then
  echo "$NUM1 is greater than $NUM2"
else
  echo "$NUM1 is less than $NUM2"
fi

FILE CONDITIONS

• -d file
  • True if the file is a directory
• -e file
  • True if the file exists (note that this is not particularly portable, thus -f is generally used)
• -f file
  • True if the provided string is a file
• -g file
  • True if the group id is set on a file
• -r file
  • True if the file is readable
• -s file
  • True if the file has a non-zero size
• -u
  • True if the user id is set on a file
• -w
  • True if the file is writable
• -x
  • True if the file is an executable

FILE="new-test"
if [ -f "$FILE" ]
then 
  echo "$FILE is a file."
else  
  echo "$FILE is not a file."

CASE STATEMENT

 read -p "are you above 18 ?" ANSWER
 case "$ANSWER" in 
   [yY] | [yY][eE][sS])
     echo "you are eligible to vote. :)"
       ;;
   [nN] | [nN][oO])
     echo "Sorry, you are not eligible to vote."
     ;;
   *)
     echo "Please enter (y/yes) or (n/no) in answer."
     ;;
 esac 

FOR LOOP

 JOBS="engineer plumber manager student teacher"
 for JOB in $JOBS
   do 
     echo "I am $JOB"
 done   

FOR LOOP TO RENAME FILES

• will add new to the every file name

 FILES=$(ls *.txt)
 NEW="new"
 for FILE in $FILES
   do
     echo "Renaming $FILE to $NEW-$FILE"
       mv $FILE $NEW-$FILE
 done

WHILE LOOP

• read through a file line by line

 LINE=1
 while read -r CURRENT_LINE
   do
     echo "$LINE: $CURRENT_LINE"
     ((LINE++))
 done < "./new-1.txt"  

FUNCTION

• without parameter

 function sayHello(){
   echo "Hello Sobhit"
 }
 sayHello

• with parameters

 function greet(){
   echo "Hello, I am $1 and i am $2";
 }

 greet "27" "software developer"

RUN CHAIN OF COMMANDS

mkdir Hello_folder
touch Hello_folder/world.txt
echo "world.txt file created"
echo "Hello Mr Sobhit Kumar" >> "Hello_folder/hello.txt"