Question

Linux Script
Convert String (Part 1)

Write a simple shell script that takes a permission string expressed as -rwxrwxrwx and prints out whether or not theobject is a directory, file or link. The string is read in from standard input.

Convert String (Part 2)

Modify the script so its able to print out the octal permission which the string represents along with the file type. This is in addition to part 1.

Convert String (Part 3)

For the script in Part 2, modify the script so it can read in as many strings till the end of file is reached.

Convert String (Part 4)

Modify the script so it reads the output of ls -la, converts the permission string into octal, prints out the type of object and also the name of the file.

Answer 1

Permission.sh:

#!/bin/bash

filetype(){ #Function for checking filetype
   if [ $1 == "-" ] #Check if character matches
   then
       f_type="file" #Assign type if it does
   else
   if [ $1 == "d" ]
   then
       f_type="directory"
   else
   if [ $1 == "l" ]
   then
       f_type="link"
   fi #End if else
   fi
   fi
}

stringtobinary(){ #Convert rwx to binary
   binary=""
   if [ ${1:0:1} == "r" ] #Check if first character is r
   then
       binary="${binary}1" #Append 1 if it matches otherwise 0
   else
       binary="${binary}0"
   fi
   if [ ${1:1:1} == "w" ] #Check if second character is w
   then
       binary="${binary}1" #Append 1 if it matches otherwise 0
   else
       binary="${binary}0"
   fi
   if [ ${1:2:1} == "x" ] #Check if third character is x
   then
       binary="${binary}1" #Append 1 if it matches otherwise 0
   else
       binary="${binary}0"
   fi
   return $binary
      
}

binarytooctal(){ #Convert binary to octal
   oct=2
   h=`expr ${1:0:1} \* $oct \* $oct` # First digit x 8^2
   d=`expr ${1:1:1} \* $oct` # Second digit x 8^1
   u=`expr ${1:2:1} + 0` # Third digit x 8^0
   res=$((h + d + u)) # Add all variables
   return $res #Return result
}

octal(){
   stringtobinary ${1:1:3} #Invoke function
   user=$? #Store returned value
   stringtobinary ${1:4:3}
   group=$?
   stringtobinary ${1:7:3}
   other=$?
   if [ $user != "0" ] #Check if binary representation is only 0
   then
       binarytooctal $user #Invoke function
       user=$? #Store returned value
   fi
   if [ $group != "0" ]
   then
       binarytooctal $group
       group=$?
   fi
   if [ $other != "0" ]
   then
       binarytooctal $other
       other=$?
   fi
   octal_res="${user}${group}${other}" #Concatenate result to form a string
}

ans="y" #initialize global variables
f_type=""
octal_res=""

while [ $ans == "y" ] #Loop until user doesn't exit
do
   echo "1.Print permission filetype"
   echo "2.Print octal permission and filetype"
   echo "3.Take multiple permissions, print octal permissions and filetypes"
   echo "4.ls -la to octal permissions"
   echo "Enter your choice:"
   read choice #Take input from user
  
   if [ $choice == "1" ] || [ $choice == "2" ] #Check if choice is 1 or 2
   then
       echo "Enter the permission" #Print statement
       read string #Read input
       identifier=${string:0:1} #Take the first character from the string
       filetype $identifier $string #Invoke function and pass 2 arguments
       echo "Object Type: $f_type" #Display result
       if [ $choice == "2" ]
       then
           octal $string
           echo "The octal representation is ${octal_res}"
       fi
   else
   if [ $choice == "3" ]
   then
       echo "1.Read permissions from file"
       echo "2.Enter permissions manually"
       echo "Enter your choice:"
       read choice
       if [ $choice == "1" ]
       then
           echo "Enter the filename:"
           read filename
           while read -r line #Read input file line by line
           do
               filetype ${line:0:1} $line
               octal $line
               echo "$line   $octal_res   ${f_type}"
           done < "$filename" #Execute loop until end of file is reached
       else
       if [ $choice == "2" ]
       then
           echo "Enter the number of permissions"
           read num
           for ((i=1;i<=num;i++))
           do   #For each iteration take input from user
               echo "Enter the permission:"
               read string
               identifier=${string:0:1}
               filetype $identifier $string
               octal $string
               echo "$string   $octal_res   ${f_type}"
           done
       fi
       fi
   else
   if [ $choice == "4" ]
   then
       echo "Permission   Octal   Filetype   Filename"
       ls -la| while read line #Read ls -la as list of lines
       do
           fields=($line) #Separate each line into tokens
           if [ ${fields[0]} != "total" ]
           then
               permission=${fields[0]} #Store permission
               filetype ${permission:0:1} ${fields[0]}
               octal ${fields[0]}
               echo "${permission}   ${f_type}   ${octal_res}   ${fields[8]}"
           fi
       done
              
   fi
   fi
   fi
   echo "Do you want to continue?(y/n)"
   read ans
done #End while loop
  

Output: