Bash script to open, read, and write then save

bash

I'm new on this bash script thing. Can you show me some example on writing Bash script. I want to write a script that can read from a filename and save it to a variable; increment the value of the variable and write that variable back to the file and save it. This is what I have started and stuck on it so far.

#!/bin/bash
# if file exist

#echo "Testing \ "$1""
if [ -f "$1" ]; then
 echo "$1 does exist"
else
 echo "$1 does not exist!" 
 echo "Creating $1"
 touch $1

 echo "This is test" > $1
 exit 1
fi

#echo "Testing \ "$2""
if [ "$2" == "" ]; then
 echo "Enter the filename"
elif [ -f "$2" ]; then
 echo "$2 Fille does exist" 
else
 echo "$2 File doesn't exist"
 echo "Creating $2"
 touch $2
exit 1
fi

counter=1
echo -n "Enter a file name : "
read file

if  [ ! -f $file ]
then
    echo "$file not a file!"
    exit 1
fi

Best Answer

Here is your script with some modifications. 

#!/bin/bash
# if file exist

#echo "Testing \ "$1""
if [ "$1" == "" ]
then
    read -r -p "Enter the filename" file1
else
    file1=$1
fi

if [ -f "$file1" ]
then
    echo "$file1 does exist"
else
     echo "$file1 does not exist!" 
     echo "Creating $file1"
     echo "1" > "$file1"
     exit 1
fi

#echo "Testing \ "$2""
if [ "$2" == "" ]
then
    read -r -p "Enter the filename" file2
else
    file1=$2
fi

if [ -f "$file2" ]
then
    echo "$file2 does exist"
else
     echo "$file2 does not exist!" 
     echo "Creating $file2"
     echo "1" > "$file2"
     exit 1
fi

if [ "$3" == "" ]
then
    read -r -p "Enter the filename" file3
else
    file3=$3
fi

# the following assumes that the data from the file is an integer
# and that it consists of only one line containing one value
# similar techniques can be used to do something much more powerful
data1=$(<"$file1")
data2=$(<"$file2")
# it's usually a good idea to validate data, but I have not included any validation
((data3 = data1 + data2))
echo "$data3" > "$file3"    # overwrite the previous contents of the file with the new value

As long as neither file1 or file2 changes, the contents of file3 will always be the same on repeated runnings of the script above. If file1 or file2 don't exist, a "1" is written to them as a default value.

Here is an improved version of the script using functions:

#!/bin/bash
checkarg () {
    local filename=$1
    if [ "$filename" == "" ]
    then
        read -r -p "Enter the filename" filename
    fi
    echo "$filename"
}

checkfile () {
    local filename=$1
    if [ -f "$filename" ]; then
        echo "$filename does exist"
    else
         echo "$filename does not exist!" 
         echo "Creating $filename"
         echo "1" > "$filename"
         # you could remove this exit if you want the script to continue
         # with newly created files instead of exiting
         exit 1
    fi
}

file1=$(checkarg "$1")
checkfile "$file1"

file2=$(checkarg "$2")
checkfile "$file2"

file3=$(checkarg "$3")

data1=$(< "$file1")
data2=$(< "$file2")
((data3 = data1 + data2))
echo "$data3" > "$file3"
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