When running bash script there is a logical error - bash

Question

So I have fixed my first issue with the script taking input but now no matter what letter I enter it only adds the numbers.
here is the code. any help would be greatly appreciated.
#!/bin/bash
add() {
expr $x + $y
}
sub() {
expr $x - $y
}
mult() {
expr $x * $y
}
div() {
expr $x / $y
}
echo "Enter a for add, s for subtract, m for multiply or d for divide and 2 numbers"
read choice x y
if [ $choice=="a" ]
then
add
else
if [ $choice == "s" ]
then
sub
else
if [ $choice == "m" ]
then
mult
else
if [ $choice == "d" ]
then
div
fi
fi
fi
fi

Answer 1

It seems like you want to get x and y from first and second arguments ( $1 and $2 ) and to read the operation ( a, s, d, m ) from stdin.
I modified your code a bit, to overcome the problems in the original script and to provide the result based on my assumptions:
#!/bin/bash
# First number.
x=$1
# Second number.
y=$2
# Result of either addition, subtraction, division or multiplication of $x and $y.
result=0
# Reads operation from user.
read -ep "Enter a for add, s for subtract, m for multiply or d for divide: " operation
case $operation in
a) result=$(( x + y ));;
s) result=$(( x - y ));;
d) result=$(( x / y ));;
m) result=$(( x * y ));;
*) printf '%s: %s\n' "$operation" "Unknown operation" >&2; exit 1;;
esac
printf 'result: %s\n' "$result"
Usage example: ( script name is sof.sh )
./sof.sh 5 4
Enter a for add, s for subtract, m for multiply or d for divide: a
result: 9
./sof.sh 5 4
Enter a for add, s for subtract, m for multiply or d for divide: m
result: 20
./sof.sh 5 4
Enter a for add, s for subtract, m for multiply or d for divide: s
result: 1
./sof.sh 5 4
Enter a for add, s for subtract, m for multiply or d for divide: d
result: 1
P.S.
Please note the following:
expr is a program used in ancient shell code to do math. In POSIX shells like bash, use $(( expression )). In bash, ksh88+, mksh/pdksh, or zsh, you can also use (( expression )) or 'let
expression'.
Though not used originaly in the script, while programming in Bash it is worth knowing that [[ is a bash keyword similar to (but more powerful than) the [ command. See this Bash FAQ and Test and conditionals.
Unless you're writing for POSIX sh, it is recommended to use [[.

Answer 2

First of all, you want the script to read the values from the standard input, but you are recovering it from the arguments.
Second, you are not passing parameters to the functions.
Third, you are not using parameters inside the functions.
Fourth, you are not letting spaces between operators when using expr.
NOTE: Rany Albeg Wein remarked that this bash guide is outdated, and he recommends this one. Also i recommend the GNU official guide (other formats).
So, assuming that you want to use your script like ./my-script.sh m 2 3 , here is your code, but working:
#!/bin/bash
add() {
expr $1 + $2
}
sub() {
expr $1 - $2
}
mult() {
expr $1 \* $2
}
div() {
expr $1 / $2
}
echo "Enter a for add, s for subtract, m for multiply or d for divide and 2 numbers"
x=$2
y=$3
if [ $1 == "a" ]
then
add $x $y
else
if [ $1 == "s" ]
then
sub $x $y
else
if [ $1 == "m" ]
then
mult $x $y
else
if [ $1 == "d" ]
then
div $x $y
fi
fi
fi
fi
And finally this is your script minimally modified to read the data from the standard input:
#!/bin/bash
add() {
echo "Result:"
expr $1 + $2
}
sub() {
echo "Result:"
expr $1 - $2
}
mult() {
echo "Result:"
expr $1 \* $2
}
div() {
echo "Result:"
expr $1 / $2
}
echo "Enter a for add, s for subtract, m for multiply or d for divide and 2 numbers"
read operation
echo "Read first parameter"
read x
echo "Read second parameter"
read y
if [ $operation == "a" ]
then
add $x $y
else
if [ $operation == "s" ]
then
sub $x $y
else
if [ $operation == "m" ]
then
mult $x $y
else
if [ $operation == "d" ]
then
div $x $y
fi
fi
fi
fi
Also, if you had some troubles, you could add debugging messages to the script just setting #!/bin/bash -xv at the beginning of the script.