shell script issue with positional arguments and if else

Question

I'm trying to write a script that will accept positional arguments (command-line arguments) that specify a mathematical operation (add, sub, mult, div, mod) and two values. The script is supposed to get this input, compute the result, and display the answer. I'm not sure where I went wrong because when I try to run my script, this is what I get

./trial.sh mod 15 10
./trial.sh: 6:        if [[ mod == add ]] then: not found
Result:  25
./trial.sh: 9:        elif [[ mod == sub ]] then: not found
Result:  5
./trial.sh: 12:        elif [[ mod == mult ]] then: not found
Result:  150
./trial.sh: 15:        elif [[ mod == div ]] then: not found
Result:  1
./trial.sh: 18:        elif [[ mod == mod ]] then: not found
Result:  5
./trial.sh: 21:        else: not found
error
./trial.sh: 23: fi: not found

This is the code I used that presented that result

#!/bin/bash
        a=$1
        b=$2
        c=$3
        result=0        
"       if [[ $a == add ]] then"
           result=$((b + c))
           echo "Result: " $result
"       elif [[ $a == sub ]] then"
           result=$((b - c))
           echo "Result: " $result
"       elif [[ $a == mult ]] then"
           result=$((b * c))
           echo "Result: " $result
"       elif [[ $a == div ]] then"
           result=$((b / c))
           echo "Result: " $result
"       elif [[ $a == mod ]] then"
           result=$((b % c))
           echo "Result: " $result
"       else"
           echo "error"
        "fi"

If you look up e.g. How do I compare two string variables in an 'if' statement? you'll see that none of them use quotes around " if [[ $a == add ]] then". You shouldn't either. — that other guy
– that other guy, Commented May 6, 2021 at 20:41
Syntax error: "elif" unexpected (expecting "then") is what shows up when I remove the "" — Toaster1
– Toaster1, Commented May 6, 2021 at 20:45
Great. If you Google that error you get BASH script expecting then, when I need else, or if you run it through ShellCheck you get "SC1010: Use semicolon or linefeed before 'then'". — that other guy
– that other guy, Commented May 6, 2021 at 20:49

webb · Accepted Answer · 2021-05-06 21:05:52Z

4

Your sample code is incorrect in several ways. You wrapped some of your statements in double-quotes to make the errors stop, but this is a bit like taking the engine out of a car because it's making a weird noise.

Here are three things you can do immediately to make your code work:

take away the double-quotes so that your code is code statements again and not strings. If you want to temporarily disable some shell code, put # before it to comment it out, e.g.,

# don't wrap broken code in double-quotes:
"if[x= 1]] then rm -rf /"
# instead, comment it out, like this:
# if[x= 1]] then rm -rf /

in bash if...then, a newline or semicolon is required before then , e.g.,

if [[ $a == 1 ]] then   # wrong

if [[ $a == 1 ]]; then  # ok

if [[ $a == 1 ]]        # also ok
then

change #!/bin/sh to #!/bin/bash because many variants of sh don't support double-bracket [[ tests.

answered May 6, 2021 at 21:05

webb

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1 Comment

Toaster1 Over a year ago

This helped. Thank you.

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