bash

scottmusician · 04-01-2018, 09:34 AM

Hi everyone,

I'm just starting to learn bash script programming, and I'm having trouble making one of my first shell scripts to function correctly. There's probably just some syntax errors - so I'm hoping that you can help.

It's a really simple script (actually it's one of the workshop quizes, as a part of the Linux Foundation's LS101 course) - all it does is ask for a few variables, and then calculate some simple arithmetic based on them. Whilst I could simply look at the lab answer provided in my course, I'd much prefer to learn about *why* my code isn't functioning 100%!

The script is below. Whilst it executes, there's a few things that it doesn't do, which I want it to:

(1) so I'm not getting an answer (ie. $total isn't being calculated). Is there something wrong about passing the variables to the functions (or maybe the if/elif/fi loop) ?

(2) in order to loop the error checking, maybe I could use a while $operator !== -style statement?

(3) is there a way that I can use a regular expression to simplify the error checking? I've tried using

Code:

if [[ $operator !== {aAmMsSdD} ]]

but that did not seem to work.
thank you again for your thoughts!

Cheers,

Scott

Code:

# functions:

adding() {
total = $(( $num1 + $num2 )) 
}

subtraction() {
total = $(( $num1 - $num2 ))
}

multiply() {  
total = $(( $num1 * $num2 ))
}

divide() {
total = $(( $num1 / $num2 ))
}

echo "would you like to [a]dd, [s]ubtract, [m]ultiply, or [d]ivide?"
echo "Enter a, s, m, or d:"
read operator

# check for input validation (ie. give an error if %operator != a,m,s, or d)

if [[ $operator != a ]] &&  [[ $operator != s ]] && [[ $operator != d ]] && [[ $operator != m ]] ; then  echo "operator must be a, s, m, or d" ; echo "enter a, s, m, d:" ; read operator
fi

echo "enter number 1"
read num1

echo "enter number 2"
read num2

# calculations

if [[ $operator == a ]] ; then adding $num1 $num2
elif [[ $operator == s ]] ; then subtraction $num1 $num2
elif [[ $operator == m ]] ; then multiply $num1 $num2
elif [[ $operator == d ]] ; then divide $num1 $num2 
fi
# show answers

echo $num1 $operator $num2 :
echo "The answer is: $total"

pan64 · 04-01-2018, 09:56 AM

first of all I would suggest you to use shellcheck to check the syntax of your script (see www.shellcheck.net if not installed).
And there is a site www.tldp.org to check, for example: http://tldp.org/LDP/Bash-Beginners-Guide/html/

Regarding your questions:
1: yes, you pass variables not by name, but by position, see positional parameters ($1, $2 ...)
2: !== is invalid, you need to check (for example man bash) about the valid comparators and tests.
3: yes, you may try regexp, but I would rather suggest you to use case:

Code:

case $op in
   a) ...
   b) ...
...
esac

BW-userx · 04-01-2018, 04:28 PM

http://tldp.org/LDP/abs/html/comparison-ops.html
http://tldp.org/LDP/abs/html/ops.html
http://tldp.org/LDP/abs/html/arithexp.html
http://tldp.org/LDP/abs/html/functions.html

that should keep you busy for a bit.

jlinkels · 04-01-2018, 07:45 PM

Look at my signature. Especially the bash -x The pointers to documentation were already provided.

jlinkels

MadeInGermany · 04-03-2018, 11:59 AM

Functions do have local parameters.

Code:

adding() {
total = $(( $1 + $2 )) 
}

case $operator in
(a)
  ...
;;
(s) 
  ...
;;
esac

I suggest ( ) not the old imbalanced ) notation.
And indention only for the
...
shell code.
Use a while loop, testing the input with the == or != operator within [[ ]] (where the 2nd operand is a glob, just like the operand in ( ) in a case-esac).

Code:

echo "Would you like to [a]dd, [s]ubtract, [m]ultiply, or [d]ivide?"
while
  echo "Enter a, s, m, or d:"
  read operator
  [[ $operator != a|s|m|d ]]
do
  echo "operator must be a, s, m, or d" 
done

secomax · 05-01-2018, 05:28 AM

I would like to add another useful resource that may help https://likegeeks.com/bash-functions/
It discusses how to pass arrays to functions and some other advanced topics about Bash functions.

Regards,

ntubski · 05-01-2018, 06:27 AM

secomax, are you affiliated with that website? Because reviving an old thread with what amounts to just a link looks pretty close to spam.

secomax · 05-01-2018, 09:00 AM

I was searching for Bash functions and I thought that would be an addition to anyone searches the forum for it like me.