How to repeat prompt to user in a shell script?

Question

In a set of if/elif/else/fi statements, I have made 'else' present the user with an error message, but I also want it to take the user back to the question which was asked before the if/else statements so that they can try to answer it again.

How do I take the user back to a previous line of code? Or, if this is not possible, is there another way to do this?

Answer 1

I think the easiest way would be to wrap the prompting code into a function, and then drop it into an until loop.

Since all you need really is to call the function until it succeeds, you can put the noop command ":" in the until loop.

Something like this:

#!/bin/bash

getgender() {
  read -p "What is the gender of the user? (male/female): " gender
  case "$gender" in
    m|M)
      grouptoaddto="boys"
      return 0
      ;;
    f|F)
      grouptoaddto="girls"
      return 0
      ;;
    *)
      printf %s\\n "Please enter 'M' or 'F'"
      return 1
      ;;
  esac
}

until getgender; do : ; done
sudo usermod -a -G "$grouptoaddto" "$username"

---

The point here is the function called with until, so it is repeatedly called until it succeeds. The case switch within the function is just an example.

---

Simpler example, without using a function:

while [ -z "$groupname" ]; do
  read -p "What gender is the user?" answer
  case "$answer" in
    [MmBb]|[Mm]ale|[Bb]oy) groupname="boys" ;;
    [FfGg]|[Ff]emale|[Gg]irl) groupname="girls" ;;
    *) echo "Please choose male/female (or boy/girl)" ;;
  esac
done
sudo usermod -a -G "$groupname" "$username"

In this last example, I'm using the -z switch to the [ (test) command, to continue the loop as long as the "groupname" variable has zero length.

The keynote is the use of while or until.

To translate this last example into human readable pseudocode:

While groupname is empty,

  ask user for gender.

  If he answers with one letter "m" or "B",
    or the word "Male" or "boy",
    set the groupname as "boys".

  If she answers with one letter "F" or "g",
    or the word "female" or "Girl",
    set the groupname as "girls".

  If he/she answers anything else, complain.

(And then repeat, since groupname is still empty.)

Once you have groupname populated,
  add the user to that group.

---

Yet another example, without the groupname variable:

while true; do
  read -p "What gender is the user?" answer
  case "$answer" in
    [MmBb]|[Mm]ale|[Bb]oy)
      sudo usermod -a -G boys "$username"
      break
      ;;
    [FfGg]|[Ff]emale|[Gg]irl)
      sudo usermod -a -G girls "$username"
      break
      ;;
    *) echo "Please choose male/female (or boy/girl)" ;;
  esac
done

Answer 2

flag=1
while [ ${flag} -eq 1 ]
do
  read -p "Please answer B or G " bg
  if [ "${bg}" = B ] || [ "${bg}" = b ] 
  then
    flag=0
    groupname=boys
  else 
    if [ "${bg}" = G ] || [ "${bg}" = g ]
    then
      flag=0
      groupname=girls
    fi
  fi
done
sudo usermod -a -G ${groupname} $username

this is the simplest way I can think of, while clearly showing what it happening.