How can I use a variable as a case condition?

Question

I am trying to use a variable consisting of different strings separated with a | as a case statement test. For example:

string="\"foo\"|\"bar\""
read choice
case $choice in
    $string)
        echo "You chose $choice";;
    *)
        echo "Bad choice!";;
esac

I want to be able to type foo or bar and execute the first part of the case statement. However, both foo and bar take me to the second:

$ foo.sh
foo
Bad choice!
$ foo.sh
bar
Bad choice!

Using "$string" instead of $string makes no difference. Neither does using string="foo|bar".

I know I can do it this way:

case $choice in
    "foo"|"bar")
        echo "You chose $choice";;
    *)
        echo "Bad choice!";;
esac

I can think of various workarounds but I would like to know if it's possible to use a variable as a case condition in bash. Is it possible and, if so, how?

Answer 1

The bash manual states:

case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac

Each pattern examined is expanded using tilde expansion, parameter and variable expansion, arithmetic substitution, command substitution, and process substitution.

No «Pathname expansion»

Thus: a pattern is NOT expanded with «Pathname expansion».

Therefore: a pattern could NOT contain "|" inside. Only: two patterns could be joined with the "|".

This works:

s1="foo"; s2="bar"    # or even s1="*foo*"; s2="*bar*"

read choice
case $choice in
    $s1|$s2 )     echo "Two val choice $choice"; ;;  # not "$s1"|"$s2"
    * )           echo "A Bad  choice! $choice"; ;;
esac

Using « Extended Globbing »

However, word is matched with pattern using « Pathname Expansion » rules.
And « Extended Globbing » here, here and, here allows the use of alternating ("|") patterns.

This also work:

shopt -s extglob

string='@(foo|bar)'

read choice
    case $choice in
        $string )      printf 'String  choice %-20s' "$choice"; ;;&
        $s1|$s2 )      printf 'Two val choice %-20s' "$choice"; ;;
        *)             printf 'A Bad  choice! %-20s' "$choice"; ;;
    esac
echo

String content

The next test script shows that the pattern that match all lines that contain either foo or bar anywhere is '*$(foo|bar)*' or the two variables $s1=*foo* and $s2=*bar*

---

Testing script:

shopt -s extglob    # comment out this line to test unset extglob.
shopt -p extglob

s1="*foo*"; s2="*bar*"

string="*foo*"
string="*foo*|*bar*"
string='@(*foo*|*bar)'
string='*@(foo|bar)*'
printf "%s\n" "$string"

while IFS= read -r choice; do
    case $choice in
        "$s1"|"$s2" )   printf 'A first choice %-20s' "$choice"; ;;&
        $string )   printf 'String  choice %-20s' "$choice"; ;;&
        $s1|$s2 )   printf 'Two val choice %-20s' "$choice"; ;;
        *)      printf 'A Bad  choice! %-20s' "$choice"; ;;
    esac
    echo
done <<-\_several_strings_
f
b
foo
bar
*foo*
*foo*|*bar*
\"foo\"
"foo"
afooline
onebarvalue
now foo with spaces
_several_strings_

Answer 2

You can use the extglob option:

shopt -s extglob
string='@(foo|bar)'

Answer 3

You need two variables for case because the or | pipe is parsed before the patterns are expanded.

v1=foo v2=bar

case foo in ("$v1"|"$v2") echo foo; esac

---

foo

Shell patterns in variables are handled differently when quoted or unquoted as well:

q=?

case a in
("$q") echo question mark;;
($q)   echo not a question mark
esac

---

not a question mark

Answer 4

Here is a POSIX solution using eval :

#!/bin/sh
string="foo|bar"
read choice
eval "
case \$choice in
    $string)
        echo \"You chose \$choice\";;
    *)
        echo \"Bad choice!\";;
esac
"

Explanation : the eval command makes the shell first expand the arguments so we are left with

case $choice in
    foo|bar)
        echo "You chose $choice";;
    *)
        echo "Bad choice!";;
esac

and then it interprets again the command and executes it.

Answer 5

If You want a dash-compatible work-around, You could write:

  string="foo|bar"
  read choice
  awk 'BEGIN{
   n=split("'$string'",p,"|");
   for(i=1;i<=n;i++)
    if(system("\
      case \"'$choice'\" in "p[i]")\
       echo \"You chose '$choice'\";\
       exit 1;;\
      esac"))
     exit;
   print "Bad choice"
  }'

Explanation:

• awk is used to split string and test each part separately. If choice matches the currently tested part p[i], the awk-command will be ended with exit in line 11.
• For the very test, the shell's case is used (within a system-call), as asked by @terdon. This keeps the possibility to modify the intended test-string for example to foo*|bar in order to match also for foooo ("search pattern"), as the shell's case allows.
• If instead you would prefer regular expressions, you could omit the system-call and use awk's ~or match instead.

Answer 6

One can use AWK's syntax

if($field ~ /regex/)

as well as

if($i ~ var)

to compare variable to input (var and star-argument list $*)

parse_arg_exists() {
  [ $# -eq 1 ] && return
  [ $# -lt 2 ] && printf "%s\n" "Usage: ${FUNCNAME[*]} <match_case> list-or-\$*" \
  "Prints the argument index that's matched in the regex-case (~ patn|patn2)" && exit 1
  export arg_case=$1
  shift
  echo "$@" | awk 'BEGIN{FS=" "; ORS=" "; split(ENVIRON["arg_case"], a, "|")} {
    n=-1
    for(i in a) {
     for(f=1; f<=NF; f++) {
      if($f ~ a[i]) n=f
     }
    }
  }
  END {
    if(n >= 0) print "arg index " n "\n"
    }'
 unset arg_case
}
string="--dot|-d"
printf "testing %s\n" "$string"
args="--dot -b -c"; printf "%s\n" "$args"
parse_arg_exists "$string" "$args"
args="-b -o"; printf "%s\n" "$args"
parse_arg_exists "$string" "$args"
args="-b -d -a"; printf "%s\n" "$args"
parse_arg_exists "$string" "$args"

Prints out:

testing --dot|-d
--dot -b -c
arg index 1
 -b -o
-b -d -a
arg index 2

Answer 7

If you don't want to assign multiple variables for each option and only have one variable containing all the choices in a single string, then a suggestion is by using =~ operator to treat "foo|bar" as a regular expression in an if statement. You can then expand the desired choices. "foo|bar|more|another-choice" This also supports inputs with a dash in the pattern.

References:

• https://linuxhint.com/equal-tilde-bash/

#!/bin/bash
string="^(foo|bar)$"
read choice
if [[ "${choice}" =~ ${string} ]]; then
  "You chose $choice"
else
  echo "Bad choice!"
fi