How to combine AND and OR condition in bash script for if condition?

Question

I was trying to combine logical AND & OR in a bash script within if condition. Somehow I am not getting the desired output and it is hard to troubleshoot. I am trying to validate the input parameters passed to a shell script for no parameter and the first parameter passed is valid or not.

if [ "$#" -ne 1 ] && ([ "$1" == "ABC" ] || [ "$1" == "DEF" ] || [ "$1" == "GHI" ] || [ "$1" == "JKL" ]) 
then
echo "Usage: ./myscript.sh [ABC | DEF | GHI | JKL]"
exit 1
fi

Can anyone point out what is going wrong here?

score 15 · Accepted Answer · edited May 23 '17 at 12:22

The immediate problem with your statement is one of logic: you probably meant to write:

if [ "$#" -ne 1 ] || ! ([ "$1" = "ABC" ] || [ "$1" = "DEF" ] || [ "$1" = "GHI" ] || [ "$1" = "JKL" ]) 
then
  echo "Usage: ./myscript.sh [ABC | DEF | GHI | JKL]" >&2
  exit 1
fi

That is: abort, if either more than 1 argument is given OR if the single argument given does NOT equal one of the acceptable values.

Note the ! to negate the expression in parentheses and the use of the POSIX-compliant form of the string equality operator, = (rather than ==).

However, given that you're using Bash, you can make do with a single [[ ... ]] conditional and Bash's regular-expression matching operator, =~:

if [[ $# -ne 1 || ! $1 =~ ^(ABC|DEF|GHI|JKL)$ ]] 
then
  echo "Usage: ./myscript.sh [ABC | DEF | GHI | JKL]" >&2
  exit 1
fi

If POSIX compliance is not required, [[ ... ]] is preferable to [ ... ] for a variety of reasons. In the case at hand, $# and $1 didn't need quoting, and || could be used inside the conditional.

Note that =~ as used above works in Bash 3.2+, whereas the implicit extglob syntax used in anubhava's helpful answer requires Bash 4.1+;
in earlier versions you can, however, explicitly enable (and restore to its original value after) the extglob shell option: shopt -s extglob.

score 8 · Answer 2 · answered Jun 28 '16 at 05:07

8

BASH actually allows use of extended glob inside [[ ... ]] and have && inside as well.

So you can do:

if [[ $# -ne 1 && $1 == @(ABC|DEF|GHI|JKL) ]]; then
   echo "Usage: ./myscript.sh [ABC | DEF | GHI | JKL]"
   exit 1
fi

answered Jun 28 '16 at 05:07

anubhava

761,203
64
569
643

1

Yes that will also work but this one is shorter since it has only one `[[ ... ]]` expression – anubhava Jun 28 '16 at 05:17
However I think in this code it should be `$1 != ...` but I just replicated OP's conditions – anubhava Jun 28 '16 at 05:22
1

++, but please note that the implicit `extglob` syntax only works in Bash 4.1+. – mklement0 Jun 28 '16 at 05:51
2

Prior to 4.1, you just need to explicitly enable it with `shopt -s extglob`. – chepner Jun 28 '16 at 12:23

score 2 · Answer 3 · answered Jun 28 '16 at 05:24

2

A few things:

[...] in bash is equivalent to the same test command (check the man page), so those && and || are not logical operators, but rather the shell equivalents
Parentheses in POSIX shell are not a grouping operator. They will work here, but they open a subshell, you are better off using standard test options of -a and -o (making your if statement if [ "$#" -ne 1 -a $ "$1" == "ABC" -o "$1" == "DEF" -o "$1" == "GHI" -o "$1" == "JKL" $ ], though based on your logic, it sounds like you actually want something like if [ "$#" -ne 1 -o $ "$1" != "ABC" -a "$1" != "DEF" -a "$1" != "GHI" -a "$1" != "JKL" $ ]. You probably can get better results with a case statement like follows:

usage() {
    echo "Usage: ./myscript.sh [ABC | DEF | GHI | JKL]"
}


if [ "$#" -ne 1 ] 
then
    usage
    exit 1
fi

case "$1" in
    ABC)
        echo "Found ABC"
        ;;
    DEF)
        echo "Found DEF"
        ;;
    GHI)
        echo "Found GHI"
        ;;
    JKL)
        echo "Found JKL"
        ;;
    *)
        usage
        exit 1
        ;;
esac

If you want to pass a set of possible static arguments in, you might want to look at the getopts special shell command.

answered Jun 28 '16 at 05:24

Taywee

1,313
11
17

Good points; quibble: the [POSIX spec. for `test`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html) itself warns against use of `-a`, `-o`, and `()`: "The XSI extensions specifying the -a and -o binary primaries and the '(' and ')' operators have been marked obsolescent. (Many expressions using them are ambiguously defined by the grammar depending on the specific expressions being evaluated.) " – mklement0 Jun 28 '16 at 06:03
That's a good quibble. How would you alternately do this with just POSIX? command groups (`{...}`), &&, and ||? – Taywee Jun 28 '16 at 06:47
1

@Taywee: correct, you would use `[ ... ] && { [ ... ] || [ ... ]; }` – chepner Jun 28 '16 at 12:29
1

You can also shorten the `case` statement with `ABC|DEF|GHI|JKL) echo "Found $1" ;;`. – chepner Jun 28 '16 at 12:29
Command groups are a good idea (to avoid a subshell); a pitfall worth pointing out: with `test`, `-a` has higher precedence than `-o`, but the shell's `&&` and `||` have _equal_ precedence. – mklement0 Jun 28 '16 at 13:34
1

@chepner I know, I'm operating under the assumption that they'll probably do different things and might want to be handled independently. – Taywee Jun 28 '16 at 20:47

How to combine AND and OR condition in bash script for if condition?

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