Using the not equal operator for string comparison

Question

I tried to check if the PHONE_TYPE variable contains one of three valid values.

if [ "$PHONE_TYPE" != "NORTEL" ] || [ "$PHONE_TYPE" != "NEC" ] ||
   [ "$PHONE_TYPE" != "CISCO" ]
then
    echo "Phone type must be nortel,cisco or nec"
    exit
fi

The above code did not work for me, so I tried this instead:

if [ "$PHONE_TYPE" == "NORTEL" ] || [ "$PHONE_TYPE" == "NEC" ] ||
   [ "$PHONE_TYPE" == "CISCO" ]
then
    :        # do nothing
else
    echo "Phone type must be nortel,cisco or nec"
    exit
fi

Are there cleaner ways for this type of task?

score 268 · Accepted Answer · edited Feb 12 '19 at 20:07

I guess you're looking for:

if [ "$PHONE_TYPE" != "NORTEL" ] && [ "$PHONE_TYPE" != "NEC" ] &&
   [ "$PHONE_TYPE" != "CISCO" ]

The rules for these equivalents are called De Morgan's laws and in your case meant:

not(A || B || C) => not(A) && not(B) && not (C)

Note the change in the boolean operator or and and.

Whereas you tried to do:

not(A || B || C) => not(A) || not(B) || not(C)

Which obviously doesn't work.

score 56 · Answer 2 · edited Jun 19 '19 at 09:09

56

A much shorter way would be:

if [[ ! $PHONE_TYPE =~ ^(NORTEL|NEC|CISCO)$ ]]; then 
  echo "Phone type must be nortel, cisco or nec."
fi

^ – To match a starting at the beginning of line
$ – To match end of the line
=~ - Bash's built-in regular expression comparison operator

edited Jun 19 '19 at 09:09

serghei

145
6

answered Mar 14 '13 at 15:26

0x80

853
6
7

3

The question doesn't mention bash, and in fact deliberately only mentions `shell` script, both in the tags, and in the question. This answer would fail in a POSIX environment. – Jack_Hu May 16 '21 at 17:21
If you're using Bash (recommended) as this answer, there's an even cleaner syntax you can use, please see my answer below. – Jesse Nickles Apr 23 '23 at 11:23

Runium · Answer 3 · 2013-03-14T11:35:19.643

Good answers, and an invaluable lesson ;) Only want to supplement with a note.

What type of test one choose to use is highly dependent on code, structure, surroundings etc.

An alternative could be to use a switch or case statement as in:

case "$PHONE_TYPE" in
"NORTEL"|"NEC"|"CISCO")
    echo "OK"
    ;;
*)
    echo "Phone type must be nortel,cisco or nec"
    ;;
esac

As a second note you should be careful by using upper-case variable names. This is to prevent collision between variables introduced by the system, which almost always is all upper case. Thus $phone_type instead of $PHONE_TYPE.

Though that one is safe, if you have as habit using all upper case, one day you might say IFS="boo" and you're in a world of hurt.

It will also make it easier to spot which is what.

Not a have to but a would strongly consider.

It is also presumably a good candidate for a function. This mostly makes the code easier to read and maintain. E.g.:

valid_phone_type()
{
    case "$1" in
    "NORTEL"|"NEC")
        return 0;;
    *)
        echo "Model $1 is not supported"
        return 1;;
    esac
}

if ! valid_phone_type "$phone_type"; then
    echo "Bye."
    exit 1
fi

+1 for use of case statement, avoiding the complexity of regular expression. — Robin A. Meade, Dec 06 '21 at 18:17
The double-quotes in `case "$var" in ...` are not necessary because word-splitting doesn't occur in that context. — Robin A. Meade, Dec 06 '21 at 18:18

score 13 · Answer 4 · answered Mar 14 '13 at 09:57

13

You should use ANDs, not ORs.

if [ "$PHONE_TYPE" != "NORTEL" ] && [ "$PHONE_TYPE" != "NEC" ] && [ "$PHONE_TYPE" != "CISCO" ]
then

or

if [ "$PHONE_TYPE" != "NORTEL" -a "$PHONE_TYPE" != "NEC" -a "$PHONE_TYPE" != "CISCO" ]
then

answered Mar 14 '13 at 09:57

jlliagre

60,319
10
115
157

Will · Answer 5 · 2018-04-13T16:35:11.843

To correct an above answer (as I can't comment yet):

PHONE_TYPE="NORTEL"
if [[ $PHONE_TYPE =~ ^(NORTEL|NEC|CISCO|SPACE TEL)$ ]]; then 
  echo "Phone type accepted."
else
  echo "Error! Phone type must be NORTEL, CISCO or NEC."
fi

Please note that you need at least bash 4 for this use of =~
It doesn't work in bash 3.

I tested on MS Windows 7 using bash 4.3.46 (works fine) and bash 3.1.17 (didn't work)

The LHS of the =~ should be in quotes. Above, PHONE_TYPE="SPACE TEL" would match too.

score 1 · Answer 6 · edited Apr 23 '23 at 12:03

1

Less portable for POSIX, but works in Bash:

if [[ $PHONE_TYPE != @(NORTEL|NEC|CISCO) ]]; then 
    echo 'Phone type must be NORTEL, CISCO, or NEC' >&2
    exit 1
fi

edited Apr 23 '23 at 12:03

Kusalananda

320,670
36
633
936

answered Apr 23 '23 at 11:22

Jesse Nickles

165
1
11

If anyone wants to know what the mods changed, my answer was `if [[ "${PHONE_TYPE}" != @(NORTEL|NEC|CISCO)$ ]]; then` which I still believe is much better syntax when using Bash... – Jesse Nickles Apr 25 '23 at 18:36

score -1 · Answer 7 · answered Apr 10 '19 at 09:25

Just a variation proposal based on @0x80 solution:

# define phone brand list
phoneBrandList=" NORTEL NEC CISCO" ## separator is space with an extra space in first place

# test if user given phone is contained in the list
if [[ ${phoneBrandList} =~ (^|[[:space:]])"${userPhoneBrand}"($|[[:space:]]) ]]; then
    echo "found it !"
fi

Swapnil · Answer 8 · 2015-07-08T19:26:19.173

-2

Use [[ instead

if [[ "$PHONE_TYPE" != "NORTEL" ]] || [[ "$PHONE_TYPE" != "NEC" ]] || 
   [[ "$PHONE_TYPE" != "CISCO" ]]
then
echo "Phone type must be nortel,cisco or nec"
exit 1
fi

edited Jul 08 '15 at 19:26

answered Jul 08 '15 at 19:20

Swapnil

121
3

2

This is, of course, wrong. `[[` vs `[` doesn't help with the logic being off. – ilkkachu Feb 12 '19 at 20:10

Using the not equal operator for string comparison

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