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About linux : Unexpected-operator-in-shell-programming-duplicate

Question Detail

My code:

    #!/bin/sh
    #filename:choose.sh
    read choose
    [ "$choose" == "y" -o "$choose" == "Y" ] && echo "Yes" && exit 0
    [ "$choose" == "n" -o "$choose" == "N" ] && echo "No"  && exit 0
    echo "Wrong Input" && exit 0

But when I execute

    sh ./choose.sh

terminal prompt me that

   [: 4: n: :Unexpected operator
   [: 5: n: :Unexpected operator

Is there any mistake in my bash script?
Thanks!

Question Answer

There is no mistake in your bash script. But you are executing it with sh which has a less extensive syntax 😉

So, run bash ./choose.sh instead 🙂

POSIX sh doesn’t understand == for string equality, as that is a bash-ism. Use = instead.

The other people saying that brackets aren’t supported by sh are wrong, btw.

To execute it with Bash, use #!/bin/bash and chmod it to be executable, then use

./choose.sh

you can use case/esac instead of if/else

case "$choose" in
  [yY]) echo "Yes" && exit;;
  [nN]) echo "No" && exit;;
  * ) echo "wrong input" && exit;;
esac

you have to use bash instead or rewrite your script using standard sh

sh -c 'test "$choose" = "y" -o "$choose" = "Y"'

In fact the “[” square opening bracket is just an internal shell alias for the test command.

So you can say:

test -f "/bin/bash" && echo "This system has a bash shell"

or

[ -f "/bin/bash" ] && echo "This system has a bash shell"

… they are equivalent in either sh or bash. Note the requirement to have a closing “]” bracket on the “[” command but other than that “[” is the same as “test”. “man test” is a good thing to read.

Do not use any reserved keyword as the start of any variable name:
eg HOSTNAME will fail as HOST {TYPE|NAME} are reserved

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