checking arguments in a shell script

Question

I'm having a surprising large amount of trouble finding a guide on this.

what's wrong with this shell script:

if ["$1" == "-t"]
then
        echo "t"
elif ["$1" == "-r"]
then
        echo "r"
fi

It's supposed to be a very simple test. if the argument is -t, do one thing, if it's -r do the other.

check it with shellcheck.net for the basic errors. It should be trivial! Hint: [ ] (<strike>statements</strike>) commands need spaces around the brackets. — fedorqui
– fedorqui, Commented May 6, 2015 at 16:01
@fedorqui (Yeah, this is a nit, but ...) [] is not a statement. [ is a command. Too many people get confused thinking that [ is part of the shell grammar, and calling it a "statement" reinforces that error. — William Pursell
– William Pursell, Commented May 6, 2015 at 16:04
Never chain if/else like this: case $1 in; -t) echo t;; -r) echo r;; esac — William Pursell
– William Pursell, Commented May 6, 2015 at 16:05

Community · Accepted Answer · 2017-05-23 11:43:39Z

3

[ is a command (see it with man [ or directly to man test). Thus, you need spaces around them in order to be interpreted properly:

if [ "$1" == "-t" ]
then
        echo "t"
elif [ "$1" == "-r" ]
then
        echo "r"
fi

Or better use POSIX compliant =: if [ "$1" = "-t" ] ....

See interesting comments by Charles Duffy and William Pursell below.

edited May 23, 2017 at 11:43

CommunityBot

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answered May 6, 2015 at 16:05

fedorqui

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2 Comments

Charles Duffy Over a year ago

If we're going to try to be pedantically correct, might be preferable to use the (POSIX-compliant) string comparison operator = rather than the extension ==.

William Pursell Over a year ago

Another nit: [] is also not a command! [ is a command, and ] is the last argument to it. It's probably easier to say [] is a command, though, but...let the reader beware!

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