Why should there be spaces around '[' and ']' in Bash?

I was trying to write a Bash script that uses an if statement.

if[$CHOICE -eq 1];

The script was giving me errors until I gave a space before and after [ and before ] as shown below:

if [ $CHOICE -eq 1 ];

My question here is, why is the space around the square brackets so important in Bash?

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最佳答案

Once you grasp that [ is a command, a whole lot becomes clearer!

[ is another way to spell "test".

help [

However while they do exactly the same, test turns out to have a more detailed help page. Check

help test

...for more information.

Furthermore note that I'm using, by intention, help test and not man test. That's because test and [ are shell builtin commands nowadays. Their feature set might differ from /bin/test and /bin/[ from coreutils which are the commands described in the man pages.

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From another question:

A bit of history: this is because '[' was historically not a shell-built-in but a separate executable that received the expresson as arguments and returned a result. If you didn't surround the '[' with space, the shell would be searching $PATH for a different filename (and not find it) . – Andrew Medico Jun 24 '09 at 1:13

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[ is a test command. So it requires space.

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[ is a command and $CHOICE should be an argument, but by doing [$CHOICE (without any space between [ and $CHOICE) you are trying to run a command named [$CHOICE. The syntax for command is:

command arguments separated with space
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It's worth noting that [ is also used in glob matching, which can get you into trouble.

$ echo [12345]
[12345]
$ echo oops >3
$ echo [12345]
3

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