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?
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.
/usr/bin/[ and was a hardlink to /usr/bin/test, when invoked as [ you should include a closing brace as last argument. Now it's a shell builtin.
[ requires that its last argument be ].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
[ 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 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 
[12345] so the shell leaves it unexpanded. But when there is a file named 3, that's what the wildcard expands to.[ is a test command. So it requires space.
if command; then commands_if_exit_code_is_0; else commands_if_exit_code_is_nonzero; fi;.commandcan be any command, and[is just one of them.$CHOICEis"foo", you are attempting to run the command["foo".[[obeys the same requirements; you need spaces on both sides, and the command needs to end with]]. The doubled version is somewhat more versatile and robust; see the link by @codeforester above for a more nuanced discussion.