How to use Bash to create a folder if it doesn't already exist?

#!/bin/bash
if [!-d /home/mlzboy/b2c2/shared/db]; then
    mkdir -p /home/mlzboy/b2c2/shared/db;
fi;

This doesn't seem to work. Can anyone help?

First, in Bash [ is just a command, which expects string ] as a last argument, so the whitespace before the closing bracket (as well as between ! and -d which need to be two separate arguments too) is important:

if [ ! -d /home/mlzboy/b2c2/shared/db ]; then
  mkdir -p /home/mlzboy/b2c2/shared/db;
fi

Second, since you are using -p switch for mkdir, this check is useless, because this is what it does in the first place. Just write:

mkdir -p /home/mlzboy/b2c2/shared/db;

and that's it.

There is actually no need to check whether it exists or not. Since you already wants to create it if it exists , just mkdir will do

mkdir -p /home/mlzboy/b2c2/shared/db

Simply do:

mkdir /path/to/your/potentially/existing/folder

mkdir will throw an error if the folder already exists. To ignore the errors write:

mkdir -p /path/to/your/potentially/existing/folder

No need to do any checking or anything like that.

For reference:

-p, --parents no error if existing, make parent directories as needed http://man7.org/linux/man-pages/man1/mkdir.1.html

You need spaces inside the [ and ] brackets:

#!/bin/bash
if [ ! -d /home/mlzboy/b2c2/shared/db ] 
then
    mkdir -p /home/mlzboy/b2c2/shared/db
fi

Cleaner way, exploit shortcut evaluation of shell logical operators. Right side of the operator is executed only if left side is true.

[ ! -d /home/mlzboy/b2c2/shared/db ] && mkdir -p /home/mlzboy/b2c2/shared/db