Shell Spacing in Square Brackets

aces are required after [ and before ].

1. [ is actually the name of a command, an alias for test. It’s not a special symbol, it’s just a command with an unusual name.

   $ help '['
   [: [ arg... ]
       Evaluate conditional expression.
   
       This is a synonym for the "test" builtin, but the last argument must
       be a literal `]', to match the opening `['.

2. Because it’s an ordinary command name and not a special character, a space is required after the [. If you omit the space and write [foo the shell will search the $PATH for a command named [foo.

   $ [ foo = foo ] && echo true
   true
   $ [foo = foo] && echo true
   [foo: command not found

3. For readability’s sake, [ expects its last argument to be exactly ]. Being an ordinary command-line argument, ] must have a space before it. If there’s no space then the bracket will become the last character of the previous argument, and [ will complain about its last argument not being ].

   $ [ foo = foo]
   bash: [: missing `]'
   $ [ foo = 'foo]'
   bash: [: missing `]'

[[ is a bash enhancement with more features than [, namely saner handling of unquoted variable names. It requires the a space on both ends, same as [. However [[ is in fact special shell syntax and is parsed differently. It’s not an “ordinary command” the way [ is.

1. || operators for boolean tests and < and > for string comparisons. [ cannot do this because it is a regular command and &&, ||, <, and > are not passed to regular commands as command-line arguments.
2. It has a wonderful =~ operator for doing regular expression matches. With [ you might write

   if [ "$answer" = y -o "$answer" = yes ]

   With [[ you can write this as

   if [[ $answer =~ ^y(es)?$ ]]

   It even lets you access the captured groups which it stores in BASH_REMATCH. For instance, ${BASH_REMATCH[1]} would be “es” if you typed a full “yes” above.

3. You get pattern matching aka globbing for free. Maybe you’re less strict about how to type yes. Maybe you’re okay if the user types y-anything. Got you covered:

   if [[ $ANSWER = y* ]]

Keep in mind that it is a bash extension, so if you are writing sh-compatible scripts then you need to stick with [. Make sure you have the #!/bin/bash shebang line for your script if you use double brackets.