UNIX for Dummies Questions & Answers

Hello,

I have done a great deal of research on this and still cannot come to an understanding of the following:

In if-else statements in shell scripting I have seen examples of single brackets, double brackets, single parantheses, and double paratheses. When should each one be used?

And what is the difference between using ' and " quotes in these transactions?

single brackets is the test utility in posix compliant shells, double brackets is a more efficient built-in statement, but it is part of ksh and bash, not posix. The same goes for the arithmetic double parentheses, also just ksh/bash. The single parentheses is a different cattle of fish and is not a test statement but it is used for subshells.

The single quote is literal quote without variable expansion and such, unlike the double quote...

Single square brackets delimit a "test" statement.
Double square brackets delimit a Conditional Expression.

Historically both invoked external commands but nowadays both are built into the shell (for example: ksh, POSIX sh). In ancient Bourne shell "[[" is actually a program.

There is some commonality in the syntax for a "test" and a "conditional expression" but there is also much unique syntax so it is important to know which one to use. The two have separate sections in a shell manual.


The "type" command can tell you how your shell treats square brackets.
For example in ksh:

Code:

type [
[ is a shell builtin.

type [[
[[ is a keyword.

type test
test is a shell builtin.