bash/sh if statement syntax

Question

In various guides and scripts I come across people tend to use different syntax of if statements. What's the difference and what are best practices? I believe all the following statements, and many more variants, will return true:

bar="foo"
if [ "foo" = "foo" ]
if [[ "foo" == $bar ]]
if [ "foo" = "$bar" ]
if [[ "foo" = "$bar" ]]
if [[ "foo" -eq $bar ]]

Answer 1

As I understand it

= expects strings

-eq expects integers

"$bar" is for literal matches, i.e. z* may expand but "z*" will literally match the wildcard char.

The difference between [] and [[]] is that in the latter word splitting and path name expansion are not done, but are in the former.

Plus [[]] allows the additional operators :

&& (AND), || (OR), > (String1 lexically greater than String2), < (String1 lexically less than String2)

The == comparison operator behaves differently within a double-brackets test than within single brackets.

[[ $a == z* ]] # True if $a starts with an "z" (pattern matching).

[[ $a == "z*" ]] # True if $a is equal to z* (literal matching).

[ $a == z* ] # File globbing and word splitting take place.

[ "$a" == "z*" ] # True if $a is equal to z* (literal matching).

Check out http://tldp.org/LDP/abs/html/comparison-ops.html for more info