I looked at bash man page and the [[ says it uses Conditional Expressions. Then I looked at Conditional Expressions section and it lists the same operators as test (and [).
So I wonder, what is the difference between [ and [[ in Bash?
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[[ is bash's improvement to the [ command. It has several enhancements that make it a better choice if you write scripts that target bash. My favorites are:
It is a syntactical feature of the shell, so it has some special behavior that [ doesn't have. You no longer have to quote variables like mad because [[ handles empty strings and strings with whitespace more intuitively. For example, with [ you have to write
if [ -f "$file" ]
to correctly handle empty strings or file names with spaces in them. With [[ the quotes are unnecessary:
if [[ -f $file ]]
Because it is a syntactical feature, it lets you use && and || operators for boolean tests and < and > for string comparisons. [ cannot do this because it is a regular command and &&, ||, <, and > are not passed to regular commands as command-line arguments.
It has a wonderful =~ operator for doing regular expression matches. With [ you might write
if [ "$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.
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.
In bash, contrary to [, [[ prevents word splitting of variable values.
[ is the same as the test builtin, and works like the test binary (man test)
[ in all the other sh-based shells in many UNIX-like environments&& and || operators must be in separate brackets. ! outside the first bracket to use the shell's facility for inverting command return values.== and != are literal string comparisons[[ is a bash
== and != apply bash pattern matching rules, see "Pattern Matching" in man bash=~ regex match operator!, &&, and || logical operators within the brackets to combine subexpressionsAside from that, they're pretty similar -- most individual tests work identically between them, things only get interesting when you need to combine different tests with logical AND/OR/NOT operations.
The most important difference will be the clarity of your code. Yes, yes, what's been said above is true, but [[ ]] brings your code in line with what you would expect in high level languages, especially in regards to AND (&&), OR (||), and NOT (!) operators. Thus, when you move between systems and languages you will be able to interpret script faster which makes your life easier. Get the nitty gritty from a good UNIX/Linux reference. You may find some of the nitty gritty to be useful in certain circumstances, but you will always appreciate clear code! Which script fragment would you rather read? Even out of context, the first choice is easier to read and understand.
if [[ -d $newDir && -n $(echo $newDir | grep "^${webRootParent}") && -n $(echo $newDir | grep '/$') ]]; then ...
or
if [ -d "$newDir" -a -n "$(echo "$newDir" | grep "^${webRootParent}")" -a -n "$(echo "$newDir" | grep '/$')" ]; then ...
Source: Stackoverflow.com