What's the difference between [ and [[ in Bash

问题:

This question already has an answer here:

• Is double square brackets [[ ]] preferable over single square brackets [ ] in Bash? 7 answers

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?

回答1:

[[ 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:

1. 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 ]]
   

2. 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.

3. 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.

4. 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.

See also

• Bash FAQ - \"What is the difference between test, [ and [[ ?\"
• Bash Practices - Bash Tests
• Server Fault - What is the difference between double and single brackets in bash?

回答2:

• [ is the same as the test builtin, and works like the test binary (man test)
  • works about the same as [ in all the other sh-based shells in many UNIX-like environments
  • only supports a single condition. Multiple tests with the bash && and || operators must be in separate brackets.
  • doesn\'t natively support a \'not\' operator. To invert a condition, use a ! outside the first bracket to use the shell\'s facility for inverting command return values.
  • == and != are literal string comparisons
• [[ is a bash
  • is bash-specific, though others shells may have implemented similar constructs. Don\'t expect it in an old-school UNIX sh.
  • == and != apply bash pattern matching rules, see \"Pattern Matching\" in man bash
  • has a =~ regex match operator
  • allows use of parentheses and the !, &&, and || logical operators within the brackets to combine subexpressions

Aside 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.

回答3:

In bash, contrary to [, [[ prevents word splitting of variable values.

回答4:

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 ...