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

 

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?

[[ 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.

    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?
  • [ 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.

    在bash中，与[ ， [[防止变量值的字分割相反。

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