请参考:
http://mywiki.wooledge.org/BashFAQ/031
附stackoverflow.com上一哥们的总结:
[[ 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.