Bash 中[ 和 [[ 的区别

[[ 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

   1	if [ -f "$file" ]

   to correctly handle empty strings or file names with spaces in them. With [[ the quotes are unnecessary:

   1	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

   1	if [ "$answer" = y -o "$answer" = yes ]

   With [[ you can write this as

   1	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:

   1	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?

原问题链接：https://stackoverflow.com/questions/3427872/whats-the-difference-between-and-in-bash