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 writeif [ -f "$FILE" ]to correctly handle empty strings or file names with spaces in them. With
[[the quotes are unnecessary:if [[ -f $FILE ]]Because it is a syntactical feature, it lets you use
&&and||operators for boolean tests and<and>for string comparisons.[cannot do this because it is a regular command and&&,||,<, and>are not passed to regular commands as command-line arguments.It has a wonderful
=~operator for doing regular expression matches. With[you might writeif [ "$ANSWER" = y -o "$ANSWER" = yes ]With
[[you can write this asif [[ $ANSWER =~ ^y(es)?$ ]]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.
