Bash double bracket

Since sh on macOS is bash pretending to be sh, double brackets will still work with the wrong shebang, but then your script might break on other platforms where a different shell might be pretending to be sh. Consistent behavior across platforms is the main point why sh is. In the previous article, we looked at various ways square brackets are used at the command line, including globbing. If you’ve not read that article, you might want to start there.

Square brackets can also be used as a command.

POSIX version, this is supported by bash and other shells(zsh,ksh). In bash , for numeric comparison we use eq, ne,lt and gt, with double brackets for comparison we can use ==, ! Double square bracket conditionals. In turn double round brackets. Latest bash version corrected that and allow using Perl-style =~ operator instead. I am confused by the usage of brackets, parentheses, curly braces in Bash , as well as the difference between their double or single forms.

Is there a clear explanation?

Difference between single and double square. Thus, the single bracket and double bracket execute different code. The test and single bracket are the most portable as they exist as separate and external commands. Using a wildcard in a condition to match the beginning of a string.

The double brackets turns the test into a regex,. The asterisk is taken literally, because the double - bracket syntax does not expand filenames. At this stage of our Bash basics series, it would be hard not to see some crossover between topics. For example, you have already seen a lot of brackets in the examples we have shown over the past several weeks, but the focus has been elsewhere.

I was looking at a script in my little book on bash and saw that one of the if statements had parentheses instead of brackets for the condition. Can somebody tell me the difference between double brackets and single brackets , when doing a test. I have always been acustomed to using single brackets and have not encountered any issues to date. This is obviously more.

It adds meaning to doubling up different brackets , and a dollar sign in front means something even more different. An the brackets are used differently than many other languages. How to use double or single brackets , parentheses, curly braces (4).

AlastairIrvine: I mention portability in the first sentence of my answer, the question title and body contain the word Bash and the question is tagged bash , and the double bracket structure provides clear advantages in many ways.

The -a and -o operators, and () grouping, are defined by POSIX but only for strictly limited cases, and are marked as deprecated. The right angle bracket symbol () is used to redirect output to a disk file. The left angle bracket symbol () is used to redirect input from a disk file.

How-to: Use parentheses to group and expand expressions. Grouping a (list of commands) in parentheses causes them to be executed as if they were a single unit. When commands are groupe redirections can be applied to the entire command list. For example, the output of all the commands in the list can be redirected to a single stream.

As double parentheses is builtin to Bash it also runs slighly more efficiently (though to be honest, with the raw computing power of machines these days the difference in performance is really insignificant). The test of a vocation is the love of the drudgery it involves ~ Logan Pearsall. Related linux commands: Examples of test constructs - Advanced Bash -Scripting Guide.

Conditionally perform a command.