Bash – the difference between double and single square brackets in bash

bashposixscriptingshell

I just wondered what exactly the difference between

[[ $STRING != foo ]]

and

[ $STRING != foo ]

is, apart from that the latter is POSIX-compliant, found in sh and the former is an extension found in bash.

Best Answer

There are several differences. In my opinion, a few of the most important are:

[ is a builtin in Bash and many other modern shells. The builtin [ is similar to test with the additional requirement of a closing ]. The builtins [ and test imitate the functionality /bin/[ and /bin/test along with their limitations so that scripts would be backwards compatible. The original executables still exist mostly for POSIX compliance and backwards compatibility. Running the command type [ in Bash indicates that [ is interpreted as a builtin by default. (Note: which [ only looks for executables on the PATH and is equivalent to type -P [. You can execute type --help for details)
[[ is not as compatible, it won't necessarily work with whatever /bin/sh points to. So [[ is the more modern Bash / Zsh / Ksh option.
Because [[ is built into the shell and does not have legacy requirements, you don't need to worry about word splitting based on the IFS variable to mess up on variables that evaluate to a string with spaces. Therefore, you don't really need to put the variable in double quotes.

For the most part, the rest is just some nicer syntax. To see more differences, I recommend this link to an FAQ answer: What is the difference between test, [ and [[ ?. In fact, if you are serious about bash scripting, I recommend reading the entire wiki, including the FAQ, Pitfalls, and Guide. The test section from the guide section explains these differences as well, and why the author(s) think [[ is a better choice if you don't need to worry about being as portable. The main reasons are:

You don't have to worry about quoting the left-hand side of the test so that it actually gets read as a variable.
You don't have to escape less than and greater than < > with backslashes in order for them not to get evaluated as input redirection, which can really mess some stuff up by overwriting files. This again goes back to [[ being a builtin. If [ (test) is an external program the shell would have to make an exception in the way it evaluates < and > only if /bin/test is being called, which wouldn't really make sense.

Related Solutions

Linux – the difference between a ‘Login’ and an ‘Interactive’ bash shell

In your login shell's profile files you can set up some things that you will use during your session, and which only need to be done once. Some ideas:

create a temporary file containing the IP address you connected from, later on you can include it in some scripts setting firewall rules.
run ssh-agent, ask for your SSH keys, and store the SSH agent environment variables in a file.
if that is a limited machine, and your co-workers want to be aware of each others logins, write(1) messages informing them of your login.

In a non-login shell's files (.bashrc) you should configure your shell:

Fancy prompt
set aliases
set history options
define custom shell functions
export environment variables, (maybe PAGER, EDITOR if system-wide settings suck)
load ssh-agent variables saved in .bash_profile

Usually, you would include .bashrc from .bash_profile with the following. Then login shell gets all your customizations, and .bash_profile file does not have to duplicate things that are already in .bashrc.

[[ -f ~/.bashrc ]] && . ~/.bashrc

Linux – the difference between unlink and rm

Both are a wrapper to the same fundamental function which is an unlink() system call.

To weigh up the differences between the userland utilies.

rm(1):

More options.
More feedback.
Sanity checking.
A bit slower for single calls as a result of the above.
Can be called with multiple arguments at the same time.

unlink(1):

Less sanity checking.
Unable to delete directories.
Unable to recurse.
Can only take one argument at a time.
Marginally leaner for single calls due to it's simplicity.
Slower when compared with giving rm(1) multiple arguments.

You could demonstrate the difference with:

$ touch $(seq 1 100)
$ unlink $(seq 1 100)
unlink: extra operand `2'

$ touch $(seq 1 100)
$ time rm $(seq 1 100)

real    0m0.048s
user    0m0.004s
sys     0m0.008s

$ touch $(seq 1 100)
$ time for i in $(seq 1 100); do rm $i; done

real    0m0.207s
user    0m0.044s
sys     0m0.112s

$ touch $(seq 1 100)
$ time for i in $(seq 1 100); do unlink $i; done

real    0m0.167s
user    0m0.048s
sys     0m0.120s

If however we're talking about an unadulterated call to the system unlink(2) function, which I now realise is probably not what you're accounting for.

You can perform a system unlink() on directories and files alike. But if the directory is a parent to other directories and files, then the link to that parent would be removed, but the children would be left dangling. Which is less than ideal.

Edit:

Sorry, clarified the difference between unlink(1) and unlink(2). Semantics are still going to differ between platform.

Best Answer

Related Solutions

Linux – the difference between a ‘Login’ and an ‘Interactive’ bash shell

Linux – the difference between unlink and rm

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