Bash – Are Wildcard Expansions Guaranteed to Be in Order?

bashfileslarge-datawildcard

Is the expansion of a wildcard in Bash guaranteed to be in alphabetical order? I am forced to split a large file into 10 Mb pieces so that they can be be accepted by my Mercurial repository.

So I was thinking I could use:

split -b 10485760 Big.file BigFilePiece.

and then in place of:

cat BigFile | bigFileProcessor

I could do:

cat BigFilePiece.* | bigFileProcessor

in its place.

However, I could not find anywhere that guaranteed that the expansion of the asterisk (aka wildcard, aka *) would always be in alphabetical order so that .aa came before .ab (as opposed to be timestamp ordering or something like that).

Also, are there any flaws in my plan? How great is the performance cost of cating the file together?

Best Answer

Yes, globbing expansion is alphabetical.

From the Bash man page:

Pathname Expansion

After word splitting, unless the -f option has been set, bash scans each word for the characters *, ?, and [. If one of these characters appears, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of file names matching the pattern.

Related Solutions

Bash Scripting – How to Determine if a Bash Variable is Empty

This will return true if a variable is unset or set to the empty string ("").

if [ -z "${VAR}" ];

Difference Between Double and Single Square Brackets in Bash – Scripting Shell POSIX

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.

Best Answer

Pathname Expansion

Related Solutions

Bash Scripting – How to Determine if a Bash Variable is Empty

Difference Between Double and Single Square Brackets in Bash – Scripting Shell POSIX

Related Topic