September 25, 2013

A bashism a week: aliases

In a response to my blogpost about bashisms in function names, reader Detlef L pointed out in a comment that aliases allow non-standard characters in their names, contrary to functions. They could then be used to, for example, set an alias of the run-parts(1) command (cf. the blog post).

Aliases indeed allow characters such as commas ( , ) to be used in the alias name. However, aliases are an extension to the POSIX:2001 specification and are therefore bashisms. Moreover, the characters set defined by POSIX does not include dashes.

Last but not least, aliases belong to the list of shell features that are usually "disabled" when the shell is run in non-interactive mode. I.e.

$ bash <<EOF
alias true=false;
if true; then echo alias disabled; else echo alias enabled; fi

alias disabled
$ bash -i <<EOF # force interactive mode
alias true=false;
if true; then echo alias disabled; else echo alias enabled; fi

$ alias true=false;
$ if true; then echo alias disabled; else echo alias enabled; fi
alias enabled
$ exit

To add to the fun of different behaviours, other shells always expand aliases.

If you decide to play with aliases you should note one thing: they are only enabled from the line after the definition. E.g. note the difference below

$ dash -c 'alias foo="echo foo"; foo'
dash: foo: not found
$ dash -c 'alias foo="echo foo";

September 23, 2013

Worse than no support

You are in need of support and you go ask in project x's forums (mailing list, online board, etc.) and get something worse than no support:

Responses from an incompetent with plenty of time and lots of FUD to spread.

Now that we have "umarell" thanks to Enrico Zini and other Italian folks, we need a name for the above.

Apologies to some readers who might be disappointed by this post. I don't like calling people names but I've seen too many of the above in less than two hours not to be tired.

September 18, 2013

A bashism a week: testing strings

The bashism of this week deals again with tests. How do you compare two strings to tell which one goes before or after the other in the alphabet in a shell script?
(also know as lexicographic comparison)

If you are familiar with perl and the relationship between its comparison operators and that of shell scripts, you would probably write the test as follows:

$ test bar '<' foo
$ test foo '>' bar

And yes, that would work, but not with all shells. I'm afraid to tell you that the < and > comparison operators are not required by the POSIX:2001 specification and are, therefore, bashisms.

What does this have to do with perl? well, the shell way is the inverse of the perl way.
What? In Perl you test the equality of two numbers with '==', in shell you use '-eq'; in perl the equality of strings is 'eq', in shell it is '='. In shell scripts you can compare two numbers with -gt (test 1 -gt 0) while in perl the equivalent comparison is with > (1 > 0).

So in this case the perl way is "bar lt foo" and based on the above the shell way should be "bar < foo". However, care must be taken when using such operators in shell scripts. Since < and > are used for redirections in shell scripts, one must quote them even when using bash. An alternative is to use the [[ special test function of bash which alters the shell syntax.

This time I didn't write a function to portably (or "somewhat portably") replace such comparisons. Feel free to share your solution in the comments, with bonus points if you come up with a solution without using external commands.

September 04, 2013

A bashism a week: tilde expansion

Did you know that you can get a user's home directory with the "~username" tilde expansion? You probably do, but how about other tilde expansions?

Thanks to a few bashisms you can make your script even more difficult to read by using "~+" to get $PWD, and "~-" to get $OLDPWD.

Are you using bash and want to access the directories stack created when using the pushd and popd bashisms? The ~i (where i is an integer) expansion can give you that. It gives you forward (~+i) and backward (~-i) access to the directories stack.

But beware, if the directories stack is smaller than the number you used, there won't be any expansion.

When using tildes in shell scripts, make sure you quote to avoid unwanted expansions. Note that the posh shell in wheezy and older do support those non-POSIX expansions.

in the first examples the tilde is quoted for easier reading; the expansion doesn't occur if the tilde is quoted.