Re: Q re: if test of command in bash script

[Ken Irving <fnkci@uaf.edu>]

On Tue, Oct 28, 2008 at 01:57:40PM -0500, Kent West wrote:
> Ken Irving wrote:
> ...
> > I'd tend to format the above code a little differently, but here's
> > the same thing with the `if' using the mkdir command directly:
> >
> >     if [ -d $targetDir ]; then
> >             echo -e "YES, '$targetDir exists!\n\n"
> 
> Ah, the "-e" enables things like newlines and the bell character. No
> wonder my bell character never sounded in another part of my script; I
> just assumed it was something turned off in my terminal settings.

I use it a lot (but I think just needed one \n above), and often without
quoting the whole thing, e.g.,

    echo -e \\nleading blank line to separate output from previous lines

> ...
> I at first liked the format
> 
> if test
> then
>   statement 1
> else
>   statement 2
> fi
> 
>  because I don't have to remember the semi-colon (when it's used; when
> it's not, etc), and because it sort of conceptually separates the test
> from the consequences of the test, but I'm beginning to swing around to
> your format of
> 
> if test; then
>   statement 1
> else
>   statement 2
> fi
> 
> Is there any advantage/disadvantage to one format over the other?

No difference, bash just uses the `;' to mean the same thing as a 
newline.  I just like that the latter form is a bit more concise looking
(to me, anyway).

If there's no else condition to worry about, the && and || operators
can be used to good effect, e.g.,

    test -d $targetdir || {
        test -n "$OK_TO_CREATE_DIR" || {
            echo directory not found
            exit 1
            }
        mkdir -p $targetdir || exit 1 # (mkdir itself emits a complaint)
        # ASSERT: mkdir was succesful
        }
    # ASSERT: directory exists

Ken

-- 
Ken Irving, fnkci@uaf.edu, 907-474-6152
Water and Environmental Research Center
Institute of Northern Engineering
University of Alaska, Fairbanks