Re: equivalent of Linux readlink -f in pure bash?

[Bernd Eggink]

On 09.08.2011 15:50, Steven W. Orr wrote:
> On 8/9/2011 5:29 AM, Bernd Eggink wrote:
> > On 09.08.2011 03:44, Jon Seymour wrote:
> > > Has anyone ever come across an equivalent to Linux's readlink -f
> > > that is implemented purely in bash?
> >
> > You can find my version here:
> >
> > http://sudrala.de/en_d/shell-getlink.html
> >
> > As it contains some corrections from Greg Wooledge, it should
> > handle even pathological situations. ;)
> >
> > Bernd
>
> I'd just like to make a couple of suggestions for your script (I hope
>  these are welcome):

You are welcome!

> *) You reset OPTIND to 1 but you didn't declare it local. This will
> cause any caller of getlink which uses getopts to reset its variable
> to 1. (I mention this because it cost me a couple of hours a while
> back.)

The reason I didn't declare OPTIND local is that OPTIND is handled 
specially by the shell; there is always exactly _one_ instance of this 
variable. In other words, OPTIND is always global, even if declared 
local (which is indeed pretty weird). Try this:

-------------------------------
function f
{
    local OPTIND=1

    echo "\$1=$1"
}

while getopts "abcdefg" opt
do
    echo "opt=$opt"
    f $opt
done
--------------------------------

Calling the sript like this works fine:
        script -a -b -c

But calling it like this leads to an endless loop:
        script -abc

One could of course save and restore the original:

-------------------------------
function f
{
    local oldind=$OPTIND

    OPTIND=1
    echo "\$1=$1"
    OPTIND=$oldind
}
-------------------------------

However, this also loops endlessly. The reason is most likely that bash 
maintains an additional internal variable holding the index of the 
current character, relative to the current word. While this variable is 
not directly accessible by the user, it is set to 0 whenever OPTIND is 
assigned a value.

So the only safe way is to _never_ use getopts within another getopts 
block, but always wait until the first one has finished.

> When calling getopts, especially from a function that is intended to
> not be used at a top level for processing command line options, you
> should declare local copies of OPTIND, OPTARG and OPTERR.
>
> *) To remove the trailing slashes, instead of
>
> while [[ $file == */ ]] do file=${file%/} done
>
> file=${file##*/} # file name
>
> just say file="${file%${file##*[!/]}}"

Yes, you can do that, but I find my version a bit more legible. Also, 
for file=/ it returns a single slash, while yours returns an empty 
string. (Hmm... the next statement in my script also creates an empty 
string, but this is a bug and will be fixed).

> *) Instead of
>
> [[ ! -d $dir ]] && { ret=1 break }
>
> how about this for slightly cleaner?
>
> [[ -d $dir ]] || { ret=1 break }

I think that's just a matter of taste.

Greetings,
Bernd

--
http://sudrala.de