[GTALUG] Help need in bash

[Seneca Cunningham]

> On May 31, 2018, at 14:40, Stephen via talk <talk at gtalug.org> wrote:
> 
> I want to build a script to convert any *.png files to *.jpg files using convert.
> 
> Doing manually works fine.
> 
> Building a script has presented problem. I am well versed in programming but a novice with bash.
> 
> My loop through the files works fine.
> 
> #!/bin/bash
> cd /big1/memes/;
> 
> shopt -s nullglob
> for f in "*.png"
> do
>    echo $f
> done
> 
> I get a list of filenames with an extension of .png
> 

In this case, the value of $f is ‘*.png’.  Your loop iterates once, and the list of PNG files is generated as a shell expansion before evaluating the “echo”.

> But when I try to get the basename with:
> 
> #!/bin/bash
> cd /big1/memes/;
> 
> shopt -s nullglob
> for f in "*.png"
> do
>    echo $f
>    b=${f%.png}
>    echo $b
> done
> 
> I first get a list of the .png files, then a list of all files, both the existing .jog and the .png. This gets a WTF reaction from me.

Once more, $f is ‘*.png’ and is being expanded for the execution of the echo.  Your brace expansion looks to be converting ‘*.png’ to the string ‘*’, and assigning that to b.  The when evaluating the line “echo $b”, the value of $b, ‘*’, is expanded prior to execution.

> Can someone explain and help me get closer to getting this to work?

You were a touch sloppy with your quoting.  You should have left “*.png” unquoted in the for loop so that the list of file names to process would expand there, and stuck $f and $b in double quotes so their values would not expand.

Seneca