If a file exists in a directory do...?

Question

I am trying to make an if-statement that says:

If a file in this directory has .PDF extension do this, otherwise do this...

And I'm having a hard time figuring out how to do this in bash. I checked here : https://stackoverflow.com/questions/3856747/check-whether-a-certain-file-type-extension-exists-in-directory

first but the solutions listed either don't work or give errors I don't know how to fix.

Attached below is my script. I slightly modified the script I was working on in my previous question here: Change only the extension of a file

The script is below:

#!/bin/bash
#shebang for bourne shell execution
echo "Hello this is task 1 of the homework"
 #Initial prompt used for testing to see if script ran


#shopt allows configuration of the shell -s dotglob allows the script to run on dot-files
shopt -s dotglob

#loop to iterate through each file in Task1 directory and rename them
#loop runs if there is a file type of ".PDF" in the folder Task1, if there isn't displays that there i
sn't a file of that type and terminates 
if [ [ -n $(echo *.PDF) ] ] # what should I put here???  
then
        for file in Task1/*.PDF;
        do

                mv "$file" "${file%.PDF}.pdf"
                echo "$file has been updated"
        done
else
                echo "No files of that type..."
fi
~           

**Edit 1: **

As per Ipor Sircer's answer below I changed my script to the following:

#!/bin/bash
#shebang for bourne shell execution
echo "Hello this is task 1 of the homework"
 #Initial prompt used for testing to see if script ran


#shopt allows configuration of the shell -s dotglob allows the script to run on dot-files
shopt -s dotglob

#loop to iterate through each file in Task1 directory and rename them
#loop runs if there is a file type of ".PDF" in the folder Task1, if there isn't displays that there isn't a file of that type and terminates 
if [ ls -1 *.PDF|xargs -l1 bash -c 'mv $0 ${0%.PDF}.pdf' ]
then
        for file in Task1/*.PDF;
        do

                mv "$file" "${file%.PDF}.pdf"
                echo "$file has been updated"
        done
else
                echo "No files of that type..."
fi

I get the following errors:

Hello this is task 1 of the homework

./Shell1.sh: line 12: [: missing `]'

mv: cannot stat ']': No such file or directory

No files of that type...

Edit 2

As per Eric Renouf's comment fixing the spacing gives me the following script:

#!/bin/bash
#shebang for bourne shell execution
echo "Hello this is task 1 of the homework"
 #Initial prompt used for testing to see if script ran


#shopt allows configuration of the shell -s dotglob allows the script to run on dot-files
shopt -s dotglob

#loop to iterate through each file in Task1 directory and rename them
#loop runs if there is a file type of ".PDF" in the folder Task1, if there isn't displays that there isn't a file of that type and terminates 
if [[ -n $(echo *.PDF) ]]
then
        for file in Task1/*.PDF;
        do

                mv "$file" "${file%.PDF}.pdf"
                echo "$file has been updated"
        done
else
                echo "No files of that type..."
fi

However if I run it twice I see the following:

Hello this is task 1 of the homework

mv: cannot stat 'Task1/*.PDF': No such file or directory

Task1/*.PDF has been updated

Why don't I just see the else echo since there are no files of type .PDF in the folder anymore?

Answer 1

shopt -s nullglob

set -- Task1/*.PDF

if (( $# > 0 )); then
   printf 'There are %d PDF files in "Task1"\n' "$#"
else
   echo 'There are no PDF files in "Task1"'
fi

The set will set the positional parameters to the names of the individual PDF files available in the Task1 sub-directory. This is the same as if you had invoked the script with ./script Task1/*.PDF.

The number of positional arguments is held in $# and we test to see if this is greater than zero. If it is, we report the number of PDF files available. Otherwise we report that there are none. You may choose to do whatever you want in place of these two actions.

If the task is just to lowercase the .PDF suffix of any such file, then it is unnecessary to first check whether there are any:

shopt -s nullglob

for fpath in Task1/*.PDF; do
  mv "$fpath" "${fpath%.PDF}.pdf"
done

Setting the nullglob shell option ensures that we don't end up with a value of * in fname if there weren't any files matching the pattern.

Answer 2

ls -1 *.PDF|xargs -l1 bash -c 'mv $0 ${0%.PDF}.pdf'

Answer 3

Could you just use:

[ -e *.PDF ]

instead of

[ [ -n $(echo *.PDF) ] ] 

e.g.

[ -e *.PDF ] && echo 'yes'