Trouble with error handling in my first bash script - linux

Question

OK, so I am a total beginner with bash scripts and I am aware that the question is probably phrased a bit awkwardly, but I'll be as clear as I can!
I have written the following script to create a backup of repositories in a folder. The script is as follows:
#!/bin/bash
SVNREPO="/var/svn"
TEMP="/var/tmp"
BACKUP="/home/helix/backups"
cd $SVNREPO
if [ $# -eq 0 ]; then
for REPO in *; do
ARRAY+=($REPO)
done
else
for REPO in $#; do
ARRAY+=($REPO)
done
fi
for REPO in ${ARRAY[#]}; do
svnadmin dump $SVNREPO/$REPO -r HEAD | gzip > $TEMP/$REPO.svn.gzip sd
cp $TEMP/$REPO.svn.gzip $BACKUP/$REPO.svn.gzip
rm $TEMP/$REPO.svn.gzip
done
This script successfully produces .gzip backups of the all of the repositories in 'var/svn' when the script is called with no arguments, and creates .gzip backups of the specific repositories that are called as arguments. Great!
However, if the script is run with an argument that does not correspond to a repository that exists, then the program will crash with the error message: svnadmin: E000002: Can't open file '/var/svn/ada/format': No such file or directory. What I am trying to achieve is to catch this error and print a more user friendly output to the console. I have been trying to do this using 'trap'.
First I added the following line:
trap 'echo ERROR! The repository or repositories that you are trying to backup do not exist!' ERR
...and then I pushed the error to /dev/null at this point in the final for loop:
svnadmin dump $SVNREPO/$REPO -r HEAD 2>/dev/null | gzip > $TEMP/$REPO.svn.gzip
I pushed to the /dev/null file at the place I did because this is where the program errors out. However, the script no longer seems to work. What am I doing wrong here? Is it an issue to do with having the 2>/dev/null in the middle of a line? If so, how could I refactor this code so that it doesn't require the pipe in the middle of the line?
Many thanks for any help, I hope my question is reasonably clear! To confirm, the final non-working code is as follows:
#!/bin/bash
SVNREPO="/var/svn"
TEMP="/var/tmp"
BACKUP="/home/helix/backups"
cd $SVNREPO
if [ $# -eq 0 ]; then
for REPO in *; do
ARRAY+=($REPO)
done
else
for REPO in $#; do
ARRAY+=($REPO)
done
fi
trap 'echo ERROR! The repository or repositories that you are trying to backup do not exist!' ERR
for REPO in ${ARRAY[#]}; do
svnadmin dump $SVNREPO/$REPO -r HEAD 2>/dev/null | gzip > $TEMP/$REPO.svn.gzip sd
cp $TEMP/$REPO.svn.gzip $BACKUP/$REPO.svn.gzip
rm $TEMP/$REPO.svn.gzip
done

Answer 1

I do not know exactly how trap command works, but I will suggest another way that might solve your problem in another way:
First, before your for loop, add this line:
set -o pipefail
This means that when any command in a pipe fails, the last exit code ($?) will contain the error code if any failed.
On the line right after your svnadmin call, I would suggest adding this:
if [ $? -ne 0 ]; then
echo "ERROR! Received error code $? for repository '$REPO'."
continue
fi
You can of course alter the error message to your taste. The functionality should be clear: If svnadmin or bzip fails, it will print an error message and continue to next item in the for loop.
Hope this helps.

Answer 2

Using #cdarke suggestion of checking whether a file exists, I now have it working with the following code:
#!/bin/bash
SVNREPO="/var/svn"
TEMP="/var/tmp"
BACKUP="/home/helix/backups"
cd $SVNREPO
if [ $# -eq 0 ]; then
for REPO in *; do
ARRAY+=($REPO)
done
else
for REPO in $#; do
ARRAY+=($REPO)
done
fi
for REPO in ${ARRAY[#]}; do
if [ -f $SVNREPO/$REPO/format ]; then
vnadmin dump $SVNREPO/$REPO -r HEAD 2>/dev/null | gzip > $TEMP/$REPO.svn.gzip
cp $TEMP/$REPO.svn.gzip $BACKUP/$REPO.svn.gzip
rm $TEMP/$REPO.svn.gzip
else
echo ERROR! The repository $REPO does not exist. No backup has been made for this argument.
fi
done