I have the following script I use for backup:
#######
#!/bin/sh

BACKUPLIST=/etc/backup.lst
HOSTNAME=`hostname`
YEAR=`date +%Y`
MONTH=`date +%B`
BACKUPDIR=/srv/misc/backup/${YEAR}/${MONTH}
DATE=`date +%Y%m%d`

mkdir -m 2775 -p $BACKUPDIR

for entry in `cat $BACKUPLIST`; do
name=`echo $entry | cut -d':' -f1`
startdir=`echo $entry | cut -d':' -f2`
bkdir=`echo $entry | cut -d':' -f3`
exlist=`echo $entry | cut -d':' -f4`
if [ -n $exlist -a -f $exlist ]; then
cd $startdir; tar -X $exlist -cf - $bkdir | bzip2 -c > ${BACKUPDIR}/${HOSTNAME}-${name}-${DATE}.tar.bz2
else
cd $startdir; tar cf - $bkdir | bzip2 -c > ${BACKUPDIR}/${HOSTNAME}-${name}-${DATE}.tar.bz2
fi
done
##########

Now, /etc/backup.lst has a list of directories I want to backup, and
looks like this
john:/home:john:/home/john/backupex.lst
vcs:/srv:/srv/vcs:

That is, some entries have 4th field, some has not.
The problem is that my test
if [ -n $exlist -a -f $exlist ]
does not work. For some reason, this always evaluate to true, and then
the tar command fails. Why is this?

My understanding is -n will check if $exlist is empty or not, and if it
is, it will fail. And even if it doesn't, -f $exlist should fail since
it wouldn't be able to find that file.

--
John L. Fjellstad
web: http://www.fjellstad.org/ Quis custodiet ipsos custodes


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On Fri, Dec 01, 2006 at 03:42:18PM -0800, John L Fjellstad wrote:
> I have the following script I use for backup:
> #######
> #!/bin/sh
>
> BACKUPLIST=/etc/backup.lst
> HOSTNAME=`hostname`
> YEAR=`date +%Y`
> MONTH=`date +%B`
> BACKUPDIR=/srv/misc/backup/${YEAR}/${MONTH}
> DATE=`date +%Y%m%d`
>
> mkdir -m 2775 -p $BACKUPDIR
>
> for entry in `cat $BACKUPLIST`; do
> name=`echo $entry | cut -d':' -f1`
> startdir=`echo $entry | cut -d':' -f2`
> bkdir=`echo $entry | cut -d':' -f3`
> exlist=`echo $entry | cut -d':' -f4`
> if [ -n $exlist -a -f $exlist ]; then
> cd $startdir; tar -X $exlist -cf - $bkdir | bzip2 -c > ${BACKUPDIR}/${HOSTNAME}-${name}-${DATE}.tar.bz2
> else
> cd $startdir; tar cf - $bkdir | bzip2 -c > ${BACKUPDIR}/${HOSTNAME}-${name}-${DATE}.tar.bz2
> fi
> done
> ##########
>
> Now, /etc/backup.lst has a list of directories I want to backup, and
> looks like this
> john:/home:john:/home/john/backupex.lst
> vcs:/srv:/srv/vcs:
>
> That is, some entries have 4th field, some has not.
> The problem is that my test
> if [ -n $exlist -a -f $exlist ]
> does not work. For some reason, this always evaluate to true, and then
> the tar command fails. Why is this?

Not sure, but it'll work if you enclose the variable in quotes.

> My understanding is -n will check if $exlist is empty or not, and if it
> is, it will fail. And even if it doesn't, -f $exlist should fail since
> it wouldn't be able to find that file.

My guess is that the variable expands to nothing, and then the -n
test is applied to the next token it sees, perhaps the -a?

I've run into the same issue recently, and would like to somehow become
less baffled by the ins and outs of shell (bash) scripting. Presumably
there's a statement in bash(1) that would clarify what's going on.

Ken

--
Ken Irving, fnkci@uaf.edu


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On Fri, Dec 01, 2006 at 06:51:05PM -0900, Ken Irving wrote:
> > The problem is that my test
> > if [ -n $exlist -a -f $exlist ]
> > does not work. For some reason, this always evaluate to true, and then
> > the tar command fails. Why is this?
>
> Not sure, but it'll work if you enclose the variable in quotes.
>
> > My understanding is -n will check if $exlist is empty or not, and if it
> > is, it will fail. And even if it doesn't, -f $exlist should fail since
> > it wouldn't be able to find that file.
>
> My guess is that the variable expands to nothing, and then the -n
> test is applied to the next token it sees, perhaps the -a?
>
> I've run into the same issue recently, and would like to somehow become
> less baffled by the ins and outs of shell (bash) scripting. Presumably
> there's a statement in bash(1) that would clarify what's going on.

This may be off the mark, but browsing the bash source I see this
comment in subst.c:

/* Partially and wholly quoted strings which expand to the empty
string are retained as an empty arguments. Unquoted strings
which expand to the empty string are discarded. The single
exception is the case of expanding "$@" when there are no
positional parameters. In that case, we discard the expansion. */

This sounds like it matches the symptoms anyway, though it still
seems odd to me.

Ken

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Ken Irving <fnkci@uaf.edu> writes:

> Not sure, but it'll work if you enclose the variable in quotes.

That worked perfectly. Thanks.

--
John L. Fjellstad
web: http://www.fjellstad.org/ Quis custodiet ipsos custodes


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