On Mon, 30 Oct 2006 07:47:22 -0500, Barry Margolin wrote:

snip...
>> #!/bin/sh
>>
>> LOCKDIR=/var/spool/cron/lastrun
>> LOCKFILE=${LOCKDIR}/lock
>>
>> echo "braces no quotes $LOCKFILE"
>>
>> LOCKFILE="${LOCKDIR}"/lock
>> echo "braces quote $LOCKFILE"
>>
>> LOCKFILE=$LOCKDIR/lock
>> echo "no braces no quote $LOCKFILE"
>>
>> LOCKFILE="$LOCKDIR"/lock
>> echo "no braces quote $LOCKFILE"
>
> None of the above examples need the braces or quotes.

Don't I feel like the dunce! Why do you think it WAS coded that way? Are
braces always "safe" to use -- that is, will bash always properly handle
it as a variable and expand it properly? What about quoted braces and
variables? When would they be required?

> But here are some examples where they're necessary:
>
> LOCKFILE=${LOCKDIR}1/lock
>
> Without the braces, it would look for a variable named LOCKDIR1.
>

Yes, I experienced that! Took a while to figure it out! So, the braces
encapsulate the variable.

> LOCKFILE="$LOCKDIR/lock file with spaces"
>
> Without the quotes, it would set the environment variable to
> $LOCKDIR/lock while trying to execute the command line "file with
> spaces".
>
Again, would braces be superfluous here?

> STRINGWITHSPACES="foo bar"
> echo $STRINGWITHSPACES
> echo "$STRINGWITHSPACES"
>
> The first one loses the multiple spaces between the words.
>

Tricky. Must be hell to debug something like that!

> However, you don't need it here:
>
> OTHERSTRING=$STRINGWITHSPACES
>

I love consistency! What if you DID use quotes? Same result?

> because word splitting of the assignment portion of a command is done
> before variable expansion.
>

thanks for the explanation!

Is there an easy way to generalize when braces are used? I have seen
assignments in the same script that inconsistently use it. Very hard for
an end user to understand!

For example, look at this line from run-crons:

# base is one of hourly, daily, etc. single word, no spaces
# lockdir has slashes but no spaces

if [ -e ${LOCKDIR}/cron.$BASE ]

Is BASE not quoted or braced because it is a single word with no escape
type chars? Why is LOCKDIR in braces? Why are quoted UNnecessary?

--
Peter