Shell Programming and Scripting

Good afternoon all,
I'm hoping my newbie question can help bolster someone's street_cred.sh today.

I'm trying to "fingerprint" SQL on its way into the rdbms for a benchmarking process (so I can tie the resource allocation back to the process more precisely).

To do this, I'm essentially picking out the SQL into a variable, and trying to replace the first whitespace character after the first word with my fingerprint, in this case, a benchmark Id and a Run Id.

Oddly, the big hangup seems to be the relatively simple sed substitution... This works just fine...

Code:

bmid=001;runid=000001;taskbody="Select Date;";echo $taskbody | sed "s/ /&\/\* bm=$bmid run=$runid \*\/ /";

...outputting the expected results:

Code:

Select /* bm=001 run=000001 */ Date;

But when I try to use command substitution to save the results back to a variable (I have further processing to do), the substitution for the astrisk "\*" part of my sed command is being treated instead as an "ls \*" function.

Thus this:

Code:

bmid=001;runid=000001;taskbody="Select Date";taskbody=`echo $taskbody | sed "s/ /&\/\* bm=$bmid run=$runid \*\/ /"`; echo $taskbody

...results in this:

Code:

Select /bin /boot /data /dev /etc /home /lib /lib64 /lost+found /media /mnt /opt /proc /root /sbin /srv /sys /tmp /usr /var bm=001 run=000001 */ Date

I tried double (and triple, and quadruple) escape characters, to see if they'd persist outside of the backquotes, but it doesn't seem to make any difference.

I searched around the forums, but I didn't see anything threads about "/*" comment character escape problems. So either I'm the first person ever to do this (right...) or I'm missing something blindingly obvious (more likely).

Thoughts?

Thanks!
Stephen

Your problem is that within double quotes, the shell will expand file globbing "wildcards" and you'll get a list of files and directories in the present directory for your "*". Usually this is solved by using single quotes, no expansion done within singles by the shell, but that won't allow you to use your variables as you'd like. I'm not a huge fan of this, but it does work if you "mix" your quotes:

Code:

sed 's/ /&\/\* bm='"$bmid"'run='"$runid"' \*\/ /'

Single quotes round everything except the variables; doubles round those. I don't like this because it's a maintenance headache. I also like to make a practice of not using escapes if I don't have to -- easier to read:

Code:

sed 's! !&/* bm='"$bmid"'run='"$runid"' */ !'

wil do the same thing. When using sed on file paths, or anything with slants, use an alternate delimiting character (!) in this case. It still uses the mixed quotes, but IMHO it's a bit easier to read.

Hey agama ~

Your point on alternate delimiters is a great one - It does make the line much easier to read.

Unfortunately, mixing the quotes didn't solve the problem - this revised code containing your changes (and simplified further with hardcoded values) still interprets asterisk as list commands:

Code:

tb=`echo "Select Date" | sed 's! !&\/\* bm=001 run=000001 \*\/ !'`; echo $tb

It seems to be related to the backquote / command substitution...
If I copy-out the variable-set part of the line, the code returns the expected results - except those results are not saved back into the variable, they're simply echo'ed to the terminal.

Thus, this sub-string of the above code returns the correct value:

Code:

echo "Select Date" | sed 's! !&\/\* bm=001 run=000001 \*\/ !'

...except the variable $tb isn't updated. Wrap the tb=`[]` around it, and it misbehaves.

I might be confused on the way command substitution works... I thought it basically ran everything within the backquotes first, then stdout the results like any completed function. This is why I'm confused why the bottom code works fine, but the top one does not. Maybe someone needs to correct my thinking...



Thanks!

Your problem isn't where you think it is. Your backquoted command is working just as you expect it to, but when you echo $tb you are expanding the contents of $tb and then passing them to echo. Since the contents of the variable contains asterisks, they are expanded on the echo command line, not as a part of the sed pipeline. Try this:

Code:

tb=`echo "Select Date" | sed 's! !&\/\* bm=001 run=000001 \*\/ !'`; echo "$tb"

You should be able to step back and insert the variables.

---------- Post updated at 21:04 ---------- Previous update was at 20:58 ----------

And your perception of how back quoting works is just fine. If you are using Kshell or bash, $(command) is the preferred usage; back ticks are deprecated in Kshell (and I assume in bash, but I don't know that for fact).

Aha - that's the ticket! Thanks for the help Agama, worked like a charm!

Also, thanks for the depreciation tip - I'll make a point of moving everything over to $(command) syntax.

Thanks!
Stephen