Wonderful people:
I am trying to make an extremely simple script to toggle the gamma settings in my lcd monitor between two values.
This is what I did with no success so far:

The problem, I think is the following:
when I run xgamma, I get the following line:

but this is not being taken as the actual output of the command in the script.

Any help, ideas, etc.?

The double quotes on PresentValue="$(xgamma)" are essential in case the returned value contains whitespace (which it does). Changing from ` ` to $( ) is good practice.

The single quotes around $PresentValue in the DEBUG print are there to make any leading or trailing whitespace characters visible.

Catkin:
Thanks for your answer, but I am still stuck.
If I do:

I get:

If I do

I also get nothing.

Am I doing something (else) wrong?

Did you read the post you are replying to? Do not use backticks, use parentheses.

NO: PresentValue=`xgamma`

YES: PresentValue="$(xgamma)"

Then test to see if you have captured what you think you have.

should be:

Your test print:

Should be:

lutusp:
I did.
I did what Catkin suggested and also what you suggested.
I am still stuck.

As I said, the problem seems to be the following:

obviously returns the output of ls.

returns nothing.

(PS: I thank you for your help and willingness to help people who are not as learned as you are).

Ha! I just figured it out. It happens that I have "xgamma" on this system, and its output is going to "stderr", not "stdout". Unices have two pre-opened streams for CLI applications:

stdout: normal text output.

stderr: error messages.

For some reason, xgamma is printing to stderr instead of stdout. But you can re-route stderr to stdout. Like this:

PresentValue="$(xgamma 2>&1)"; echo "$PresentValue"

And please stop using backticks. It's unprofessional.

WOW! I have been screwing with this for the past hour! What a brain-burner :| I thought wtf is wrong here!?
Nice one, Paul

Then the DEBUG is doing exactly what it should and identifying the problem -- $PresentValue is not being set.

In this situation = and == work identically, as described in the GNU Bash Reference, where it says "‘=’ may be used in place of ‘==’ for strict posix compliance". Some prefer the == because it is obviously not an assignment operator. Others prefer = for historical reasons (and takes less typing!)

The quotes around the string constant can be double or single. If you use single quotes then it is easier to read -- the string is used exactly as it stands until the closing single quote whereas a double quoted string has to be scanned for characters that are special to the shell such as $, \, ` etc.
That is correct if you want the value of $PresentValue to be displayed within double quotes. There is nothing wrong with if you want the value of $PresentValue to be displayed within single quotes. I prefer the latter -- it is "cleaner", both in the script and in the output.

Thank you all guys. It worked wonderfully.
Thanks also for the indications on the equal sign.
(I'm sorry about the lack of professionality of the backticks. As is obvious, I'm not a professional; this is not what I do in real life; it's just a wfie-and-kids-gone-to-bed-hobby).

Yes, and this is extremely bad advice to be giving newbies, many of whom are studying computer programming, and many of whom will take it to mean there is no difference bewtween '=' and '==' in general. The fact that Bash accepts both only shows a lack of consistency and intellectual discipline.

Same reply! This is very bad advice for newbies, who may not know the general difference between the two, and who may therefore not understand that:

$ echo "$PresentValue"

and

$ echo '$PresentValue'

produce different results, and why. For these students, explaining that --

$ echo "$PresentValue"

-- and --

$ echo "'$PresentValue'"

-- both work after a fashion is a conversation not worth having, and one that can only sow confusion.

This syntactical ambiguity will confuse relative newcomers to the world of computer programming, and has no other purpose.

Utter nonsense. If it were true, the single-quoted version would not have printed the string's contents. In fact, in this example the single-quoted and double-quoted versions are both scanned for special sequences and the only difference between the two is the fact that one prints single quotes and the other prints double quotes -- that's all.

Thanks for sharing your personal tastes. This is about making computer programming and shell scripting comprehensible to newbies. Consistent syntax furthers that goal. Not making side trips to Alice's wonderland furthers that goal.

lutusp:
If your concern is giving good advice to newbies like me, I would suggest considering manners: I've been a teacher for a long time, and one thing I've learnt is that politeness is the requisite for learning (on both parts).
Asking the person who asked a question if he has bothered to read something or that some of his habits (such as using backticks) are not, I think, conducive to effectively sharing your knowledge.

That said, I thank you again for the information and advice.

See GNU Bash Reference

The prior poster's claim was that within this string --

-- the single-quoted section would not be scanned for special sequences. The prior poster was mistaken. I said so.

See GNU Bash Reference

Ah! Misunderstanding! I presume you are referring to When I wrote that I was referring to the "-> Red 0.900, Green 0.900, Blue 0.900 " above and not the "DEBUG: PresentValue: '$PresentValue'" below. Sorry for the ambiguity.