Need help with failing simple if statement

Shell Programming and Scripting
1

So I'm writing a script that takes in arguments given from the shell. The last argument I'm using as a tag to tell my script, via if statements, to do one thing or another. A sample input would be:

script.scr infile.txt outfile.txt F

The "F" at the end there I expect to be one of two tags "F" or "F.p". In my script, just to give an error if the tags are neither of two things, I have an if statment like:

if ($#argv = 3)
   set tags = $argv[3]
else set tags = ""
if ( $tags != "F" || $tags != "F.p" ) then
    echo Invalid tag entry.
    exit
endif

The problem is that the echo and exit will execute even if a correct tag is given. I use similar if statements to perform different actions on those files based on the tag given. For example:

if ($tags == "F") then
   ...
else if ($tags == "F.p") then
   ...
else if ($tags == "") then
   ...
endif

I'm sure that there's just a little thing I'm doing wrong, and any help is appreciated.

2

Using negative and Or conditions can get confusing.
Can you show an example?

This is confusing...

( $tags != "F" || $tags != "F.p" )

if F.p, then will be true for first condition
if F, then true for second condition

I am thinking you want && and not || for logic.

3

Haha, you're right I was thinking of &&. Now my only other problem is that second part. Where $tags == "F" and the like don't execute properly. Using -vx will even show "if ( F == F ) then" but it won't execute the code block even though the statement is clearly true.

4

Something wrong with the structure here, should be: if ( ) then ... else ... endif:

csh has bad structure control, here the result is nearly unpredictable.
Then, $#argv = 3 is an assignment, you want a comparison.
Then, assignments should be var=value, no spaces!

if ($#argv == 3) then
   set tags=$argv[3]
else
   set tags=""
endif

Last but not least, you should quote variables references like "$var" whenever possible:

if ( "$tags" != "F" || "$tags" != "F.p" ) then