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Shell advanced syntax?

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# 8  
Old 10-18-2010
Quote:
Originally Posted by Scrutinizer
I think it is equivalent to this:
Code:
if [ $COMPILER = "sun" ]; then
  if [[ -f /sunstudio12/bin/sunf90 ]]; then
    export IMDI_COMPILER=/sunstudio12/bin/sunf90
  else
    echo "Please specify location!"
    exit
  fi
  if [[ -z "$SZIP_ROOT" ]]; then
    SZIP_ROOT=/szip-2.1
  fi
elif [ $COMPILER = "intel" ]; then

Quote:
Originally Posted by ctsgnb
I agree with the dude Scrutinizer Smilie
That's close but not quite correct. The original version's echo executes if the export command fails. This interpretation's does not.

Regards,
Alister
This User Gave Thanks to alister For This Post:
alt
# 9  
Old 10-18-2010
Now I agree with Dude Alister ... LoL
This User Gave Thanks to ctsgnb For This Post:
alt
# 10  
Old 10-18-2010
A few alternatives:

Code:
if [[ -f /sunstudio12/bin/sunf90 ]]; then
    export IMDI_COMPILER=/sunstudio12/bin/sunf90
if [ $? -ne 0 ]; then
    echo "Please specify location!"
    exit
fi

...or...

Code:
if ! { [[ -f /sunstudio12/bin/sunf90 ]] && export IMDI_COMPILER=/sunstudio12/bin/sunf90; }; then
    echo "Please specify location!"
    exit
fi

...or...

Code:
if ! [[ -f /sunstudio12/bin/sunf90 ]] || ! export IMDI_COMPILER=/sunstudio12/bin/sunf90; then
    echo "Please specify location!"
    exit
fi

Regards,
Alister
These 3 Users Gave Thanks to alister For This Post:
alt methyl Scott
# 11  
Old 10-18-2010
And which one is the best then ? (less parsing & less resource consumming) ?
This User Gave Thanks to ctsgnb For This Post:
alt
# 12  
Old 10-18-2010
Quote:
Originally Posted by alt
because I am expecting no return from export command.
Every single command run on a UNIX/Linux/POSIX system will return an exit status. If that exit status is 0, the shell treats that as a boolean true. Any other exit status is treated as a boolean false.

Regards,
Alister

---------- Post updated at 02:21 PM ---------- Previous update was at 02:14 PM ----------
Quote:
Originally Posted by alister
That's close but not quite correct. The original version's echo executes if the export command fails. This interpretation's does not.

Regards,
Alister

Actually, export always returns a 0 exit status, so it will never fail. Scrutinizer's code should work just as the original.

That said, I'll leave my post as is since that's a very unusual corner case. In just about every other situation, a command can return a non-zero exit status and the constructs would not be logically equivalent.

Regards,
Alister
This User Gave Thanks to alister For This Post:
alt
# 13  
Old 10-18-2010
Quote:
Originally Posted by alister
Every single command run on a UNIX/Linux/POSIX system will return an exit status. If that exit status is 0, the shell treats that as a boolean true. Any other exit status is treated as a boolean false.

Regards,
Alister
I certainly wouldn't challenge your assertion of that. But I would challenge any assertion that said return codes are always meaningful:

Code:
/root # yum install jiggery-pokery
Loaded plugins: fastestmirror, priorities
Determining fastest mirrors
 * addons: mirror.atrpms.net
 * base: mirror.switch.ch
 * extras: mirror.atrpms.net
 * updates: ftp.halifax.rwth-aachen.de
addons                                                   |  951 B     00:00     
addons/primary                                           |  202 B     00:00     
base                                                     | 2.1 kB     00:00     
base/primary_db                                          | 1.6 MB     00:01     
extras                                                   | 2.1 kB     00:00     
extras/primary_db                                        | 188 kB     00:00     
updates                                                  | 1.9 kB     00:00     
updates/primary_db                                       | 670 kB     00:00     
Setting up Install Process
No package jiggery-pokery available.
Nothing to do
/root # echo $?
0

Or perhaps alt's confusion is based on the interpretation of
Code:
if [[.....]] ; then
   [[ -f ... ]] && \
      export  ... || \
      { 
      echo ...; exit; 
      }

where the || relates not to the failure of the export command, but more to that of the test which precedes it?
This User Gave Thanks to Scott For This Post:
alt
# 14  
Old 10-18-2010
Your quite right Alister, however I suspect my interpretation is what the author of the script intended to write. Otherwise there is no logic to the error message and the exit statement. This illustrates how easily these logical ANDs and ORs get misinterpreted. That is precisely why I dislike them especially when there is a x && y || z concoction..

--edit--
Actually I think I was correct after all:
If the test fails then the statement after the && does not get executed since the left side is false. And then the part after the || gets excecuted since its left side was false and also the export will never fail, so that will not trigger the right side of the or.

OK, only now I read your added remark to your post. Probably all of us are right then Smilie
Last edited by Scrutinizer; 10-18-2010 at 03:42 PM..
This User Gave Thanks to Scrutinizer For This Post:
Scott
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