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Shell advanced syntax?


 
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# 1  
Old 10-18-2010
Shell advanced syntax?

I am not an expert of shell scripting, but I can do some simple things. Now, I read a script written by others and I need some help from the experts of this forum.

Please help me to understand what is going on in this cycle:
Code:
if [ $COMPILER = "sun" ]; then
[[ -f /sunstudio12/bin/sunf90 ]] && \
      export IMDI_COMPILER=/sunstudio12/bin/sunf90 || \
      { 
      echo "Please specify location!"; exit; 
      }
    [[ -z "$SZIP_ROOT" ]] && SZIP_ROOT=/szip-2.1
elif [ $COMPILER = "intel" ]; then
....

In particular, I do not understand this synax: [[...]] && \ export... ||,
where the || operand is used after the export command. It rise by me the question if the export command return any Boolean value?

Thank you in advance for help!
# 2  
Old 10-18-2010
Code:
[[...]] && export... ||,

is one line form of

Code:
if [[ ... ]] ; then
export...
else
echo "Please ...
fi

the \ is used for cosmetic reasons : when a line is too long, you can break it by typing <enter> but in order to "desactivate" the <enter> you have to protect it first by a \

so the line
Code:
[[...]] && export... || cmd...

can be put one several line to make it readable when commands are too long:
Code:
[[...]] && \
export... || \
cmd...


Last edited by ctsgnb; 10-18-2010 at 12:03 PM..
These 2 Users Gave Thanks to ctsgnb For This Post:
# 3  
Old 10-18-2010
still unclear

Thank you very much for good answer!

But I would like to point out two things that are still unclear to me:

1. Actually, I have two ifs or not? I mean when typing
Code:
if [[...]]; then
   [[....]]
elif....

2. Mainly, what is a bit strange for me, is the following:
Code:
if [[.....]] ; then
   [[ -f ... ]] && \
      export  ... || \
      { 
      echo ...; exit; 
      }

because I am expecting no return from export command. So, why should I ask like this:
Code:
 [[...]] AND export VAR OR...

# 4  
Old 10-18-2010
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

This User Gave Thanks to Scrutinizer For This Post:
# 5  
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


I agree with the dude Scrutinizer Smilie
This User Gave Thanks to ctsgnb For This Post:
# 6  
Old 10-18-2010
[[ xxx ]] is test [ xxx ], see man test or test in man ksh.

I call this "procedure as logic". Run the expresions and commands connected by && in order and keep going as long as commnads return true). You see this in PERL and C/C++ occasionally. Every command returns a logical value, the state of $?, 0 = true. I prefer, for each step, "if ( !next_command );then print error on next command and exit". It keeps the indentedness under control, separates the steps and reads easily. I call it "Cut them off at the pass!" (prevent complexity by doing simple, important things first, and when they are out of the way, the rest is simpler).
This User Gave Thanks to DGPickett For This Post:
# 7  
Old 10-18-2010
Quote:
[[ xxx ]] is test [ xxx ], see man test or test in man ksh.
Sorry, but this is absolute tosh. A Conditional Expression is not the same as a "test" and they have their own syntax and rules. Admittedly there is some small overlap in the syntax for certain conditions (such as "-f").
This User Gave Thanks to methyl For This Post:

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