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What does : do in this instance

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# 1  
Old 05-07-2009
What does : do in this instance
Guys please see below functions to return a status depending on user input. Both seem to work the same.

The second way has a : line which i can't understand or see in a ksh manual anywhere. Instead of doing the variable change if its empty on this line the first function simply does it on the case line. They both work but which is best and what does the : do?

Code:
confirm_yes()
{
          messages="$1"
          while :
          do
              print "${messages} [yn,] (default y)"
              read ans
              case ${ans:=y}
                   N*|n*) return 1 ;;
                   Y*|y*) return 0 ;;
                   *) print "You have entered an invalid option. Try again"
              esac
          done
}

Second way

Code:
confirm_yes()
{
          Valid=1
          while [ $Valid = 1 ]
          do
              echo "${1} [yn,] (default y)"
              read ans
              :      ${ans:=y}
              case ${ans}
                   N*|n*) return 1 ;;
                   Y*|y*) return 0 ;;
                   *) echo "You have entered an invalid option. Try again"
              esac
          done
}

This is for ksh script and function is called by :-

Code:
if confirm_yes "Would you like to unfreeze me?" ;then
        blah blah blah.......
else
        woa woa woa.....
fi

Cheers
# 2  
Old 05-07-2009
: is an alias for "true". See "man true" (though it is also built into ksh). It is a command which does nothing and returns exit status 0. The loop will keep going until it is exited.
In the second example the ":" is being used cleverly as a null command to allow the "${ans:=y}" default substitution to work without giving a syntax error.
# 3  
Old 05-07-2009
In your context the ':' evaluates to 'true' making the 'while' loop an 'endless loop'.
In ksh manual look for 'Special Commands' - you'll see something like:
Code:
: [ arg ... ]
           The command only expands parameters.

# 4  
Old 05-07-2009
Ok thanks.

I'm actually using it in my example then for the while section instead of saying while valid = 1 i'm just saying while true do bla bla

Its just a case of looping then until input is taken.

I personally thing code example one is better but i guess there's not much in it.

cheers
# 5  
Old 05-08-2009
: is a comment that is evaluated. It's not an alias for true really.

: is often times used to set default values for variables.

: ${val:=5}

if val has a value before this, then val is unchanged, if it's empty or not set, val=5

In the olden days before #!/bin/sh, a bourne shell script programmer might begin their script with : at the top to avoid starting the script with #, which meant the script was a csh script.

If you like readability, I sort of like this infinite loop construct:
Code:
while [ T ]; do
....
done

But arguably using : is much more efficient time wise.
# 6  
Old 05-08-2009
Quote:
: is a comment that is evaluated. It's not an alias for true really.
No.

login to a ksh shell,
Code:
alias | grep true
# or try
type true

see what you get.

I don't have bash handy to verify : in that shell.
# 7  
Old 05-08-2009
Quote:
Originally Posted by jim mcnamara
No.

login to a ksh shell,
Code:
alias | grep true
# or try
type true

see what you get.

I don't have bash handy to verify : in that shell.
No, it's neither a comment nor an alias (well, maybe in your case it's). My 'alias | grep true' returns nothing. And 'type true' returns 'true is a shell builtin'.

As the 'Good Book' (man ksh) says, ':' is a 'Special Command' on the same level as '.', 'alias', 'bg', 'break', eval and exec
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