There are many ways of doing this.

1. Use the inbuilt features of the shell. Commands (and functions) have exit values. Usually, an exit value of 0 means the command was successful and a value > 0 means it failed. This is checked by the shell when you use the || or && operators. So, to run a command only if the previous one was successful, you would use &&, ro run it unless the previous was successful, you use ||. For example:

   
   
   

   $ foo
   
   The last command was succesful
   
   echo 'foo' && echo "The last command was succesful"
   
   

   
   
   

   If you use a command that does not exist, it will obviously fail:

   
   
   

    $ echooo 'foo' && echo "The last command was succesful"
   
    echooo: command not found
   
   

   
   
   

   Note that the second message is not printed. If you use || instead:

   
   
   

   $ echooo 'foo' || echo "The last command failed"
   
   echooo: command not found
   
   The last command failed
   
   

   
   
   

   You can combine these into your shell function:

   
   
   

   function isitthere
   
   {
   
       [ -f thefile ]
   
   }
   
   
   
   isitthere && echo "The file exists" || echo "the file does not exist"
   
   

   
   
   

   The test operator (also known as [ ]) returns an exit value of 0 on success and 1 on failure. This means that your function can be simplified to the above.




2. Use the return command. This is actually exactly the same as the example above, I am simply stating it more explicitly. It also allows you to return more complex values for different fail cases.

   
   
   

   function isitthere
   
   {
   
       if [ -f <thefile> ]; then
   
           return 0
   
       else
   
           return 1
   
       fi
   
   }
   
   
   
   if [ $(isitthere) ]; then
   
       echo "Your program is properly installed"
   
   else
   
       echo "Your program isn't properly installed.
   
   fi
   
   

   
   
   

   I used the same syntax as you to illustrate, but this could be shortened to

   
   
   

   isitthere && echo "Your program is properly installed" || echo "Your program isn't properly installed."
   
   




3. Have your function print something to standard output and then act accordingly.

   
   
   

   function isitthere
   
   {
   
       [ -f thefile ] && echo yes || echo no
   
   }
   
   
   
   if [ $(isitthere) = "yes" ];
   
   then
   
       echo "Your program is properly installed"
   
   else
   
       echo "Your program isn't properly installed."
   
   fi
   
   

   
   
   

   In this example, we're using the standard output of the function and not its return value. To capture the output of a command (or function) you need to use command substitution. This allows you to run a command and save the output as a variable. For example

   
   
   

   $ foo=$(echo bar)
   
   $ echo $foo
   
   bar
   
   

   
   
   

   So, the test if [ $(isitthere) = "yes" ] checks whether the output of the functin was yes and acts accordingly.

Note that I have used both [ -f thefile ] && echo yes || echo no and the more complex

if [ -f <thefile> ]; then

    return 0

else

    return 1

fi

The two are basically equivalent, I used both only to illustrate that. You can use whichever you prefer.