[SOLVED] BASH

Garrett85 · 01-08-2013, 11:12 PM

I wrote a date validation program and it worked just fine but the guy teaching me to program want me to modify the program so that all varaible within a function are local and variables the function workes on must be refered by the function as command line arguments ($1 $2, etc) rather than their outside world name ${VARIABLE}. I have also been told that rather than using an else clause with in an if statement to try and set a defualt value first and let the if change that value if the condition is true. Below is one of my functions. The variable I need back from this is DAY and it will hold GOOD or BAD. Since I cannot use global variables within a function I would have to rename DAY="DAY" to local DAY=${DAY} within the function. But then how wold I give it a defualt value since if the next line is DAY="BAD" DAY would no longer point to the value of ${DAY}? I though maybe I could pre set the value befor the function is called, in the outside world I could go ahead and type DAY="BAD" but in order to retrieve the vale to the outside from within the function, don't I need to do something like this DAY=get_day()? if that's the case DAY would immediatly lose it's default value of "BAD" before it ever got to the function. I need some help here. Thanks.

Code:

get_day() {
	DAY="BAD"
	FIRSTDIGIT=${MM:0:1}
	SECONDDIGIT=${MM:1:1}

	if [[ ${FIRSTDIGIT} -eq 0 ]]; then #{
		if [[ ${DD} -gt 0 ]] && [[ ${DD} -le ${CALENDER[SECONDDIGIT-1]} ]]; then #{
			DAY="GOOD"
		fi #}
	else #{
		if [[ ${DD} -gt 0 ]] && [[ ${DD} -le ${CALENDER[MM-1]} ]]; then #{
			DAY="GOOD"
		fi #}
	fi #} }
}

grail · 01-09-2013, 01:40 AM

Your explanation seems a little confusing, but my understanding would be:

1. get_day function to return whether day is GOOD or BAD .... to me this would mean placing an echo at the end of the function as you need to return a string

2. internally to the function you will need local variables for the following:

DAY - would pick an alternate name here so to not get confused ... maybe retval (I am using lower case as upper is usually reserved for environment variables)
FIRSTDIGIT
SECONDDIGIT
DD
CALENDER - this one may be tricky as it is an array ... is it used elsewhere?

---
I wanted to mention previously that I am not sure why you use the outer if that tests FIRSTDIGIT ... if you test as a number it will remove the leading zero

Hope some of that helps

If not, please advise which points I am mistaken about?

Garrett85 · 01-09-2013, 09:50 AM

If in the if statement within the function I use echo to return a value, will that not print that value to the screen as echo typacally does? Because I wouldnt actaully be wanting to print anything to the screen from within a function. The reason I bother to remove the 0 from MM is because my script was not getting the right index from CALENDER if 02 was input instead of just 2. I was something that I hadnt thought of originally until I tested it and it didn't work, I stipped the 0 off of MM using FIRST & SECONDDIGIT and it worked after that.

Quote:

Originally Posted by grail

Your explanation seems a little confusing, but my understanding would be:

1. get_day function to return whether day is GOOD or BAD .... to me this would mean placing an echo at the end of the function as you need to return a string

2. internally to the function you will need local variables for the following:

DAY - would pick an alternate name here so to not get confused ... maybe retval (I am using lower case as upper is usually reserved for environment variables)
FIRSTDIGIT
SECONDDIGIT
DD
CALENDER - this one may be tricky as it is an array ... is it used elsewhere?

---
I wanted to mention previously that I am not sure why you use the outer if that tests FIRSTDIGIT ... if you test as a number it will remove the leading zero

Hope some of that helps

If not, please advise which points I am mistaken about?

Ramurd · 01-09-2013, 10:09 AM

Try it out:

Code:

output=$(echo "foo bar")

You'll see it won't echo to the screen, but into the variable; which you can try out with

Code:

echo "output = ${output}"

so, no: calling a function does not necessarily have to output to the screen.
Since you cannot "return" a string, and the scope of a variable is tricky at best, even while using global variables... I think this is your best bet.

Take this example, as how tricky scoping of variables can be (run it in a directory with at least a few files)

Code:

NUMBER=0
ls | while read fn
do
    ((NUMBER++))
done
echo $NUMBER

The value of ${NUMBER} you expected might not really be the number you got... This is something how bash traditionally works with pipes; there are workarounds.

David the H. · 01-09-2013, 03:03 PM

parameter substitution

Bash has a built in substitutions for defining values under various conditions, such as if a string is empty.

Code:

$ aloha(){ local foo=${foo:-goodbye} ; echo "$foo" ;}

$ foo=hello ; aloha
hello

$ unset foo ; aloha
goodbye