BASH:Rewrite a function using parameter expansion instead of changing IFS

Mes9 · 08-19-2012, 11:10 AM

I am learning bash from a book and im stuck at this exercise from the current chapter.
The exercise is: rewrite function isvalidip using parameter expansion instead of changing IFS.

Here is the funtion:

Code:

isvalidip() #@ USAGE: isvalidip DOTTED-QUAD
{
 case $1 in
    ## reject the following:
    ##  empty string
    ##  anything other than digits and dots
    ##  anything not ending in a digit
    "" | *[!0-9.]* | *[!0-9]) return 1 ;;
 esac

## Change IFS to a dot, but only in this function
local IFS=.


## Place the IP address into the positional parameters;
## after word splitting each element becomes a parameter
set -- $1

[ $# -eq 4 ] && ## must be four parameters
                ## each must be less than 256
# A default of 666 (which is invalid) is used if a parameter is empty
# All four parameters must pass the test
[ ${1:-666} -le 255 ] && [ ${2:-666} -le 255 ] &&
[ ${3:-666} -le 255 ] && [ ${4:-666} -le 255 ]
}

Here is what I've done so far(knowing this has probably nothing to do with the solution.

Code:

pre=:
post=:
#var=192.168.0.123


if [ -z "$*" ]
 then
 printf "You need to type command line arguments\n" >&2
 exit 1
else
 for var in $@
 do
    printf "$pre%s$post\n" "$var"
##printf "$pre%s$post\n" "${var:4:3}"
 done
fi
unset var

rg421 · 08-20-2012, 08:37 AM

Hello,

If your code has nothing to do with the solution, why do you post it ?

If I read correctly, you must write a function, so you could do, at least something like this:

Code:

function anotherIsValidIp() # anotherIsValidIp DOTTED-QUAD
{case $1 in
# ...
# I let you copy
esac

myIp="$1"
## Some other nice stuff and that's where I can't figure what to do
}

So, now is the difficult part. It is an exercice, so you probably have to read your book. But you have an answer in the bash man

Code:

$ man bash

You can find it at chapter Parameter Expansion, and particularly at the definition of ${parameter#word}, ${parameter##word}, or ${parameter%word} or ${parameter%%word}

Good luck,
rg

Mes9 · 08-20-2012, 01:48 PM

My understanding is that it has to do with word-splitting.

PTrenholme · 08-20-2012, 02:15 PM

Look at your description (in your book) of ${parameter//pattern/string} construct, and consider what would happen if your function called another with an argument in that form. (There's no reason why a function you write can't call another function that you also write.)

Oh, and the doubled / is not a typo.

David the H. · 08-20-2012, 03:43 PM

It seems to me that the requirement is...you have an ip address string, and you need to separate this string into four sections. Now, using parameter substitution, how do you get each of the four sections?

So just complete the ${}'s, below. (Hint, it will probably actually take two steps each to get the middle two.)

Code:

ip='123.45.54.321'

first=${}
second=${}
third=${}
fourth=${}

echo "$first $second $third $fourth"

Then it's just a matter of looping through them for the final part of the function. Although actually it would be easier to store them in an array rather than four separate variables.

Incidentally, a cleaner way would be to still use IFS, but with read instead:

Code:

ip='123.45.54.321'
IFS=. read -a slice <<<"$ip"
printf '%s\n' "${slice[@]}"

Mes9 · 09-05-2012, 03:04 PM

Thank you all for the reply's. Think it is going to take some time to work through this.

Mes9 · 09-26-2012, 02:37 PM

Quote:

Originally Posted by Mes9

Thank you all for the reply's. Think it is going to take some time to work through this.

I am going through some other books, so it will be awhile until I go back to this question, thank you again for the feedback.

Habitual · 09-26-2012, 02:50 PM

"Splitting a string into fields" section at http://mywiki.wooledge.org/BashFAQ/100?

Edit: err, maybe not. ("instead of changing IFS")
I'm too eager to help sometimes.