Checking input is Numerical

Shell Programming and Scripting
1

Hey guys, i was looking for some examples of how can i check if the use input is just using numerical. i came across an example using tr :

echo "read this"
read this

if [ -z "`echo "$this" | tr -d [[:digit:]]`" ] ;

then
echo "True - only alpha and numeric"
else
echo "False - there are NON alpha and numeric stuff here!"
fi

Could somebody explain to me how the code works and why -z and -d must be used.

2

if you had searched the forum you could find the script below by Franklin which is better than the above one:-

#!/bin/bash

shopt -s extglob

INPUT="123"

case $INPUT in
   ( +([[:digit:]])  ) echo "$INPUT is all numbers" ;;
   ( +([[:alpha:]])  ) echo "$INPUT is all characters" ;;
   ( +([[:alnum:]])  ) echo "$INPUT is alphanumeric" ;;
   (                *) echo "$INPUT is empty or something unknown" ;;
esac

exit 0

for the tr -d & if [ -z ]..
just see your system manual using below.

man if
man tr
3

From man sh:

            -z string     True if the length of string is zero.

And from man tr:

       -d, --delete
              delete characters in SET1, do not translate

:slight_smile:

4

its the -d command which i do not get it. a syntax for a normal tr command would be

tr [set1][set2]

so how does it work in the code above. basically, the input of $this is being deleted only if it is a numerical number since we specified [[:digit:]]?. am i getting it right?

hey admad, thanks for your solution. could i ask, why must we surround the option with +()?

---------- Post updated at 03:47 AM ---------- Previous update was at 03:41 AM ----------

There another problem which i am facing. I would like to key in a a input which gets to read decimals and numerical numbers like 23.23 or 24. but i would also like to do a check to make sure that no characters like "ad.23" or "aec" is being inputted.

Do you guys have a solution to this?

5

Read this:-


Some versions of sed, ed, and ex support escaped versions of the extended Regular Expressions
described above, as do the GNU utilities.
POSIX Character Classes. [:class:]
This is an alternate method of specifying a range of characters to match.
� 
[:alnum:] matches alphabetic or numeric characters. This is equivalent to A-Za-z0-9. � 
[:alpha:] matches alphabetic characters. This is equivalent to A-Za-z. � 
[:blank:] matches a space or a tab. � 
[:cntrl:] matches control characters. � 
[:digit:] matches (decimal) digits. This is equivalent to 0-9. � 
[:graph:] (graphic printable characters). Matches characters in the range of ASCII 33 - 126. This
is the same as [:print:], below, but excluding the space character.
� 
[:lower:] matches lowercase alphabetic characters. This is equivalent to a-z. � 
[:print:] (printable characters). Matches characters in the range of ASCII 32 - 126. This is the
same as [:graph:], above, but adding the space character.
� 
[:space:] matches whitespace characters (space and horizontal tab). � 
[:upper:] matches uppercase alphabetic characters. This is equivalent to A-Z. � 
[:xdigit:] matches hexadecimal digits. This is equivalent to 0-9A-Fa-f.
POSIX character classes generally require quoting or double brackets ([[ ]]).
bash$ grep [[:digit:]] test.file
abc=723
These character classes may even be used with globbing, to a limited extent.

bash$ ls -l ?[[:digit:]][[:digit:]]?
-rw-rw-r--    1 bozo  bozo         0 Aug 21 14:47 a33b
6

If you do:

man tr

you'll find that tr -d [set1] means delete the characters in set 1.
The code above is in fact broken:

  1. they are testing for numerical values not alphanumerical
  2. They left out single quotes so the command was not working right:

Try this:

echo "read this"
read this
if [ -z $(echo "$this" | tr -d '[[:digit:]]') ] ;
then
  echo "True - only numeric or the string was empty"
else
  echo "False - there is NON numeric stuff here!"
fi

But you do not need an external command, e.g.:

echo "read this"
read this
case $this in
  *[^0-9]*|"") echo "False - there is NON numeric stuff here or string is empty"    ;;
  *)           echo "True - only numeric"  ;;
esac
7

Hey Scrutinizer, just let me check how this code works

echo "$this" |

*this echo out the content inside $this and pipes it into the next set of command
 

tr -d '[[:digit:]]')

*this command will check if the data from the $this is in digits , and if it is, will delete them. if not, it will not delete them , thus the data will not be 0 and this  the message that there is a non-numeric stuff is inside.

did i get it right?

8

Correct, the outcome will not be "" (empty) after the operation if there are non-digits in the string.