Check a string in bash for unwanted characters

Question

Hello,
I'm trying to do a bash script that checks a variable against a list of unwanted characters, e.g. to parse a file name.
This doesn't really sound like a difficult task, but for some reason, whatever I've tried so far does not work, including my last attempt, shown below. Perhaps I'm doing something silly here - and I'm getting tired of it. What would be the best way for instance to parse a file name for invalid characters, or to accomplish or fix the below?
#!/bin/bash
read -p "Enter a filename: " fname
invalid_chars=", . ! @ # \$ % ^ & \* ( ) + = ? { } [ ] | ~"
i=0
while (( i <= ${#fname} )); do
   char=${fname:$i:1}
   for char in `echo $invalid_chars`; do
     echo "$char"
   done
   (( i += 1 ))
doneThanks.

Answer 1

Meanwhile I figured out the mistake I made in the for loop. The below finally works catching the list of characters. It won't catch * and ? though. I wonder if there wasn't an easier way to do it, beside using "grep".
#!/bin/bash
f_varcheck()
  ifs_orig=$IFS
  count=0
  score=0
  while (( count <= ${#1} )); do
    char=${1:$count:1}
    if [ "$score" = "0" ]; then
      wanted_char='~,!,@,#,$,%,^,&,(,),+,`,='
      IFS=$','
      for item in `echo "$wanted_char"`; do
        [ "$item" = "$char" ] && score=1
      done
    fi
    if [ "$score" = "0" ]; then
      wanted_char='{,},|,[,],\,:,",;,<,>,., ,/,'
      IFS=$','
      for item in `echo "$wanted_char"`; do
        [ "$item" = "$char" ] && score=1
      done
    fi
    if [ "$score" = "0" ]; then
      wanted_char=','
      IFS=$' '
      for item in `echo "$wanted_char"`; do
        [ "$item" = "$char" ] && score=1
      done
    fi
  if [ "$score" = "0" ]; then
    (( count += 1 ))
  else
    break
  fi
  done
  IFS=$ifs_orig
  [ "$score" = "1" ] && return 1 || return 0
read -p "Enter a filename: " fname
if ! f_varcheck "$fname"; then
   echo "Invalid character \`$char\` found."
else
   echo "Input is ok."
fi
{code}
Edited by: Dude on Feb 11, 2012 8:51 AM