#!/bin/sh
# validAlphaNum - Ensures that input consists only of alphabetical
# and numeric characters.

validAlphaNum()
{
 # Validate arg: returns 0 if all upper+lower+digits, 1 otherwise

 # Remove all unacceptable chars
 compressed="$(echo $1 | sed -e 's/[^[:alnum:]]//g')"

 if [ "$compressed" != "$input" ] ; then
  return 1
 else
  return 0
 fi
}

# Sample usage of this function in a script

echo -n "Enter input: "
read input

if ! validAlphaNum "$input" ; then  #// 这个有点巧妙，就是如果函数的返回值为1的话，则执行
 echo "Your input must consist of only letters and numbers." >&2
 exit 1
else
 echo "Input is valid."
fi

exit 0