Special characters in bash

# This line is a comment.

echo "A comment will follow." # Comment here.
#                            ^ Note whitespace before #

echo "The # here does not begin a comment."
echo 'The # here does not begin a comment.'
echo The \# here does not begin a comment.
echo The # here begins a comment.
echo ${PATH#*:}       # Parameter substitution, not a comment.
echo $(( 2#101011 ))  # Base conversion, not a comment.

if [ -x "$filename" ]; then    #  Note the space after the semicolon.
#+                   ^^
  echo "File $filename exists."; cp $filename $filename.bak
else   #                       ^^
  echo "File $filename not found."; touch $filename
fi; echo "File test complete."

case "$variable" in
  abc)  echo "\$variable = abc" ;;
  xyz)  echo "\$variable = xyz" ;;
esac

let "t2 = ((a = 9, 15 / 3))"
# Set "a = 9" and "t2 = 15 / 3"

for file in /{,usr/}bin/*calc
#             ^    Find all executable files ending in "calc"
#+                 in /bin and /usr/bin directories.
do
    if [ -x "$file" ]
    then
        echo $file
    fi
done

while : do
   operation-1
   operation-2
   ...
   operation-n
done
# Same as:
#    while true
#    do
#      ...
#    done

if condition
then :   # Do nothing and branch ahead
else     # Or else ...
   take-some-action
fi

: ${username=`whoami`}
# ${username=`whoami`}   Gives an error without the leading :
#                        unless "username" is a command or builtin...
: ${1?"Usage: $0 ARGUMENT"}     # From "usage-message.sh example script.

: ${HOSTNAME?} ${USER?} ${MAIL?}
#  Prints error message
#+ if one or more of essential environmental variables not set.

: > data.xxx   # File "data.xxx" now empty.
# Same effect as   cat /dev/null >data.xxx
# However, this does not fork a new process, since ":" is a builtin.

:() {
  echo "The name of this function is "$FUNCNAME" "
  # Why use a colon as a function name?
  # It's a way of obfuscating your code.
}
:
# The name of this function is :

not_empty ()
{
  :
} # Contains a : (null command), and so is not empty.