Characters significant to shells "sh" and "bash":     WH 1996/9, 2009/01
Character(s): Significance:
|             form a pipe between surrounding commands
||            do following command only if previous fails    (C logical or)
&&            do following command only if previous succeeds (C logical and)
; OR newline  terminate previous command
&             run previous command in background
>             redirect "stdout" by sending output to the following file
>>            redirect "stdout" by appending output to the following file
<             redirect "stdin" by getting input from following file
<<            take input from the "here" document (special syntax)
                 - input to come from the following lines
'   '         encloses a string preventing shell from touching it
"   "         encloses a string preventing shell from touching it EXCEPT
                 that variable subs $ and command subs `  ` are done
$             variable substitution: evaluate variable - replace by value
              By convention, an environment (exported) variable is all CAPs.
              Use ${  } where ambiguity is possible.  Modifiers available.
        Special variables:   (Note all numbers are decimal.)
        $0    command name
        $1,$2,...  parameters given with command (see also: "shift", "set")
        $*    $1 $2 ...    thus   "$*" is "$1 $2 ..."
        $@    $1 $2 ...    except "$@" is "$1" "$2" ... 
        $#    number of parameters supplied to command
        $?    exit status of last command 
        $$    process number of this shell: use to generate temporary names
        $!    process number of last command run in background
        $-    flags supplied to shell
=             Variable assignment when in a string: VARIABLE=value
              Never put spaces around '=' and quote value if has spaces
              or other characters significant to shell.
`   `         command substitution: execute the enclosed commands and replace
                 by output.  In bash can also use:   $(   )      It nests!
(   )         enclosed commands are run in subshell, a separate environment.
{   }         command grouping
      (){ }   used for shell functions:      function_name () 
                                             {  commands  ; }
.             execute commands from given file in the current shell 
                 and environment, not in a subshell:  "source" the file
#             comment
:             null command - do not use it like a comment  ": hi ; trouble"
\             escape or quote character: next character is taken literally
                 losing significance to the shell.  However, don't use inside 
                 a '   ' since no character has special significance there.
                 \newline  is ignored so can use this to continue a line.
[    ]        synonym for "test" but make sure [ and ] surrounded by spaces
[[  ]]        improved builtin "test" for "bash"  - for conditional expressions
((  ))        arithmetic expression in "bash".    $(( )) gives value 

FILE REFERENCE characters:        Note that '/' and '.' when initial
 or just after '/' must be matched explicitly.
/             directory
*             replace by any string (even a null one)
?             replace by any single character
[ ]           replace by any single character of given kind, as in:
      [abcd]         list of characters   (a,b,c,d)
      [a-z]       -  range of characters  (a,b,c,...,z)
      [!abq-z]    !  not these characters (not a,b,q,r,...,y,z)
~             "bash" only - home directory of user

"csh": !     history substitution: replace by value as determined
                 from this or previous command lines