This is an extract from the bash manual obtained by typing 'man bash'.


ALIASES
       Aliases	allow a string to be substituted for a word when it is used as
       the first word of a simple command.  The  shell	maintains  a  list  of
       aliases	that  may  be set and unset with the alias and unalias builtin
       commands (see SHELL BUILTIN COMMANDS below).  The first	word  of  each
       simple  command, if unquoted, is checked to see if it has an alias.  If
       so, that word is replaced by the text of the alias.  The characters  /,
       $,  ,  and = and any of the shell metacharacters or quoting characters
       listed above may not appear in an alias name.  The replacement text may
       contain	any  valid  shell  input, including shell metacharacters.  The
       first word of the replacement text is tested for aliases,  but  a  word
       that  is  identical to an alias being expanded is not expanded a second
       time.  This means that one may alias ls to ls  -F,  for	instance,  and
       bash  does  not try to recursively expand the replacement text.	If the
       last character of the alias value is a blank,  then  the  next  command
       word following the alias is also checked for alias expansion.

       Aliases are created and listed with the alias command, and removed with
       the unalias command.

       There is no mechanism for using arguments in the replacement text.   If
       arguments  are  needed,	a shell function should be used (see FUNCTIONS
       below).

       Aliases are not expanded when the shell is not interactive, unless  the
       expand_aliases  shell option is set using shopt (see the description of
       shopt under SHELL BUILTIN COMMANDS below).

       The rules concerning the definition and use  of	aliases  are  somewhat
       confusing.   Bash  always  reads  at  least  one complete line of input
       before executing any  of  the  commands	on  that  line.   Aliases  are
       expanded  when  a command is read, not when it is executed.  Therefore,
       an alias definition appearing on the same line as another command  does
       not  take  effect  until  the next line of input is read.  The commands
       following the alias definition on that line are not affected by the new
       alias.	This  behavior	is  also an issue when functions are executed.
       Aliases are expanded when a function definition is read, not  when  the
       function  is  executed,	because a function definition is itself a com
       pound command.  As a consequence, aliases defined in a function are not
       available  until  after	that function is executed.  To be safe, always
       put alias definitions on a separate line, and do not use alias in  com
       pound commands.

       For almost every purpose, aliases are superseded by shell functions.