Aliases allow a string to be substituted for a word that is in
a position in the input where it can be the first word of a simple
command.
Aliases have names and corresponding values that are set
and unset using the alias and unalias builtin commands
(see Shell Builtin Commands).
If the shell reads an unquoted word in the right position, it checks the word to see if it matches an alias name. If it matches, the shell replaces the word with the alias value, and reads that value as if it had been read instead of the word. The shell doesn’t look at any characters following the word before attempting alias substitution.
The characters ‘/’, ‘$’, ‘`’, ‘=’ 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 shell checks
the next command word following the alias 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,
as in csh.
If arguments are needed, use a shell function
(see Shell Functions) instead.
Aliases are not expanded when the shell is not interactive,
unless the expand_aliases shell option is set using
shopt (see The Shopt Builtin).
The rules concerning the definition and use of aliases are
somewhat confusing.
Bash always reads at least one complete line of input,
and all lines that make up a compound command,
before executing any of the commands on that line or the compound command.
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 shell reads the next line of input,
and an alias definition in a compound command does not take
effect until the shell parses and executes the entire compound command.
The commands following the alias definition
on that line,
or in the rest of a compound command,
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 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 compound commands.
For almost every purpose, shell functions are preferable to aliases.