See http://www.openwall.com/john/doc/EXAMPLES.shtml
and 
http://www.openwall.com/john/doc/MODES.shtml

Example: 
unshadow /etc/passwd /etc/shadow > myPasswords.txt
john myPasswords.txt

This will try "single crack" mode first, then use a wordlist with rules, and finally go for "incremental" mode. 
Single crack is based on account information+mangling rules (/etc/john.conf)
wordlist uses a list of words (see /usr/share/john/password.lst) + mangling rules



John the Ripper - wordlist rules syntax

Character classes.
??	matches "?"
?v	matches vowels: "aeiouAEIOU"
?c	matches consonants: "bcdfghjklmnpqrstvwxyzBCDFGHJKLMNPQRSTVWXYZ"
?w	matches whitespace: space and horizontal tabulation characters
?p	matches punctuation: ".,:;'?!`" and the double quote character
?s	matches symbols "$%^&*()-_+=|\<>[]{}#@/~"
?l	matches lowercase letters [a-z]
?u	matches uppercase letters [A-Z]
?d	matches digits [0-9]
?a	matches letters [a-zA-Z]
?x	matches letters and digits [a-zA-Z0-9]

The complement of a class can be specified by uppercasing its name. 
For example, "?D" matches everything but digits. 

Simple commands.
:	no-op: do nothing to the input word
l	convert to lowercase
u	convert to uppercase
c	capitalize
C	lowercase the first character, and uppercase the rest
t	toggle case of all characters in the word
TN	toggle case of the character in position N
r	reverse: "Fred" -> "derF"
d	duplicate: "Fred" -> "FredFred"
f	reflect: "Fred" -> "FredderF"
{	rotate the word left: "jsmith" -> "smithj"
}	rotate the word right: "smithj" -> "jsmith"
$X	append character X to the word
^X	prefix the word with character X

Length control commands.
<N	reject the word unless it is less than N characters long
>N	reject the word unless it is greater than N characters long
'N	truncate the word at length N

English grammar commands.
p	pluralize: "crack" -> "cracks", etc. (lowercase only)
P	"crack" -> "cracked", etc. (lowercase only)
I	"crack" -> "cracking", etc. (lowercase only)

Insert/delete commands.
[	delete the first character
]	delete the last character
DN	delete the character in position N
xNM	extract substring from position N for up to M characters
iNX	insert character X in position N and shift the rest right
oNX	overstrike character in position N with character X
Note that square brackets ("[" and "]") are special characters to the 
preprocessor: you should escape them with a backslash ("\") if using these 
commands. 

Charset conversion commands.
S	shift case: "Crack96" -> "cRACK(^"
V	lowercase vowels, uppercase consonants: "Crack96" -> "CRaCK96"
R	shift each character right, by keyboard: "Crack96" -> "Vtsvl07"
L	shift each character left, by keyboard: "Crack96" -> "Xeaxj85"

Memory access commands.
M	memorize the word
Q	reject the word unless it has changed

Character class commands.
sXY	replace all characters X in the word with Y
s?CY	replace all characters of class C in the word with Y
@X	purge all characters X from the word
@?C	purge all characters of class C from the word
!X	reject the word if it contains character X
!?C	reject the word if it contains a character in class C
/X	reject the word unless it contains character X
/?C	reject the word unless it contains a character in class C
=NX	reject the word unless character in position N is equal to X
=N?C	reject the word unless character in position N is in class C
(X	reject the word unless its first character is X
(?C	reject the word unless its first character is in class C
)X	reject the word unless its last character is X
)?C	reject the word unless its last character is in class C
%NX	reject the word unless it contains at least N instances of X
%N?C	reject the word unless it contains at least N characters of class C