Unix find tutorial

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Part 3: Finding files using file name or path

• Introduction
• Name predicate and shell patterns
• Nuances of usage of name predicate
• Searching fully qualified file name and path
• Regular expressions

Introduction

Find is now more then 40 years old and naturally there are several generations of it. As we are taking about GNU find there are multiple version of it too. Each has different level of support of regular expressions. Here we see the sad truth of Donald Knuth humorous definition of Unix/Linux as an OS with six different types of regular expressions.

The current versions of GNU find (as of Sept 2014 this is 4.5.11) support more then six different types of regular expression with the exception of one, the most needed type: as of August 2014 it still cannot use Perl regular expression, though. Starting in 1997, Philip Hazel developed PCRE (Perl Compatible Regular Expressions), which attempts to closely mimic Perl's regular expression functionality and is used by many modern tools including PHP and Apache HTTP Server. Unfortunately GNU find does not use it yet.

Name predicate and shell patterns

The first and probably the most popular option in finding files using regular expression is -name option which supports basic (shell-style or DOS-style) regular expressions:

-name "basic_regular_expression"

• Classes like [0-9] or [chly] match any character in the class.
• The . is not a special character; it matches only dot character
• The ? matches any single character.
• The * matches any sequence of zero to more characters.
• Unlike regular expressions, shell globbing patterns must match the entire word, so it works as if it were a regexp that starts with ^ and ends with $.

"Globbing patterns" are not as powerful as regular expressions, but they are easier to read, and they are convenient for simple matching of filenames. They also are well known by most Unix sysadmins. See below for more complete discussion.

Nuances of usage of name predicate

the most common name related predicate used with find is -name.

The -name predicate operates only of basename of the file (with the path removed). Expression is true if file name matches the shell pattern specified. For example to find files with the extension .conf in the /etc directory:

find /etc -name '*.conf'

Predicate -iname pattern does the same thing but matching is case insensitive.

Notes:

• To ignore a subtree you can use -prune .
• This idea of using 'i" prefix for ignore is used for other types of regex too. For example you can specify -regex option (or -iregex option which ignores case).
• You must quote patterns that contain metacharacters to prevent the shell from expanding them itself. Double and single quotes both work; so does escaping with a backslash.

Searching fully qualified file name and path

• -path and -ipath
• -wholename  and -wholename

 Using

-path  shell_pattern

you can search a full path of the file instead of its name.

Even more useful is predicate -wholename which searches the path+name (in case of using relative derectory path such as ./my  the path is from start of the search so this not a fully qualified file name).

Predicates -ipath and -iwholename are similar but in the latter case the match is case-insensitive.

Note: For predicates -path , -wholename, -ipath and -iwholename , a path is consists of all the directories traversed from find's start point to the file being tested, followed by the base name of the file itself. Only if search starts from the root directory these it will be equal to absolute paths

For example

cd /tmp
mkdir -p foo/bar/baz
find foo -path foo/bar -print # first find command
find foo -path /tmp/foo/bar -print # the second find command (prints nothing)
find /tmp/foo -path /tmp/foo/bar -print /tmp/foo/bar # the third find command

Notice that due to search starting point foo the second find command prints nothing, even though /tmp/foo/bar exists.

Unlike file name expansion on the command line, a * in the pattern will match both / and leading dots in file names:

find .  -path '*f'
./quux/bar/baz/f

find .  -path '*/*config'
./quux/bar/baz/.config

Regular expressions

Find defaults to basic regular expressions (DOS style regex or shell pattern matching). Here is the quote from GNU find manual (Finding Files)

find and locate can compare file names, or parts of file names, to shell patterns. A shell pattern is a string that may contain the following special characters, which are known as wildcards or metacharacters.

You must quote patterns that contain metacharacters to prevent the shell from expanding them itself. Double and single quotes both work; so does escaping with a backslash.

*

Matches any zero or more characters.

?

Matches any one character.

[string]

Matches exactly one character that is a member of the string string. This is called a character class. As a shorthand, string may contain ranges, which consist of two characters with a dash between them. For example, the class [a-z0-9_] matches a lowercase letter, a number, or an underscore. You can negate a class by placing a ! or ^ immediately after the opening bracket. Thus, [^A-Z@] matches any character except an uppercase letter or an at sign.

\

Removes the special meaning of the character that follows it. This works even in character classes.

In the find tests that do shell pattern matching ( -name , -wholename , etc.), wildcards in the pattern will match a . at the beginning of a file name. This is also the case for locate. Thus, find -name '*macs' will match a file named .emacs, as will locate '*macs' .

Slash characters have no special significance in the shell pattern matching that find and locate do, unlike in the shell, in which wildcards do not match them. Therefore, a pattern foo*bar can match a file name foo3/bar , and a pattern ./sr*sc can match a file name ./src/misc .

If you want to locate some files with the locate command but don't need to see the full list you can use the --limit option to see just a small number of results, or the --count option to display only the total number of matches.

Type of regular expression that find will use can be specified with option -regextype. In best GNU traditions you need to select from several option, only half of which are useful (see Finding Files)

• findutils-default. Default behavior if -regex or -iregex is specified, but option -regextype is not .
  • The character ‘.’ matches any single character.

    ‘+’

    indicates that the regular expression should match one or more occurrences of the previous atom or regexp.
    

    ‘?’

    indicates that the regular expression should match zero or one occurrence of the previous atom or regexp.
    

    ‘\+’

    matches a ‘+’
    

    ‘\?’

    matches a ‘?’.

  • Bracket expressions are used to match ranges of characters. Bracket expressions where the range is backward, for example ‘[z-a]’, are ignored. Within square brackets, ‘\’ is taken literally. Character classes are not supported, so for example you would need to use ‘[0-9]’ instead of ‘[[:digit:]]’.
  • GNU extensions are supported:
    1. ‘\w’ matches a character within a word
    2. ‘\W’ matches a character which is not within a word
    3. ‘\<’ matches the beginning of a word
    4. ‘\>’ matches the end of a word
    5. ‘\b’ matches a word boundary
    6. ‘\B’ matches characters which are not a word boundary
    7. ‘\`’ matches the beginning of the whole input
    8. ‘\'’ matches the end of the whole input
  • Grouping is performed with backslashes followed by parentheses ‘\(’, ‘\)’. A backslash followed by a digit acts as a back-reference and matches the same thing as the previous grouped expression indicated by that number. For example ‘\2’ matches the second group expression. The order of group expressions is determined by the position of their opening parenthesis ‘\(’.
  • The alternation operator is ‘\|’.
  • The character ‘^’ only represents the beginning of a string when it appears:
    1. At the beginning of a regular expression
    2. After an open-group, signified by ‘\(’
    3. After the alternation operator ‘\|’
  • The character ‘$’ only represents the end of a string when it appears:
    1. At the end of a regular expression
    2. Before a close-group, signified by ‘\)’
    3. Before the alternation operator ‘\|’
  • ‘*’, ‘+’ and ‘?’ are special at any point in a regular expression except:
    1. At the beginning of a regular expression
    2. After an open-group, signified by ‘\(’
    3. After the alternation operator ‘\|’
  • The longest possible match is returned; this applies to the regular expression as a whole and (subject to this constraint) to subexpressions within groups.
    
• egrep. This is the second useful option as most sysadmin know egrep well.

  The character ‘.’ matches any single character except newline.

  ‘+’

  indicates that the regular expression should match one or more occurrences of the previous atom or regexp.
  

  ‘?’

  indicates that the regular expression should match zero or one occurrence of the previous atom or regexp.
  

  ‘\+’

  matches a ‘+’
  

  ‘\?’

  matches a ‘?’.

  Bracket expressions are used to match ranges of characters. Bracket expressions where the range is backward, for example ‘[z-a]’, are ignored. Within square brackets, ‘\’ is taken literally. Character classes are supported; for example ‘[[:digit:]]’ will match a single decimal digit. Non-matching lists ‘[^...]’ do not ever match newline.

  GNU extensions are supported:

  1. ‘\w’ matches a character within a word
  2. ‘\W’ matches a character which is not within a word
  3. ‘\<’ matches the beginning of a word
  4. ‘\>’ matches the end of a word
  5. ‘\b’ matches a word boundary
  6. ‘\B’ matches characters which are not a word boundary
  7. ‘\`’ matches the beginning of the whole input
  8. ‘\'’ matches the end of the whole input

  Grouping is performed with parentheses ‘()’.

  A backslash followed by a digit acts as a back-reference and matches the same thing as the previous grouped expression indicated by that number. For example ‘\2’ matches the second group expression. The order of group expressions is determined by the position of their opening parenthesis ‘(’.

  The alternation operator is ‘|’.

  The characters ‘^’ and ‘$’ always represent the beginning and end of a string respectively, except within square brackets. Within brackets, ‘^’ can be used to invert the membership of the character class being specified.

  The characters ‘*’, ‘+’ and ‘?’ are special anywhere in a regular expression.

  The longest possible match is returned; this applies to the regular expression as a whole and (subject to this constraint) to subexpressions within groups.

• posix-awk Regular expressions compatible with the POSIX awk command (not GNU awk). Useful for heavy awk users:

  The character ‘.’ matches any single character except the null character.

  ‘+’

  indicates that the regular expression should match one or more occurrences of the previous atom or regexp.
  

  ‘?’

  indicates that the regular expression should match zero or one occurrence of the previous atom or regexp.
  

  ‘\+’

  matches a ‘+’
  

  ‘\?’

  matches a ‘?’.

  Bracket expressions are used to match ranges of characters. Bracket expressions where the range is backward, for example ‘[z-a]’, are invalid. Within square brackets, ‘\’ can be used to quote the following character. Character classes are not supported, so for example you would need to use ‘[0-9]’ instead of ‘[[:digit:]]’.

  GNU extensions are not supported and so ‘\w’, ‘\W’, ‘\<’, ‘\>’, ‘\b’, ‘\B’, ‘\`’, and ‘\'’ match ‘w’, ‘W’, ‘<’, ‘>’, ‘b’, ‘B’, ‘`’, and ‘'’ respectively.

  Grouping is performed with parentheses ‘()’. An unmatched ‘)’ matches just itself. A backslash followed by a digit matches that digit.

  The alternation operator is ‘|’.

  The characters ‘^’ and ‘$’ always represent the beginning and end of a string respectively, except within square brackets. Within brackets, ‘^’ can be used to invert the membership of the character class being specified.

  ‘*’, ‘+’ and ‘?’ are special at any point in a regular expression except:

  1. At the beginning of a regular expression
  2. After an open-group, signified by ‘(’
  3. After the alternation operator ‘|’

  The longest possible match is returned; this applies to the regular expression as a whole and (subject to this constraint) to subexpressions within groups.

• posix-basic POSIX Basic Regular Expressions. It's close to behaviour of default in -name predicate
  • * Matches any zero or more characters.
  • ? Matches any one character.
  • [string] Matches exactly one character that is a member of the string string. This is called a character class. As a shorthand, string may contain ranges, which consist of two characters with a dash between them. For example, the class [a-z0-9_] matches a lowercase letter, a number, or an underscore. You can negate a class by placing a ! or ^ immediately after the opening bracket. Thus, [^A-Z@] matches any character except an uppercase letter or an at sign.
  • \ Removes the special meaning of the character that follows it. This works even in character classes
• posix-egrep Regular expressions compatible with the POSIX egrep command
• posix-extended POSIX Extended Regular Expressions.
• emacs Useful for heavy users of Emacs.

See Regular Expressions for more information on the regular expression dialects... There are many books about regular expressions that provide a good guidance into this esoteric area. For in depth coverage of regular expression see recommendations in the page Best books about Regular Expressions

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