7.5. Filename Substitution

When evaluating the command line, the shell uses metacharacters to abbreviate filenames or pathnames that match a certain set of characters. The filename substitution metacharacters listed in Table 7.2 are expanded into an alphabetically listed set of filenames. The process of expanding the metacharacter into filenames is also called filename substitution, or globng. If a metacharacter is used and there is no filename that matches it, the shell treats the metacharacter as a literal character.

Table 7.2. Bourne Shell Metacharacters and Filename Substitution
Metacharacter
Meaning
*
Matches zero or more characters
?
Matches exactly one character
[abc]
Matches one character in the set a, b, or c
[a–z]
Matches one character in the range from a to z
[!a–z]
Matches one character not in the range from a to z
\
Escapes or disables the metacharacter

7.5.1 The Asterisk

The asterisk is a wildcard that matches for zero or more of any characters in a filename.

Example 7.14.


1   $ ls *

    abc abc1 abc122 abc123 abc2 file1 file1.bak file2 file2.bak none

    nonsense nobody nothing nowhere one

2   $ ls  *.bak

    file1.bak file2.bak

3   $ echo a*

    ab abc1 abc122 abc123 abc2

EXPLANATION

The asterisk expands to all of the files in the present working directory. All of the files are passed as arguments to ls and displayed.
All files starting with zero or more characters and ending with .bak are matched and listed.
All files starting with a, followed by zero or more characters, are matched and passed as arguments to the echo command.

7.5.2 The Question Mark

The question mark represents a single character in a filename. When a filename contains one or more question marks, the shell performs filename substitution by replacing the question mark with the character it matches in the filename.

Example 7.15.


1   $ ls

    abc  abc122  abc2  file1.bak  file2.bak  nonsense  nothing  one

    abc1  abc123  file1  file2  none  noone  nowhere

2   $ ls a?c?

    abc1 abc2

3   $ ls ??

    ?? not found

4   $ echo  abc???

    abc122 abc123

5   $ echo ??

    ??

EXPLANATION

The files in the current directory are listed.
Filenames starting with a, followed by a single character, followed by c and a single character, are matched and listed.
Filenames containing exactly two characters are listed if found. Because there are no two-character filenames, the question marks are treated as a literal filename.
Filenames starting with abc and followed by exactly three characters are expanded and displayed by the echo command.
There are no files in the directory that contain exactly two characters. The shell treats the question mark as a literal question mark if it cannot find a match.

7.5.3 The Square Brackets

The brackets are used to match filenames containing one character in a set or range of characters.

Example 7.16.


1   $ ls

    abc  abc122 abc2 file1.bak file2.bak nonsense nothing

    one  abc1 abc123 file1 file2 none noone nowhere

2   $ ls abc[123]

    abc1  abc2

3   $ ls abc[1–3]

    abc1  abc2

4   $ ls [a–z][a–z][a–z]

    abc one

5   $ ls [!f–z]???

    abc1  abc2

6   $ ls abc12[23]

    abc122 abc123

EXPLANATION

All of the files in the present working directory are listed.
All filenames containing four characters are matched and listed if the filename starts with abc, followed by 1, 2, or 3. Only one character from the set in the brackets is matched.
All filenames containing four characters are matched and listed if the filename starts with abc and is followed by a number in the range from 1 to 3.
All filenames containing three characters are matched and listed if the filename contains exactly three lowercase alphabetic characters.
All filenames containing four characters are listed if the first character is not a lowercase letter between f and z ([!f–z]), followed by three of any character (e.g., ???).
Files are listed if the filenames contain abc12 followed by 2 or 3.

7.5.4 Escaping Metacharacters

To use a metacharacter as a literal character, the backslash may be used to prevent the metacharacter from being interpreted.

Example 7.17.


1   $ ls

    abc file1 youx

2   $ echo How are you?

    How are youx

3   $ echo How are you\?

    How are you?

4   $ echo  When does this line \

    > ever end\?

    When does this line ever end?

EXPLANATION

The files in the present working directory are listed. (Note the file youx.)
The shell will perform filename expansion on the ?. Any files in the current directory starting with y-o-u and followed by exactly one character are matched and substituted in the string. The filename youx will be substituted in the string to read How are youx (probably not what you wanted to happen).
By preceding the question mark with a backslash, it is escaped, meaning that the shell will not try to interpret it as a wildcard.
The newline is escaped by preceding it with a backslash. The secondary prompt is displayed until the string is terminated with a newline. The question mark (?) is escaped to protect it from filename expansion.