Linux command line 101 #1 : Here

Metacharacters are characters with a special use other than literal.

We have two types of metacharacters :

Substitution :

• * – (star) replace any character or group of characters.
• ? – (question mark) replace a character.

Protection :

• \ – prevents the interpretation of a metacharacter.
• ` – backticks force the interpretation of the character string included between two of these characters as a command.
• ‘ – all metacharacters included between two

  apostrophes are interpreted as text.

Some examples of substitution :

I will use a directory containing the following files in my examples :

Sandrine-3:test sandrine$ ls -l
total 0
-rw-r--r--  1 sandrine  staff  0  6 jul 20:49 file_AA
-rw-r--r--  1 sandrine  staff  0  6 jul 21:13 file_Ab
-rw-r--r--  1 sandrine  staff  0  6 jul 21:13 file_Ac
-rw-r--r--  1 sandrine  staff  0  6 jul 20:49 file_bb
-rw-r--r--  1 sandrine  staff  0  6 jul 20:49 file_cc
-rw-r--r--  1 sandrine  staff  0  6 jul 20:49 files_1
-rw-r--r--  1 sandrine  staff  0  6 jul 21:15 files_2
-rw-r--r--  1 sandrine  staff  0  6 jul 21:15 files_3
Sandrine-3:test sandrine$ 

In this example, the “ls” command displays all files name with names starting with “files” with all possible endings :

Sandrine-3:test sandrine$ ls -l files*
-rw-r--r--  1 sandrine  staff  0  6 jul 20:49 files_1
-rw-r--r--  1 sandrine  staff  0  6 jul 21:15 files_2
-rw-r--r--  1 sandrine  staff  0  6 jul 21:15 files_3
Sandrine-3:test sandrine$ 

You can also do the opposite and find all files name that end with “_1” like this :

Sandrine-3:test sandrine$ ls -l *_1
-rw-r--r--  1 sandrine  staff  0  6 jul 20:49 files_1
Sandrine-3:test sandrine$ 

And with “?” you can search for all occurrences of files name beginning with “file_”, with an unknown character and ending with “b” like this :

Sandrine-3:test sandrine$ ls -l file_?b
-rw-r--r--  1 sandrine  staff  0  6 jul 21:13 file_Ab
-rw-r--r--  1 sandrine  staff  0  6 jul 20:49 file_bb
Sandrine-3:test sandrine$ 

Some examples of protective character uses :

If you wish to use the “echo” command to display a “*” you will get this :

Sandrine-3:test sandrine$ echo *
file_AA file_Ab file_Ac file_bb file_cc files_1 files_2 files_3
Sandrine-3:test sandrine$ 

To avoid interpretation of “*” as metacharacter precede it with the “\” :

Sandrine-3:test sandrine$ echo \*
*
Sandrine-3:test sandrine$ 

To force the interpretation of a character string placed it between backticks :

Sandrine-3:test sandrine$ echo ls
ls
Sandrine-3:test sandrine$ echo `ls`
file_AA file_Ab file_Ac file_bb file_cc files_1 files_2 files_3
Sandrine-3:test sandrine$

To avoid interpretation of a sting of metacharacters place it between quotes :

Sandrine-3:test sandrine$ echo ******
file_AA file_Ab file_Ac file_bb file_cc files_1 files_2 files_3
Sandrine-3:test sandrine$ echo '******'
******
Sandrine-3:test sandrine$