Red Hat System Administration I

Pathname expansion expands a pattern of special characters that represent wild cards or classes of characters into a list of file names that match the pattern. Before the shell runs your command, it replaces the pattern with the list of file names that matched. If the pattern does not match anything, then the shell tries to use the pattern as a literal argument for the command that it runs. The following table lists common metacharacters and pattern classes that are used for pattern matching.

For the next example, imagine that you ran the following commands to create some sample files:

[user@host ~]$ mkdir glob; cd glob
[user@host glob]$ touch alfa bravo charlie delta echo able baker cast dog easy
[user@host glob]$ ls
able  alfa  baker  bravo  cast  charlie  delta  dog  easy  echo
[user@host glob]$

In the next example, the first two commands use simple pattern matches with the asterisk (*) to match all the file names that start with "a" and all the file names that contain an "a", respectively. The third command uses the asterisk and square brackets to match all the file names that start with "a" or "c".

[user@host glob]$ ls a*
able  alfa
[user@host glob]$ ls *a*
able  alfa  baker  bravo  cast  charlie  delta  easy
[user@host glob]$ ls [ac]*
able  alfa  cast  charlie

The next example also uses question mark (?) characters to match some of those file names. The two commands match only file names with four and five characters in length, respectively.

[user@host glob]$ ls ????
able  cast  easy  echo
[user@host glob]$ ls ?????
baker  bravo  delta

Brace expansion is used to generate discretionary strings of characters. Braces contain a comma-separated list of strings, or a sequence expression. The result includes the text that precedes or follows the brace definition. Brace expansions might be nested, one inside another. You can also use double-dot syntax (..), which expands to a sequence. For example, the {m..p} double-dot syntax inside braces expands to m n o p.

[user@host glob]$ echo {Sunday,Monday,Tuesday,Wednesday}.log
Sunday.log Monday.log Tuesday.log Wednesday.log
[user@host glob]$ echo file{1..3}.txt
file1.txt file2.txt file3.txt
[user@host glob]$ echo file{a..c}.txt
filea.txt fileb.txt filec.txt
[user@host glob]$ echo file{a,b}{1,2}.txt
filea1.txt filea2.txt fileb1.txt fileb2.txt
[user@host glob]$ echo file{a{1,2},b,c}.txt
filea1.txt filea2.txt fileb.txt filec.txt

A practical use of brace expansion is to create multiple files or directories.

[user@host glob]$ mkdir ../RHEL{7,8,9}
[user@host glob]$ ls ../RHEL*
RHEL7 RHEL8 RHEL9

The tilde character (~), matches the current user's home directory. If it starts with a string of characters other than a slash (/), then the shell interprets the string up to that slash as a username, if one matches, and replaces the string with the absolute path to that user's home directory. If no username matches, then the shell uses an actual tilde followed by the string of characters.

In the following example, the echo command is used to display the value of the tilde character. You can also use the echo command to display the values of brace and variable expansion characters, and others.

[user@host glob]$ echo ~root
/root
[user@host glob]$ echo ~user
/home/user
[user@host glob]$ echo ~/glob
/home/user/glob
[user@host glob]$ echo ~nonexistinguser
~nonexistinguser

A variable acts like a named container that stores a value in memory. Variables simplify accessing and modifying the stored data either from the command line or within a shell script.

You can assign data as a value to a variable with the following syntax:

[user@host ~]$ VARIABLENAME=value

You can use variable expansion to convert the variable name to its value on the command line. If a string starts with a dollar sign ($), then the shell tries to use the rest of that string as a variable name and to replace it with the variable value.

[user@host ~]$ USERNAME=operator
[user@host ~]$ echo $USERNAME
operator

To prevent mistakes due to other shell expansions, you can put the name of the variable in curly braces, for example ${VARIABLENAME}.

[user@host ~]$ USERNAME=operator
[user@host ~]$ echo ${USERNAME}
operator

Variable names can contain only letters (uppercase and lowercase), numbers, and underscores. Variable names are case-sensitive and cannot start with a number.

Command substitution enables the output of a command to replace the command itself on the command line. Command substitution occurs when you enclose a command in parentheses and precede it by a dollar sign ($). The $(command) form can nest multiple command expansions inside each other.

[user@host glob]$ echo Today is $(date +%A).
Today is Wednesday.
[user@host glob]$ echo The time is $(date +%M) minutes past $(date +%l%p).
The time is 26 minutes past 11AM.