1. Shell expansions

1. Wildcard expansions:

   echo this is a test

   echo displays all the arguments that are passed to it.

   cd /usr

   ls

   echo *

   The "*" character means match any characters in a filename. The shell expands the "*" before executing the command echo.

   echo lib*

   echo *bin

   echo lib*32

   echo */share

   echo /*/*/bin

   A question mark "?" matches any single character:

   echo lib??

   echo lib???

   Character sets are enclosed in square brackets:

   echo lib[123456789]?

   echo lib[xyz][123456789]?

   Character ranges:

   echo lib[1-9][1-9]

   echo lib[a-z][1-9][1-9]

   echo lib[1-9][!2]

   Character classes:

   echo lib[[:digit:]][[:digit:]]

   echo lib[[:alpha:]][[:digit:]][[:digit:]]

   echo lib[[:alnum:]][[:alnum:]][[:alnum:]]

   echo lib[![:digit:]]*

   echo lib[36[:lower:]]*

   echo /etc/[[:upper:]]*

2. The tilde character ("~") expands to the home directory:

   echo ~

   echo ~root

3. Arithmetic expansion:

   echo $((2 + 2))

   echo $(($((5**2)) * 3))

   echo $(((5**2) * 3))

   echo Five divided by two equals $((5/2))

   echo with $((5%2)) left over

4. Brace expansion:

   echo Front-{A,B,C}-Back

   echo Number_{1..5}

   echo {01..15}

   echo {001..15}

   echo {Z..A}

   echo a{A{1,2},B{3,4}}b

   cd

   mkdir Photos

   cd Photos

   mkdir {2017..2019}-{01..12}

   ls

5. Variable expansion:

   echo $USER

   printenv | less

   echo $SUER

   When the variable does not exist, it is expanded to the empty string.

6. Command substitution:

   echo $(ls)

   ls -l $(which cp)

   echo "$(cal)"

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