Expansions in Linux

When we run a command on Linux, there are quite a few substitutions that are done by bash before execution. The process that makes this happen is called expansion.

Pathname Expansion

The mechanism by which wildcards work is called pathname expansion. For examples on how to use, check out this post

Tilde Expansion

Tilde (~) expansion, by default, points to the home directory of the current user.

Arithmetic Expansion

Arithmetic expansion supports basic arithmetic such as addition (+), subtraction (-), multiplication (*), division (/), modulo (%) and exponentiation (**). The syntax of arithmetic expansion looks like this:

$((expression))

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

Brace Expansion

This is mostly useful to make a list of files or directories. It works like this:

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

echo Number_{1..5}
Number_1 Number_2 Number_3 Number_4 Number_5

Parameter Expansion

Variables stored by the system are expanded by bash to get their values. For example, USER is a variable containing our username. We can expand it like this:

echo $USER
saikat

Command Substitution

This allows us to use the output of a command as an expansion. For example, we could

ls - $(which cp)
-rwxr-xr-x 1 root root 146880 Feb 28  2019 /bin/cp

Escaping expansion

In case we wish to escape bash expansion, we can put the string section that is likely to be expanded inside double (") quotes. For example:

Without quotes

echo $(cal)
March 2020 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

With quotes

echo "$(cal)"
     March 2020
Su Mo Tu We Th Fr Sa
 1  2  3  4  5  6  7
 8  9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30 31