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man bash
Command Substitution
Command substitution allows the output of a command to replace the com-
mand name. There are two forms:


$(command)
or
`command`

Bash performs the expansion by executing command and replacing the com-
mand substitution with the standard output of the command, with any
trailing newlines deleted. Embedded newlines are not deleted, but they
may be removed during word splitting. The command substitution $(cat
file) can be replaced by the equivalent but faster $(< file).

When the old-style backquote form of substitution is used, backslash
retains its literal meaning except when followed by $, `, or \. The
first backquote not preceded by a backslash terminates the command sub-
stitution. When using the $(command) form, all characters between the
parentheses make up the command; none are treated specially.

Command substitutions may be nested. To nest when using the backquoted
form, escape the inner backquotes with backslashes.

If the substitution appears within double quotes, word splitting and
pathname expansion are not performed on the results.