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Command Substitution - $(unix command)
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-Ian! D. Allen - idallen@idallen.ca

Command Substitution is similar to variable substitution; but, it is
the standard output of the given Unix command that is substituted.
The command line is executed and all the standard output is captured
and becomes the result of the substitution.

- syntax:  $( any Unix command goes between parentheses )

- *all* the standard output of the command(s) between parentheses is captured

- *only* the standard output, not standard error

- Typical use is to assign command output to a shell variable:

  $ users=$( who | wc -l )
  $ echo "Number of users logged in: $users"

  $ words=$( wc -w </etc/passwd )
  $ echo "The passwd file contains $words words"

- You can only capture what you can see; if the command produces no
  standard output, nothing will be captured by command substitution:

  $ foo=$( cp /etc/passwd bar )
  $ echo "$foo"
  $
  - empty, because "cp" has no standard output to capture

- You can wrap any Unix command line with $(...) and capture stdout:

  $ grep bash /etc/passwd | awk -F: '{print $1}'
  root
  postgres
  generic
  idallen

  $ list=$( grep bash /etc/passwd | awk -F: '{print $1}' )

  $ echo "The list of bash users is $list"
  The list of bash users is root
  postgres
  generic
  idallen

  Note that if the command produces separate lines of output, the captured
  stdout saved in the variable will contain separate lines of output too.

- You can wrap *any* set of one or more commands with $(...).
  (But don't wrap commands that need a keyboard, e.g. vim!)

- Only standard output is captured; standard error is not affected.

- You don't have to put the output into a variable, you can use it directly:

  $ var=$( hostname )
  $ echo "The name of this computer is $var."
  The name of this computer is idallen-home.

  $ echo "The name of this computer is $( hostname )."
  The name of this computer is idallen-home.

  $ echo "Hello.  $( pwd ) is the current directory."
  Hello.  /home/idallen is the current directory.

Remember to double-quote your variables (including command substitutions)!