Command Substitution

Command substitution is a feature that allows output to be expanded from a command. It can be used to assign the output of a command to a variable, or to imbed the output of a command within another command. The format for command substitution is:

					$(command)
				

where command is executed and the output is substituted for the entire $(command) construct. For example, to print the current date in a friendly format:

					$ echo The date is $(date)
					The date is Fri Jul 27 10:41:21 PST 1996
				

or see who is logged on:

					$ echo $(who —q) are logged on now
					root anatole are logged on now
				

Any commands can be used inside $(...), including pipes, I/O operators, metacharacters (wildcards), and more. We can find out how many users are logged on by using the who and wc –l commands:

					$ echo $(who | wc —l) users are logged on
					There are 3 users logged on
				

Bourne Shell Compatibility

For compatibility with the Bourne shell, the following format for command substitution can also be used:

						`command`
					

Using `. . .` command substitution, we could get the names of the files in the current directory like this:

						$ echo `ls` are in this directory
						NEWS asp bin pc are this directory
					

If you wanted a count of the files, a pipe to wc could be added:

						$ echo There are `ls | wc —l` files here
						There are 4 files here
					

Directing File Input

There is also a special form of the $(...) command that is used to substitute the contents of a file. The format for file input substitution is:

						$(<file)
					

This is equivalent to $(cat file) or `cat file`, except that it is faster, because an extra process does not have to be created to execute the cat command. A good use for this is assigning file contents to variables:

						$ cat foo
						a b c
						$ X=$(<foo)
						$ echo $X
						a b c
					

We will talk about this later in Chapter 3.

Arithmetic Operations

Another form of the $(...) command is used to substitute the output of arithmetic expressions. The value of an arithmetic expression is returned when enclosed in double parentheses and preceded with a dollar sign.

						$((arithmetic-expression))
					

Here are a few examples.

						$ echo $((3+5))
						8
						$ echo $((8192*16384%23))
						9
					

Performing arithmetic is discussed in detail in Chapter 6.