Command Substitution | Korn Shell. Unix and Linux Programming Manual, Third Edition

	Korn Shell: Unix and Linux Programming Manual, Third Edition, The By Anatole Olczak
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	Chapter 2. Korn Shell Basics

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.

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