Command Substitution

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.

Tilde Substitution

Tilde substitution is used to substitute the pathname of a user 's home directory for ~ user . Words in the command line that start with the tilde character cause the Korn shell to check the rest of the word up to a slash. If the tilde character is found alone or is only followed by a slash, it is replaced with the value of the HOME variable. This is a handy shortcut borrowed from the C shell. For example, to print the pathname of your home directory:

$ echo ~


/home/anatole

or to list its contents:

$ ls ~/


NEWS         bin         pc


asp          mail        src

If the tilde character is followed by a login name file, it is replaced with the home directory of that user. Here we change directory to the tools directory in smith 's home directory:

$ cd ~smith/tools


$ pwd


/home/users/admin/smith/tools

If the tilde character is followed by a + or ?/span>, it is replaced with the value of PWD (current directory) and OLDPWD (previous directory), respectively. This is not very useful for directory navigation, since cd ~+ leaves you in the current directory. The cd ~?/span> command puts you in the previous directory, but the Korn shell provides an even shorter shortcut: cd ?/span> does the same thing, except that it also displays the current directory.

$ pwd


/home/users/admin/smith/tools


$ cd /usr/bin


$ cd ~-


$ pwd


/home/users/admin/smith/tools


$ cd -


/usr/bin

Table 2.7. Tilde Substitution