$(Unix command)

The command inside the parenthesis is run, and anything the command writes to standard output (and to standard error) is returned as the value of the expression. These constructs can be nested, i.e., the Unix command can contain command substitutions.

Here are some simple examples:

PS1="$(hostname) $ "

Task 4-4 The file used in Task 4-1 is actually a report derived from a bigger table of data about albums. This table consists of several columns, or fields, to which a user refers by names like "artist," "title," "year," etc. The columns are separated by vertical bars (|, the same as the Unix pipe character). To deal with individual columns in the table, field names need to be converted to field numbers. Suppose there is a shell function called getfield that takes the field name as argument and writes the corresponding field number on the standard output. Use this routine to help extract a column from the data table.

Assume that the data table in our task is a file called albums and that it looks like this:

Coltrane, John|Giant Steps|Atlantic|1960|Ja
Coltrane, John|Coltrane Jazz|Atlantic|1960|Ja
Coltrane, John|My Favorite Things|Atlantic|1961|Ja
Coltrane, John|Coltrane Plays the Blues|Atlantic|1961|Ja
...

Here is how we would use cut to extract the fourth (year) column:

cut -f4 -d\| albums

From this line of code and the getfield routine, we can easily derive the solution to the task. Assume that the first argument to getfield is the name of the field the user wants to extract. Then the solution is:

fieldname=$1
cut -f$(getfield $fieldname) -d\| albums

If we ran this script with the argument year, the output would be:

1960
1960
1961
1961
...

Task 4-5 Assume that you are logged into a large server or mainframe that supports many simultaneous users. Send a mail message to everyone who is currently logged in.

billr      console      May 22 07:57
fred       tty02        May 22 08:31
bob        tty04        May 22 08:12

who | cut -d' ' -f1

With the above who output, this command's output would look like this:

billr
fred
bob

This leads directly to a solution to the task. Just type:

mail $(who | cut -d' '  -f1)

The command mail billr fred bob will run and then you can type your message.

Task 4-6 The ls command gives you pattern-matching capability with wildcards, but it doesn't allow you to select files by modification date. Devise a mechanism that lets you do this.

This task was inspired by the feature of the OpenVMS operating system that lets you specify files by date with BEFORE and SINCE parameters.

Here is a function that allows you to list all files that were last modified on the date you give as argument. Once again, we choose a function for speed reasons. No pun is intended by the function's name:

function lsd {
    date=$1
    ls -l | grep -i "^.\{41\}$date" | cut -c55-
}

With command substitution, you can use this function with any command that accepts filename arguments. For example, if you want to print all files in your current directory that were last modified today, and today is January 15, you could type:

lp $(lsd 'jan 15')