7.12. The here document and Input

7.12. The here document and Input

The here document accepts inline text for a program expecting input, such as mail , sort , or cat , until a user -defined terminator is reached. It is often used in shell scripts for creating menus . The command receiving the input is appended with a << symbol, followed by a user-defined word or symbol, and a newline. The next lines of text will be the lines of input to be sent to the command. The input is terminated when the user-defined word or symbol is then placed on a line by itself in the leftmost column (it cannot have spaces surrounding it). The word is used in place of Ctrl-D to stop the program from reading input.

Example 7.42.

 1   $  sort   # Sort waits for user input if it does get a file   pears   apples   bananas  ^D (Ctrl-D)  # User presses Ctrl-D to stop input to sort   (Output)   apples   bananas   pears   $  2   $  sort <<DONE   # DONE is a user-defined terminator  >  pears     >  apples  # The user input is called the "document"  >  bananas 3   DONE  # Marks end of here document   # Here is where input stops  (Output) 4  apples   bananas   pears  5   $ 

EXPLANATION

1. The UNIX/Linux sort command waits for user input because it was not provided with a filename as its argument. The user types three lines of input, and then must press Ctrl-D, the key sequence that stops input to the program. (Don't mistake Ctrl-D with Ctrl-C. Ctrl-C kills the program so that you won't get any output at all!) After the user presses Ctrl-D, the sort command will send the sorted output to the screen.

2. A here document will provide input to the sort command. The << is followed by what is called a user-defined terminator, a word or symbol. A secondary prompt appears. The following text is input for the sort command.

3. The DONE terminator marks the end of the here document . The DONE cannot have space on either side of it. The terminator replaces typing Ctrl-D. The sort command will not receive further input. When finished, sort will send its output to the screen.

4. The output from the sort program is displayed.

5. The shell prompt reappears.

If the terminator is preceded by the << “ operator, leading tabs, and only tabs, may precede the final terminator. The user-defined terminating word or symbol must match exactly from "here" to "here." The following examples illustrate the use of the here document at the command line to demonstrate the syntax. It is much more practical to use them in scripts.

Example 7.43.

  5  Hello there ellie   The time is Wed April 22 19:42:16 PST 2004   I can't wait to see you!!!  6   $ 

EXPLANATION

1. The UNIX cat program will accept input until the word FINISH appears on a line by itself.

2. A secondary prompt appears. The following text is input for the cat command. Variable substitution is performed within the here document .

3. Command substitution, `date` , is performed within the here document .

4. The user-defined terminator FINISH marks the end of input for the cat program. It cannot have any spaces before or after it and is on a line by itself.

5. The output from the cat program is displayed.

6. The shell prompt reappears.

Example 7.44.

 1   $  cat << DONE  >    Hello there     >    What's up?     >    Bye now The time is `date`. 2   >  DONE   Hello there   What's up?   Bye now The time is Thu Oct 28 19:48:23 PST 2004.   $  

EXPLANATION

1. The cat program accepts input until DONE appears on a line by itself. The << “ operator allows the input and final terminator to be preceded by one or more tabs.

2. The final matching terminator, DONE , is preceded by a tab. The output of the cat program is displayed on the screen.

7.12.1 Now What?

Now that we have completed our discussion on the interactive shell, it is time to combine many of these features in scripts, to produce programs that will automate repetitive tasks , produce information, monitor processes, and so on. If you are not a programmer, writing shell scripts may be a good place for you to start. It's fun and you get results quickly.