9.13 Multiline Commands, Secondary Prompts

Both the Bourne shell and the C shell support multiline commands. In the Bourne shell, a newline following an open quote (' or " ), a pipe symbol (| ), or a backslash (\ ) will not cause the command to be executed. Instead, you'll get a secondary prompt (from the PS2 environment variable, set to > by default) and you can continue the command on the next line. For example, to send a quick write (1.33 ) message without making the other user wait for you to type the message:

$ echo "We're leaving in 10 minutes. See you downstairs." |


> write joanne

In the C shell, you can continue a line by typing a backslash (\ ) before the newline (8.15 ) . You won't get the secondary prompt.

Obviously, this is a convenience if you're typing a long command line. It is a minor feature and one easily overlooked; however, it makes it much easier to use a program like sed (34.24 ) from the command line. For example, if you know you chronically make the typos "mvoe" (for "move") and "thier" (for "their"), you might be inspired to type the following command:

nroff
 lp
 

$ sed '


> s/mvoe/move/g


> s/thier/their/g' myfile | nroff -ms | lp

More importantly, the ability to issue multiline commands lets you use the shell's programming features interactively from the command line. In both the Bourne and the C shell, multiline programming constructs automatically generate a secondary prompt (> in the Bourne shell, ? in the C shell) until the construct is completed.

For example, here's a place to use my favorite programming construct for non-programmers, the for loop (9.12 ) :

$ for x in file1 file2 file3


> do


> sed 's/thier/their/g' $x > ,$x


> mv ,$x $x


> done


$

Or in the C shell with foreach (9.11 ) :

% foreach x (file1 file2 file3)


? sed 's/thier/their/g' $x > ,$x


? mv ,$x $x


? end


%

While a simple command like this could be saved into a shell script (1.5 ) , it is often even easier to use it interactively.

Users of sed should of course makesure their script works correctly before overwritingtheir original file . (34.3 )