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 )