15.5. Backslash

The backslash stops the shell from misinterpreting the next character if it has special meaning to the shell. The following characters may have special meaning: & * = ^ $ ` " | ?.

Issuing the echo command with an * causes a listing of the whole current directory in a squashed up format, instead of echoing just the asterisk.

						$ echo * 

conf.linuxconf conf.modules cron.daily cron.hourly cron.monthly 
cron.weekly crontab csh.cshrc default dosemu.conf dosemu.users exports 
fdprm fstab gettydefs gpm-root.c 
onf group group- host.conf hosts hosts.allow hosts.deny httpd inetd 
...
					

To disable the special meaning of the asterisk, use a backslash.

						$ echo * 
*
					

The same thing goes for the $$ command which the shell interprets as your current PID (process ID number). To disable it and just echo the $$ put a backslash in front.

						$ echo $$ 
284
$ echo $$ 
$$
					

To include octal characters when echoing, you must precede them with the backslash, otherwise the shell will just think they are ordinary numbers.

						$ echo " This is a copyright 251 sign" 
 This is a copyright 251 sign 
$ echo " This is a copyright 251 sign" 
 This is a copyright © sign
					

							$ echo -e "This is a copyright 251 sign" 
 This is a copyright © sign
						

When using the command expr, if you try to multiply using the * it will return an error. Put a backslash before the * sign, and this will work.

						$ expr 12 * 12 
expr: syntax error 

$expr12* 12
144

To include any of the metacharacters within an echo statement, you must escape them with a backslash. In the following example, the price $19.99 is to be displayed, but because we have not escaped it the shell, the shell will treat it differently.

						$ echo "That video looks a good price for $19.99" 
That video looks a good price for 9.99
					

Now if we escape the dollar sign we get a better result.

						$ echo "That video looks a good price for $19.99" 
That video looks a good price for $19.99