An Introduction to the Command-Line (on Unix-like systems)
by Oliver; 201415. Escape Sequences
Escape sequences are important in every language. When bash reads $a it interprets it as whatever's stored in the variable a. What if we actually want to echo the string $a? To do this, we use \ as an escape character:
$ a=3
$ echo $a
3
$ echo \$a
$a
$ echo "\$a" # use quotes if you like
$a
What if we want to echo the slash, too?
Then we have to escape the escape character (using the escape character!):
$ echo \\\$a # escape the slash and the dollar sign
\$a
This really comes down to parsing.
The slash helps bash figure out if your text is a plain old string or a variable.
It goes without saying that you should avoid special characters in your variable names.
In unix we might occasionally fall into a parsing tar-pit trap.
To avoid this, and make extra sure bash parses our variable right, we can use the syntax ${a} as in:
$ echo ${a}
3
When could this possibly be an issue?
Later, when we discuss scripting, we'll learn that $n, where n is a number, is the nth argument to our script.
If you were crazy enough to write a script with 11 arguments, you'd discover that bash interprets a=$11 as a=$1 (the first argument) concatenated with the string 1 while a=${11} properly represents the eleventh argument.
This is getting in the weeds, but FYI.
Here's a more practical example:
$ a=3 $ echo $a # variable a equals 3 3 $ echo $apple # variable apple is not set $ echo ${a}pple # this describes the variable a plus the string "pple" 3pple
