What is the shell mode started with the apostrophe?

Question

In a (SSH) terminal session, what is the purpose of the mode started by the key sequence apostrophe <ENTER> or more literally '<ENTER>.

What is the way to exit that mode?

Answer 1

Typing 'enter doesn't start any special SSH mode. The (normal) commands you type are interpreted by the shell on the other end of your SSH connection, not SSH itself.

' is usually called a single quote in shell terminology, " a double-quote, and ` is a backtick.

' starts a character sequence that is not interpreted by the shell. This is useful, for example, if you need to pass and argument to a program the must contain $ or another character sequence the would otherwise be interpreted by the shell, or that contains newlines. You end that sequence with another '.

" is similar, but some interpolation happens (i.e. variables are expanded).

` is different. That's for command substitution. If you type

echo `foo`

the output of running the foo program is substituted before running the echo command. Another way of doing that is by using $(...):

echo $(foo)

So if you type 'enter, you're just starting a multiline literal string. You end that either with a matching ', or with CtrlC if you want to break out.

$ echo 'hello
> this
> is 
> a
> multiline
> string
> '
hello
this
is
a
multiline
string
$ echo 'foo $PWD'
foo $PWD               # no interpolation
$ echo "foo $PWD"
foo /tmp               # interpolation

Starting a line with a ' isn't really useful by itself.