Quoting literal text

All the examples in this paragraph print the line

!"#$&'()*;<=>?  @[\]^`{|}~

A backslash quotes the next character, i.e. the next character is interpreted literally. The one exception is a newline: backslash-newline expands to the empty string.

echo \!\"\#\$\&\'\(\)\*\;\<\=\>\?\ \ \@\[\\\]\^\`\{\|\}\~

All text between single quotes (forward quotes \', also known as apostrophe) is printed literally. Even backslash stands for itself, and it’s impossible to include a single quote; instead, you can stop the literal string, include a literal single quote with a backslash, and start the literal string again. Thus the 4-character sequence '\'' effectively allow to include a single quote in a literal string.

echo '!"#$&'\''()*;<=>?  @[\]^`{|}~'
#          ^^^^

Dollar-single-quote starts a string literal $'…' like many other programming languages, where backslash quotes the next character.

echo $'!"#$&\'()*;<=>?  @[\\]^`{|}~'
#           ^^            ^^

Double quotes " delimit semi-literal strings where only the characters " \\ $ and ``` retain their special meaning. These characters need a backslash before them (note that if backslash is followed by some other character, the backslash remains). Double quotes are mostly useful when including a variable or a command substitution.

echo "!\"#\$&'()*;<=>?  @[\\]^\`{|}~"
#      ^^                 ^^  ^^
echo "!\"#\$&'()*;<=>?  @[\]^\`{|}~"
#      ^^                 ^  ^^      \[ prints \[

Interactively, beware that \! triggers history expansion inside double quotes: "!oops" looks for an older command containing oops; "\!oops" doesn’t do history expansion but keeps the backslash. This does not happen in scripts.