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echo

"..." double quote should be used in general
'...' single quote only when to force everything literal

Escape character
\\, \", \$, \`, \(, \), \[, \], \#, \*, \|, \; (or within ' ')
'&', '<', '>' (cannot use \)
'...'\''...' escaped \' must be outside single quotes

echo ... : non quote - reserve characters must be escaped, '&' must use single quoted
echo "..." : weak quote - translate variables, no escapes required
echo '...' : strong quote - all literal

echo : blank line
echo $var : var value - multi words
echo "$var" : var value - single word, keep new line
echo '$var' : $var literal
echo $var abc : concatenate var value and string
echo 'abc "def"' : double quote echo "$var \"abc\"" : escaped double quote inside double quote
echo ${var}abc or echo -e "$var \babc" : var next to string without space
echo -n "abc" : -n = no new line
echo -e "\nabc\n" : -e = enable translate backslash characters

    \a          alert (bell)  
    \b          backspace
    \c          suppress trailing new line
    \e          escape 
    \f          form feed
    \n          new line
    \r          carriage return
    \t          horizontal tab
    \v          vertical tab
    \\          backslash
    \NNN        character - ASCII code in octal
    \xnnn       character - ASCII code in hexadecimal

echo abc > file : create new file or replace if file exist with 'abc' content
echo abc >> file : create new file or append if file exist with 'abc' content