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Unix/Linux Shell Command Line Quoting
=====================================
-IAN!  idallen@freenet.carleton.ca

Ref: Chapter  2, p.37-38
     Chapter 10, very bottom of p.320; most of p.321

     http://www.washington.edu/computing/training/125/quote.html
     http://unix.about.com/library/glossary/bldef-quoting.htm
     http://www.grymoire.com/Unix/Quote.html

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Which programs treat quotes specially?
======================================

Quotes, both double and single, are treated as special characters when fed
as input to the Unix shells, usually by typing them on shell command lines
or putting them inside shell scripts.  Quotes are not special characters
when fed as input to most other programs such as "cat" or "head":

    $ echo "It's a nice day"
    It's a nice day.                <= note how shell processes quoted string

    $ cat
    "It's a nice day"               <= typed input
    "It's a nice day"               <= unchanged program output on stdout
    ^D

    $ head -1
    "It's a nice day"               <= typed input
    "It's a nice day"               <= unchanged program output on stdout
    $

The shell treats quotes specially when they are used on a command line
or in a shell script.

Most other programs do not treat quotes specially.

===============
Why use quotes?
===============

The shells use quotes to protect other special characters from expansion
by the shell.  Backslashes also protect special characters from the
shell; but, sometimes they are far less convenient to use.  For example,
one could protect all the special characters in a line of text with
backslashes:

    $ echo This\ line\ is\ a\ single\ \"argument\"\ to\ the\ \"echo\"\ command.
    This line is a single "argument" to the "echo" command.

Alternately, one could simply use single quotes to do the same thing:

    $ echo 'This line is a single "argument" to the "echo" command.'
    This line is a single "argument" to the "echo" command.

Most agree that the use of quotes is easier than using a lot of backslashes.

The quotes are used by the shells to locate a string of characters to
protect.  The quotes themselves are not part of the string; the quotes
are removed once the string of characters is collected.  This next echo
command line has six string arguments:

    $ echo "a  b"     c     'd  e'    f     "g  h"     "i  j"
    a  b c d  e f g  h i  j
    ^^^^ ^ ^^^^ ^ ^^^^ ^^^^ 
       1 2    3 4    5    6

Blanks are special characters to the shell, unless they are hidden from
the shell by quoting.  The shell splits the command line into arguments
on any un-quoted blanks.  Blanks that are quoted are not seen by the
shell; they become part of the argument string passed to the command
being invoked.  Unquoted blanks, special to the shell, simply separate
arguments.  You may type one blank to separate the arguments or a hundred
blanks; it makes no difference to the shell or to the arguments:

    $ echo a b c
    a b c

    $ echo a                        b                         c
    a b c

The three string arguments passed to the echo command in the above two
cases are identical.  There are three arguments, separated by blanks.

================
Quote Processing
================

Quotes are processed by the Unix shells from left-to-right.  Let's
examine how this works:

    $ echo 'one"two"three'four"five"six"seven'eight'nine"ten
    one"two"threefourfivesixseven'eight'nineten

The first quote found by the shell, starting from the left, is a single
quote.  The shell collects all the characters up to the next single quote.
All these characters are protected from further analysis by the shell.
The words one, two, and three are all inside this first single-quoted
string.  The double quotes, inside single quotes, have no special meaning
to the shell.  They are simply part of the string.  The single quotes
themselves, used to delimit the string, are not part of the string.

The word four is outside of any quotes.  Right after four we start a
double-quoted string that extends until the next double-quote character.
The word five is inside this double-quoted string.  The double quotes
themselves, used to delimit the string, are not part of the string.

The word six is outside of any quotes.  Right after six we start a
double-quoted string that extends until the next double-quote character.
The words seven, eight, and nine are all inside this first double-quoted
string.  The single quotes, inside double quotes, have no special meaning
to the shell.  They are simply part of the string.  The double quotes
themselves, used to delimit the string, are not part of the string.

The word ten is outside of any quotes.

The given command line argument to "echo" was built up of text that
contains three quoted strings.  Each quoted string connected immediately
to another non-blank character, and all the blanks are inside quoted
strings were protected from the shell.  Thus, the "echo" command is
handed only *one* command line argument.  (Command line arguments are
separated by blanks.  There were no unquoted blanks used.)

The fact that this one argument was made up of several pieces on the
command line, including both quoted and unquoted parts, is invisible
to the echo command.  The shell did the work; the echo command gets
its one argument.

The quotes seen by the shell are *not* part of the argument.  The
quotes delimit the string; they are not part of the string.

========================
Single vs. Double Quotes
========================

Single quotes are "stronger" than double quotes.  Nothing is special
inside single quotes; the shell treats all the characters, no matter
what they are, as part of the string being collected and it does not
expand any of them inside single quotes.

Inside double quotes, the shell still sees and expands some special
characters.  The most important character expanded by the shell even
inside double quotes is the dollar sign that signals the start of a
shell variable:

    $ echo '$SHELL'
    $SHELL                        <= single quotes prevent expansion

    $ echo "$SHELL"
    /bin/bash                     <= double quotes permit expansion

We will talk more about shell variables in a future lecture.