====== BASH Quotes examples ======

BASH uses single quotes, double quotes and backticks. They all have their uses and using the wrong ones at the wrong time will cause unpredictable results. 

This page attempts to clarify their usage.

Note, just to confuse new users, BASH uses $ as a normal user prompt as well as $ as a variable identifier.

user@ubuntu20:~$  <- User Prompt \\

root@ubuntu20:~#  <- root user prompt \\

$VARIABLE  <- Variable

There is no connection between their uses.

===== Good and Bad quotes =====

0-bad-quotes.sh
<code bash>
#!/bin/bash

goodquote="Anyone who has never made a mistake has never tried anything new. Albert Einstein."
echo "Quote 1. ${goodquote}"

echo ""

badquote=“To be is to do - Socrates, To do is to be - Sartre, Do Be Do Be Do - Sinatra”
echo "Quote 2. ${badquote}"
</code>

<code>
$ ./0-bad_quotes.sh 
Quote 1. Anyone who has never made a mistake has never tried anything new. Albert Einstein.

./0-bad_quotes.sh: line 8: be: command not found
Quote 2. 
$ 
</code>

goodquote is fine, it uses normal double quotes. 

badquote is bad because it uses [[https://www.businesslegions.com/blog/2014/01/17/simple-explanation-quotation-marks-66-99/|"66's and 99's"]] which are not valid quotes. Usual source of these is MS word and others, so copy and paste carefully. Better still just don't use windows.


===== Set Variables =====


1-set_variable.sh
<code bash>
#!/bin/bash

VAR1='abc 123'

VAR2="abc 123"

echo $VAR1 $VAR2

</code>

<code>
$ ./1-set_variable.sh 
abc 123 abc 123
$
</code>

Although not important here, single quotes produces a literal string whilst double quotes allows BASH to expand any variables referenced in them.


===== Print variables =====

2-print_variable.sh
<code bash>
#!/bin/bash

VAR1='abc 123 ABC 321'
VAR2="abc 123 ABC 321"

echo '$VAR1 is ' $VAR1

echo "$VAR2 is " $VAR2
</code>

<code>
$ ./2-print_variable.sh 
$VAR1 is  abc 123 ABC 321
abc 123 ABC 321 is  abc 123 ABC 321
$
</code>

VAR1 is printed with echo using single quotes ('$VAR1 is '), so it prints the literal string, $VAR1, and outside the quotes, VAR1 is expanded to the abc string.

VAR2 is printed inside double quotes, so BASH expands it to the abc string and then expands it as well outside the quotes, giving a probably unintended statement of the obvious.

Although not visible here, BASH treats VAR1 and VAR2 differently as they are declared with either single or double quotes.

===== Newlines with echo and printf =====

newlines_printf.sh

<code bash>
#!/bin/bash

VAR='abc 123 ABC 321'

LINE1="Line one."
LINE2="Line two."

echo "${LINE1}"
echo "${LINE2}"

echo ''

echo -n "${LINE1}"
echo "${LINE2}"

echo ''

printf "printf variables:- %s\n\n\n" "${VAR}"
</code>

<code>
$ ./3a-newlines_printf.sh 
Line one.
Line two.

Line one.Line two.

printf variables:- abc 123 ABC 321
$
</code>

To use ''echo'' to print multiple statements on one line, use a ''-n'' to supress newlines as ''echo'' will add them unless told not to.

On the other hand, ''printf'' needs a ''\n'' to print a new line, the default is not to include one. ''\t'' can be used to print a tab character.
Additionally, ''printf'' can use a ''%s'' (string) substitution so it then takes the second argument and uses that to replace the ''%s'' .


===== Strings or Lists? =====


3-string_or_list.sh
<code bash>
#!/bin/bash
VAR='abc 123 ABC 321'
echo '$VAR is ' "${VAR}"

echo 'Single quotes with spaces in the string causes BASH to treat it as a list.'
for WORD in ${VAR}
do
  echo '"${WORD}" is:- ' "${WORD}"
done

echo ''
echo 'Double quotes forces variable to include spaces.'
echo ''

for WORD in "${VAR}"
do
  echo '"${WORD}" is:- ' "${WORD}"
done
$
</code>

<code>
$ ./3-string_or_list.sh 
$VAR is  abc 123 ABC 321
Single quotes with spaces in the string causes BASH to treat it as a list.
"${WORD}" is:-  abc
"${WORD}" is:-  123
"${WORD}" is:-  ABC
"${WORD}" is:-  321

Double quotes forces variable to include spaces

"${WORD}" is:-  abc 123 ABC 321
</code>

In this example, the two identical loops have a different way to start the FOR loop, double quoting the FOR part ( for WORD in "${VAR}" ) causes BASH to treat the string as one string rather than using a space as an element separator in the forst example. 

This is what you usually want, so remember to use the format "${VARIABLE}".


===== Commands and paths =====


4-commands_and_paths.sh
<code bash>
#!/bin/bash

PWD=$(which pwd)

echo "pwd command lives in:- ${PWD}"


CMD=$(basename "${PWD}")
PATH=$(dirname "${PWD}")

echo "path $PATH  and command  $CMD"


echo 'How to remake absolute path and file'

echo "Is it:- $PATH/$CMD"

echo ''

echo "Or:- ${PATH}/${CMD}"

echo ''
echo ''

ENVIR='prod'

BUCKET1="env$ENVIRbucket"
echo 'No brace BUCKET1:- "env$ENVIRbucket" -> ' ${BUCKET1}

BUCKET2="env${ENVIR}bucket"
echo 'BUCKET with braces:- "env${ENVIR}bucket" -> ' ${BUCKET2}

echo ''
echo 'Quotes inside quotes'
echo 'BUCKET with braces and quotes:- \"${BUCKET2}\" -> ' ">\"${BUCKET2}\"<"
</code>

<code>
$ ./4-commands_and_paths.sh 
pwd command lives in:- /usr/bin/pwd
path /usr/bin  and command  pwd
How to remake absolute path and file
Is it:- /usr/bin/pwd

Or:- /usr/bin/pwd


No brace BUCKET1:- "env$ENVIRbucket" ->  env
BUCKET with braces:- "env${ENVIR}bucket" ->  envprodbucket

Quotes inside quotes
BUCKET with braces and quotes:- \"${BUCKET2}\" ->  >"envprodbucket"<
</code>


===== Braces in variables =====

<code bash>
#!/bin/bash

PWD=$(which pwd)

echo "pwd command lives in:- ${PWD}"


CMD=$(basename "${PWD}")
PATH=$(dirname "${PWD}")

echo "$PATH - $CMD"


echo "Let's remake absolute path and file"

echo "Is it:- $PATH/$CMD"

echo ""

echo "Or:- ${PATH}/${CMD}"

echo ""
echo ""
echo ""

ENVIR='prod'

BUCKET1="env$ENVIRbucket"
echo 'No brace BUCKET1:- "env$ENVIRbucket" -> ' ${BUCKET1}

BUCKET2="env${ENVIR}bucket"
echo 'BUCKET with braces:- "env${ENVIR}bucket" -> ' ${BUCKET2}

echo ''
echo 'Quotes inside quotes'
echo 'BUCKET with braces and quotes:- \"${BUCKET2}\" -> ' "\"${BUCKET2}\""
</code>

<code>
$ ./5-braces.sh 
pwd command lives in:- /usr/bin/pwd
/usr/bin - pwd
Let's remake absolute path and file
Is it:- /usr/bin/pwd

Or:- /usr/bin/pwd

No brace BUCKET1:- "env$ENVIRbucket" ->  env
BUCKET with braces:- "env${ENVIR}bucket" ->  envprodbucket

Quotes inside quotes
BUCKET with braces and quotes:- \"${BUCKET2}\" ->  "envprodbucket"
</code>



===== Weird Stuff =====

6-weird-stuff.sh 
<code bash>
#!/bin/bash

WEIRD1='\\\\'

echo "No expansion setting variable:- ${WEIRD1}"

echo ''

WEIRD2="\\\\"

echo "Variable expanded when set:- ${WEIRD2}"

echo ''

WEIRD3="\\\\\" "
echo "W3"
echo "${WEIRD3}"

echo ''

echo "This works:- Why can't I write 's between single quotes?"
echo "But this works:- Why can\"t I write \"s between single quotes?"
echo ''

echo ''
echo "This works (clue):-"
echo 'Why can\t I write \s between single quotes with backslash escape?'

echo ''

echo 'This fails:-'
echo "'Why can\'t I write \'s between single quotes with backslash escape?'"
echo 'Why can\'t I write \'s between single quotes with backslash escape?'
</code>

<code>
$ ./6-weird-stuff.sh 
No expansion setting variable:- \\\\

Variable expanded when set:- \\

W3
\\" 

This works:- Why can't I write 's between single quotes?
But this works:- Why can"t I write "s between single quotes?


This works (clue):-
Why can\t I write \s between single quotes with backslash escape?

This fails:-
'Why can\'t I write \'s between single quotes with backslash escape?'
./6-weird-stuff.sh: line 33: unexpected EOF while looking for matching `''
$
</code>

===== Backticks and round brackets =====

7-backticks.sh
<code bash>
#!/bin/bash
PWD=`pwd`

echo "Current backtick directory is:- ${PWD}."

DIR=$(pwd)
echo "Dollar bracket dir is:- ${DIR}".
</code>

<code>
$ ./7-backticks.sh 
Current backtick directory is:- /home/cloudshell-user/quotes.
Dollar bracket dir is:- /home/cloudshell-user/quotes.
$
</code>