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Difference between echo `ls -l` and echo "`ls -l`" ?


 
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# 1  
Old 08-05-2014
Difference between echo `ls -l` and echo "`ls -l`" ?

Hi guys,
Been messing around with shell programming for a couple of days and I found something that was pretty odd in the behavior of the echo command. Below is an example-:

When I type the following in my /home directory from my lxterminal in Debian-:

echo "`ls -l`"

I get the following output-:
Code:
sreyan@debian:~$ echo "`ls -l`"
total 2296148
drwx------ 4 sreyan sreyan       4096 Jul 31 15:21 Desktop
-rw------- 1 sreyan sreyan     102097 Dec 26  2009 Euphoria.jpg
drwx------ 3 sreyan sreyan       4096 Feb 10 21:28 GNUstep
-rw------- 1 sreyan sreyan 1239269270 Aug 21  2009 linux.x64_11gR2_database_1of2.zip
-rw------- 1 sreyan sreyan 1111416131 Aug 21  2009 linux.x64_11gR2_database_2of2.zip
-rw-r--r-- 1 sreyan sreyan     447144 Jul 24 18:34 RedshiftGUI-0.2.1-Linux-x86_64.deb

Now this is what I expected. I know whatever is put in ` ` is suppose to interpreted as a command in the BASH echo. So, therefore this is the expected output of ls -l on my /home directory.

But it gets weird when I remove the " " (double quotes) from the echo statement.

When I run the following-:

echo `ls -l`

I get as output the following-:
Code:
total 2296148 drwx------ 4 sreyan sreyan 4096 Jul 31 15:21 Desktop -rw------- 1 sreyan sreyan 102097 Dec 26 2009 Euphoria.jpg drwx------ 3 sreyan sreyan 4096 Feb 10 21:28 GNUstep -rw------- 1 sreyan sreyan 1239269270 Aug 21 2009 linux.x64_11gR2_database_1of2.zip -rw------- 1 sreyan sreyan 1111416131 Aug 21 2009 linux.x64_11gR2_database_2of2.zip -rw-r--r-- 1 sreyan sreyan 447144 Jul 24 18:34 RedshiftGUI-0.2.1-Linux-x86_64.deb

If you look closely you will see that it is exactly identical to the output that is given above except for the fact that it contains no line breaks. My question is why ?

  1. Why are the line-breaks removed when I remove the double quotes from echo ? Whatever is there in ` `(tild signs) is supposed to be interpreted as a command right ?
  2. If removal of double quotes " " means that there is a change in the meaning of the echo, then please tell me what is the special meaning that double quotes has in echo ?
  3. Is there any difference between echo "$SHELL" and echo $SHELL ? I know the output is same but since one has double quotes and one doesn't is there any difference in the way that BASH treats them internally ?
I am using Debian Wheezy LXDE with Bash 4.2.
Thanks.
# 2  
Old 08-05-2014
Well, for starters -- it's a pretty silly thing to do, either way. Why do echo "`command`" when you could just do command ?

But the difference is, quotes prevent a string from splitting on whitespace. Spaces, tabs and newlines all count as whitespace. If you do VAR="a b c", and use $VAR without putting it into quotes, the shell will split it in three.

So, quoted, it ends up being:

Code:
echo "one
incredibly
long
string
crammed
into
a
single
argument
containing
actual,
real
line
breaks
printed
literally"

And the unquoted way ends up being:

Code:
echo "one" "incredibly" "long" "string" "where" "each" "whitespace" "separated" "section" "becomes" "its" "own" "individual" "separate" "argument"

This is also one of many reasons why for LINE in `command` is a poor idea. It splits on whitespace, not lines.
This User Gave Thanks to Corona688 For This Post:
# 3  
Old 08-06-2014
Thanks so much.
 

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