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Old 01-08-2009, 07:32 AM   #1
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Shell Programming : $1 $2 ..

Hi all,

I just started off learning shell scripting basics and am an absolute newbie..

These are the commands i typed on the bash shell..

$set I am the king
$echo $1 $2 $3 $4
I am the king // output of the above command
$echo $1 $2 $3 $4 who do u think u r!!
echo $1 $2 $3 $4 who do u think u recho $1 $2 $3 $4 //output
I am the king who do u think u recho I am the king //output

While i expected it to print "I am the king who do u think u r!!" It printed something which i found gibberish.

Why is the output so?

i even tried using a '?' but it prints just fine and also it works as expected with a '!'.

Please clarify my doubt..

Old 01-08-2009, 07:46 AM   #2
Registered: Sep 2008
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There's two reasons for this. The main one is that !! refers to the last command. Apparently the shell first shows what it is, then executes it in context:
prompt: echo this
prompt: !!
echo this
The second is because you can do this:
prompt: echo echo
So you included the previous command as a string literal in an echo statement.

If you want to actually see !! use single quotes:
prompt: echo the emperor has new clothes'!!'
the emperor has new clothes!!
There's more about this in the bash man page under Event Designators. Now try ?!echo

Last edited by mk27; 01-08-2009 at 07:48 AM.
Old 01-08-2009, 07:48 AM   #3
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The '!' character is used to find the last command you used which matches a string, or to execute the last command in the case of '!!'

To print literal '!' characters, you'll have to either escape them:
$ echo hello\!\!
or quote the string you want to echo:
$ echo 'hello!!'

To be honest I can't quite figure out exactly what's happening with the '!!' at the end of a line - seems to be doing odd things with the shell history, but I'm not sure what.

Old 01-08-2009, 07:52 AM   #4
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It happens because the double exclamation mark is expanded by the shell as "the last executed command" (see the BASH history explanation in your guide). When you use !! the shell first prints out the command to execute, then the output of the command itself. For example:
$ whoami
$ !!
In your case you get the reprint of the command line with !! expanded as the last executed command (in blue below), then its output (the output of the entire echo command - in red below).
$ set I am the king
$ echo $1 $2 $3 $4
I am the king
$ echo $1 $2 $3 $4 who do u think u r!!
echo $1 $2 $3 $4 who do u think u recho $1 $2 $3 $4
I am the king who do u think u recho I am the king
To prevent the shell to interpret !! as a history command, you have to use single quotes or escape the exclamation marks using a backslash. That is both the following commands should work as you expected:
$ echo $1 $2 $3 $4 who do u think u r'!!'
$ echo $1 $2 $3 $4 who do u think u r\!\!
Old 01-09-2009, 12:23 PM   #5
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I think This Book could also be helpful in further explaining the above question and your bash scripting.


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