Where does the 8 come from? (or what happend to the first `) Post: 302296279

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Top Forums Shell Programming and Scripting Where does the 8 come from? (or what happend to the first `) Post 302296279 by Perderabo on Tuesday 10th of March 2009 05:59:56 PM

03-10-2009

Administrator Emeritus

To answer the question, the echo man page says:

Code:

 \0n   where n is the 8-bit character whose ASCII  code
                 is the 1-, 2- or 3-digit octal number represent-
                 ing that character.

So the \0 just says that 1, 2, or 3 digits may follow. That zero in \0 is not counted as of of the digits. So
echo "\047"0
is seen by the echo command exactly like
echo "\0470"
and is interrpreted as an attempt to use a byte with the octal value "470". This is too big and so leading bits are dropped. In this case the bit represented by the 4 is dropped and character becomes "\070".

Code:

# echo "\0470" | od -to1
0000000 070 012
0000002
# bc
obase=2
ibase=8
470
100111000
#

Edit:
By the way, a couple of ways that do work:

Code:

# echo "'0'"
'0'
# echo "\00470\0047"
'0'
#

Perderabo

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LEARN ABOUT OPENSOLARIS

echo

echo(1B)					     SunOS/BSD Compatibility Package Commands						  echo(1B)

NAME

       echo - echo arguments to standard output

SYNOPSIS

       /usr/ucb/echo [-n] [argument]

DESCRIPTION

       echo writes its arguments, separated by BLANKs and terminated by a NEWLINE, to the standard output.

       echo  is useful for producing diagnostics in command files and for sending known data into a pipe, and for displaying the contents of envi-
       ronment variables.

       For example, you can use echo to determine how many subdirectories below the root directory (/) is your current directory, as follows:

	   o	  echo your current-working-directory's full pathname

	   o	  pipe the output through tr to translate the path's embedded slash-characters into space-characters

	   o	  pipe that output through wc -w for a count of the names in your path.

		    example% /usr/bin/echo "echo $PWD | tr '/' ' ' | wc -w"

       See tr(1) and wc(1) for their functionality.

       The shells csh(1), ksh(1), and sh(1), each have an echo built-in command, which, by default, will have precedence, and will be  invoked	if
       the  user  calls  echo without a full pathname. /usr/ucb/echo and csh's echo() have an -n option, but do not understand back-slashed escape
       characters. sh's echo(), ksh's echo(), and /usr/bin/echo, on the other hand, understand the  black-slashed  escape  characters,	and  ksh's
       echo() also understands a as the audible bell character; however, these commands do not have an -n option.

OPTIONS

       -n    Do not add the NEWLINE to the output.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWscpu			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       csh(1), echo(1), ksh(1), sh(1), tr(1), wc(1), attributes(5)

NOTES

       The -n option is a transition aid for BSD applications, and may not be supported in future  releases.

SunOS 5.11							    3 Aug 1994								  echo(1B)

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echo