Strange sed behaviour - UNIX for Advanced & Expert Users

vino · #1 02-07-2008

Code:

[/tmp]$ echo a.bc | sed -e "s/\|/\\|/g"
|a|.|b|c|
[/tmp]$

Is the behavior of the sed statement expected ? Or is this a bug in sed ?

OS details

Code:

Linux 2.6.9-55.0.0.0.2.ELsmp #1 SMP Wed May 2 14:59:56 PDT 2007 i686 i686 i386 GNU/Linux

bakunin · #2 02-07-2008

The problem is perhaps in the quotation you use: The shell is probably "eating" your escape chars away and sed doesn't see what you expect it to see.

I made it a habit to use always single quotes for sed-statements to avoid this. It is even possible to use single quotes when using a variable inside an sed-statement:

sed 's/'"$src"'/'"$tgt"'/g'

will change ocurrences of $src to the value of $tgt

I hope this helps.

bakunin

vino · #3 02-07-2008

Fine. The single quotes vs double quotes has an impact.

Code:

[/tmp]$ echo a.bc | sed -e "s/\|/\\|/g"
|a|.|b|c|
[/tmp]$ echo a.bc | sed -e 's/\|/\\|/g'
\|a\|.\|b\|c\|
[/tmp]$

But that does not explain those extra characters in the output. In either case I would expect the output to be a.bc and not anything which has | or \| wrapped around every character.

Am I missing something something here ?

bakunin · #4 02-07-2008

Quote:

Originally Posted by vino

Am I missing something something here ?

To be honest, now i'm astonished myself:

lacking a UNIX machine (got a day off) i fired up cygwin and tried:

Code:

# echo a.bc | sed --posix 's/\|/x/g'
xax.xbxcx

# echo 'a.bc' | sed --posix 's/\|/x/g'
xax.xbxcx

# echo 'a.bc' | sed --posix 's/|/x/g'
a.bc

# sed --version
GNU sed version 4.1.5
Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE, to the extent permitted by law.

It seems that "\|" is matching the NULL-regexp, whereas "|" is matching a single pipe character as expected. I have no idea, why this is the case, but will investigate further.

bakunin

ghostdog74 · #5 02-07-2008

my guess:
\| is used in sed (gnu) as alternation. therefore

Code:

# echo "a.bc" | sed -e 's/\|/\\|/g'
\|a\|.\|b\|c\|

seem to say "blank" or "blank" (or null?) substitute with \|, hence the result.
if really want to search for a "|", use the open square brackets

Code:

# echo "a.bc" | sed -e 's/[|]/\\|/g'
a.bc
# echo "a|bc" | sed -e 's/[|]/\\|/g'
a\|bc

vino · #6 02-07-2008

Quote:

Originally Posted by ghostdog74

my guess:
\| is used in sed (gnu) as alternation. therefore

Code:

# echo "a.bc" | sed -e 's/\|/\\|/g'
\|a\|.\|b\|c\|

seem to say "blank" or "blank" (or null?) substitute with \|, hence the result.
if really want to search for a "|", use the open square brackets

Code:

# echo "a.bc" | sed -e 's/[|]/\\|/g'
a.bc
# echo "a|bc" | sed -e 's/[|]/\\|/g'
a\|bc

Wouldn't the alternation operator require atleast two operands ?

fpmurphy · #7 02-08-2008

Humm, seems to be a GNU sed-ism

Under Interix 6.0 and ksh-93 and the OOB sed (non-GNU), it works as expected

Code:

$ echo a.bc | sed -e "s/\|/\\|/g"
a.bc
$

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