sed script fails to run in dash

sed script fails to run in dash

[Tormen]

Hi,

I am puzzled and don't know why this command does not work with dash (I
am currently using the debian stable (v7.2) version of dash (v0.5.7-3)):

echo 1>/tmp/x; reponse=$( sed -e $(wc -l </tmp/x)$'{w/dev/stdout\n;d}'
-i /tmp/x ); echo "reponse='$reponse'"

but also just this line:

    sed -e 1$'{w/dev/stdout\n;d}' -i /tmp/x

in a dash script will yield the error message:

    sed: -e expression #1, char 2: unknown command: `$'

But why ? :(

I would be really grateful for any explanation.
Thanks a lot in advance for any hint!


Tormen


P.S.: The source for the above line is:
http://stackoverflow.com/questions/13804967/bash-pop-lines-in-a-file

Re: sed script fails to run in dash

[Seb]

Le Fri, 22 Nov 2013 19:11:42 +0100
Tormen a écrit:

Hello,


> but also just this line:
> 
>     sed -e 1$'{w/dev/stdout\n;d}' -i /tmp/x
> 
> in a dash script will yield the error message:
> 
>     sed: -e expression #1, char 2: unknown command: `$'
> 
> But why ? :(

It seems dash strictly passes what you have given and which is
sed-speaking incorrect:

$ dash -c "strace sed -e 1$'{w/dev/stdout\n;d}' -i /tmp/x"
execve("/usr/bin/sed", ["sed", "-e", "1${w/dev/stdout\\n;d}", ...
                                      ^^^
Bash does not, and so passes something sed understands:

$ bash -c "strace sed -e 1$'{w/dev/stdout\n;d}' -i /tmp/x"
execve("/usr/bin/sed", ["sed", "-e", "1{w/dev/stdout \n;d}", ...
                                      ^^^
If you escape the "$" with bash, you will get your kitten back:

$ bash -c "sed -e 1\\$'{w/dev/stdout\n;d}' -i /tmp/x"
sed: -e expression #1, char 2: unknown command: `$'


Take care,
Seb.

Re: sed script fails to run in dash

[Jonathan Nieder]

Hi Tormen,

Tormen wrote:

>     sed -e 1$'{w/dev/stdout\n;d}' -i /tmp/x
>
> in a dash script will yield the error message:
>
>     sed: -e expression #1, char 2: unknown command: `$'
>
> But why ? :(

POSIX sayeth[1]:

  The '$' character is used to introduce parameter expansion, command
  substitution, or arithmetic evaluation. If an unquoted '$' is
  followed by a character that is not one of the following:

  * A numeric character
  * The name of one of the special parameters (see 2.5.2 Special
    Parameters)
  * A valid first character of a variable name
  * A <left-curly-bracket> ( '{' )
  * A <left-parenthesis>

  the result is unspecified.

A single-quote is not the name of a special parameter nor valid as the
first character of a variable name, so this is unspecified behavior
and should be avoided in scripts meant to run on an arbitrary POSIX
shell.

However the next major version of POSIX is likely to mandate support
for $'' string handling[2].  Work towards an implementation in dash would
be very welcome.

Thanks and hope that helps,
Jonathan

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06
[2] http://austingroupbugs.net/view.php?id=249

Re: sed script fails to run in dash

[Eric Blake]

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On 11/22/2013 11:11 AM, Tormen wrote:

>     sed -e 1$'{w/dev/stdout\n;d}' -i /tmp/x
> 
> in a dash script will yield the error message:
> 
>     sed: -e expression #1, char 2: unknown command: `$'
> 
> But why ? :(

Because $'' is not (yet) in POSIX.  It will be required in a future
release, but dash hasn't implemented it yet.

http://austingroupbugs.net/view.php?id=249

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: sed script fails to run in dash

[Tormen]

On 22/11/13 20:26, Eric Blake wrote:
> On 11/22/2013 11:11 AM, Tormen wrote:
>
>>     sed -e 1$'{w/dev/stdout\n;d}' -i /tmp/x
>>
>> in a dash script will yield the error message:
>>
>>     sed: -e expression #1, char 2: unknown command: `$'
>>
>> But why ? :(
> Because $'' is not (yet) in POSIX.  It will be required in a future
> release, but dash hasn't implemented it yet.
>
> http://austingroupbugs.net/view.php?id=249
Now I get how this script works ;)

Thanks !! :)

!!! And also thanks to all the other answers !!! - really appreciated! :)))