Re: [Findutils-patches] xargs problem

[aws backup]

Thank you Dale for the detailed explanation :-)

> On 20 Dec 2016, at 16:20, Dale R. Worley <address@hidden> wrote:
> 
> aws backup <address@hidden> writes:
> [adding a line break and indentation:]
>> fswatch -0 -Ie ".*\.*$" -i ".*\.mp4$" /path/to/folder |
>>   xargs -0 -n 1 -I {} /path/to/shellscript.sh
>> 
>> shellscript.sh:
>> 
>> s3cmd put {} s3://bucket/ <s3://bucket/>
>> 
>> I get following failure message:
>> 
>> ERROR: Parameter problem: Nothing to upload.
>> 
>> So I get stuck already on the first point. 
>> Sorry I am a beginner in shell scripting.
>> How do I get it into the script?
> 
> When you run a shell script, the arguments that follow the script name
> are assigned to the variables $1, $2, etc. within the script.  So you
> want to say:
> 
> fswatch -0 -Ie ".*\.*$" -i ".*\.mp4$" /path/to/folder |
>    xargs -0 -n 1 -I {} /path/to/shellscript.sh {}
> 
> (IIRC, you can exploit the defaults by saying
> "xargs -0 -n 1 /path/to/shellscript.sh".)
> 
> And have shellscript.sh say:
> 
> s3cmd put "$1" s3://bucket/ <s3://bucket/>
> 
> Then xargs runs commands like:
> 
>  /path/to/shellscript.sh /.../name-of-some-file
> 
> and when shellscript.sh runs, $1 gets replaced with
> /.../name-of-some-file, effectively:
> 
> s3cmd put /.../name-of-some-file s3://bucket/ <s3://bucket/>
> 
> There are a lot of rules regarding exactly how quoting works within
> shell scripts, but in this case the essential rules are:
> 
> - xargs does not invoke the shell to assemble and run the commands it
>  runs.  Hence, '/path/to/shellscript.sh {}' gets turned into the kernel
>  call to run the program that you'd expect, the first item being
>  /path/to/shellscript.sh and the second item being the filename that
>  xargs dissects out -- regardless of any special characters that appear
>  in the file name
> 
> - The shell picks up the second argument to the kernel call and assigns
>  it to the variable $1 without any further processing.
> 
> - When the value of $1 is substituted into the shell line
> 
> s3cmd put "$1" s3://bucket/ <s3://bucket/>
> 
> the presence of "..." around $1 causes the value of $1 to be used as the
> first argument in the kernel call to run s3cmd, regardless of any
> special characters that appear in the value of $1.  Without "...", if
> the value of $1 contained whitespace characters, the value would be
> divided into multiple arguments to the kernel call.  (And possibly other
> transformations, I'd have to read the manual page carefully to get it
> right.)
> 
> Dale