On 06/21/2010 05:05 PM, Eric Blake wrote:
On 06/21/2010 12:32 PM, Eduardo Otubo wrote:
Ouch. If the result of the `` command substitution begins with 0, you have a problem with octal numbers. Remember, $((010 + 1)) is not the same as $((10 + 1)). Perhaps you can modify the sed commands used in your script to strip leading 0?
Yes, I can strip the leading zero using sed, but I hardly believe that would be a such a return. But better fix this now than in the client screen. :)
And, rather than doing $(()) (or expr) to do +1 in the shell, where you have to worry about octal in the first place, why not just output the last value as-is (that is, drop "echo $((`" and "` +1 ))" from cmd), then do +1 in C code?
This is an option, but I really like to isolate the most I can on the shell side, returning the final value for the function. I've been keeping this pattern over all the code, so did the same here
I would much rather see it shift in the other direction - do as LITTLE work in shell as possible (since you have to carefully audit for exploits, and because it is SO expensive to fork processes), and as MUCH work in C as possible. Shortcuts like 'cmd | head -n 1' => C processing are one of the few things where doing more in shell can actually be faster, as it can lead to much less I/O, and even stop a cmd with lots of output early (due to EPIPE/SIGPIPE) rather than wasting processing power in running cmd to completion when we only need the first line in C code. But anything as complex as massaging octal strings into known binary is going to be orders of magnitude faster in C than in shell.
Yes, you're right on this point. I was just thinking on the easy side for the one who writes the code. Parsing strings in C is not exactly an easy job, mainly if you have all the facilities of a shell script on hand. This surely will be a separate patch in the future.
Also, '\ ' is not a portable sed escape sequence. Did you mean to use the C string "s/\\\\ //g" for the shell line 's/\\ //g', in order to delete backslash-space sequences from the output? (multiple instances of this pattern in your patch)
No, I meant to use the C string 's/\\ //g' for the shell line 's/\ //g' in order to delete white spaces.
But that's my point. Some versions of sed treat '\ ' as the single byte space, while others treat it as the two-byte sequence backslash-space. In short: sed 's/\ //g' _is not portable_. The only portable shell command lines for deleting spaces are: sed 's/ //g' sed s/\ //g
That is, either quote the space using '' in shell, or quote the space using \ in shell, but do NOT escape the space for sed.
Save a process: ...|sed '1d'|sed '1d' is equivalent to: ...|sed 2d
Ok, fixed. The next patch is right away. -- Eduardo Otubo Software Engineer Linux Technology Center IBM Systems & Technology Group Mobile: +55 19 8135 0885 eotubo@linux.vnet.ibm.com