man page for eval?

[conradin]

Hi all,
I am working on a Ubuntu port of some scientific equipment scripts aimed at RHEL. There seems to be some different command line functions that I'm not exactly familiar with, such as $eval
how can I find out more about the eval command?
My usual source:
http://linux.die.net/man/1/eval
seems useless in this case.

the specific is a variable assignment. where findproc is a previous line
Actualy, while I am at it, Im not exactly sure what grep $1 is either.


...
findproc="ps -e | grep $1 | awk '{ printf(\"%d \",\$1) }'"
npids=`eval $findproc`

as
usual any insight would be greatly appreciated.

[steeldriver]

iirc it's a shell builtin

http://tldp.org/LDP/abs/html/internal.html

AFAIK in newer versions of bash, it is better practice to replace that kind of thing with the $(...) syntax which allows you to assign the result of a command e.g.


var=$(ps -ef | grep whatever)Regarding your second question, $1 in the grep command would conventionally be the 1st command line argument to the script (assuming this is part of a shell script); however inside the awk command it's the first field in the input - in other words, any lines matched by grepping for the 1st shell argument get piped to awk, which prints the 1st field (column) of those lines.

[Vaphell]

eval should be avoided if possible. Executing arbitrary strings at runtime is a dirty hack, not to mention dubious security, especially when the string depends on user input in any way - allowing to eval "rm -rf /" would not be a great idea. It's not unlike the sql injection attacks which are the bane of the poorly constructed websites on the internet.

[conradin]

Thanks Guys,
I found this web-page helpful as well.
http://wiki.bash-hackers.org/rcwatson

the set -vx command was also a great thing to learn about.
Thanks again!