Actually, this is pretty straight forward. But, an "&" is never simply ignored in the shell command language. The "&" causes the previous list (where "list" in this case is a simple command that is an assignment statement) to be run asynchronously in a subshell. So, we not only have a subshell due to the parentheses, we have a sub-subshell to run the assignment statement in the background. It is not clear to me that the standards mandate whether the command substitution that is part of this command is run in the background or whether the shell can run the command substitution in the foreground and then run the assignment of the results of that command substitution to the specified variable in the background.
In either case result of the variable assignment is visible only in the sub-subshell; it is not visible in the environment of the shell that started the subshell and it is not visible in the subshell that started the asynchronous list.
First off, thank you for clearing that up. It actually never occurred to me to put an assignment (or anything else not being an external executable, for that matter) in background so i couldn't draw on experience. One thing is still not completely clear to me and i hope you can clarify. You say:
Quote:
Originally Posted by Don Cragun
In either case result of the variable assignment is visible only in the sub-subshell; it is not visible in the environment of the shell that started the subshell and it is not visible in the subshell that started the asynchronous list.
I take it, the mechanism of
is to open a subshell, execute list there and then close the subshell. Now, any command (and, as i have just learned, even an assignment) put into background is executed in its own subprocess (and as the assignment is a shell command, a subshell, yes?). Therefore:
should, as i understood it, open a subshell (running asynchronously), execute
there and open another sub-subshell in there to calculate this <something> because <something> is actually $( ... ). So the assignment happens at the first subshell and the list inside $( list; ... ) happens at the sub-subshell. Have i gotten that right or am i a hopeless case?
Anyway, i have at least tested a bit and i found:
So, putting an assignment into background does not only make no sense it even (for practical purposes) nullifies its effect because the variable gets assigned in a subshell. At least this much i take away from your explanation and this is true regardless of in which way the subshells and sub-subshells are arranged. Thank you again.
Hi All,
I need to pass a variable to perl script from bash script, where in perl i am using if condition. Here is the cmd what i am using in perl
FROM_DATE="06/05/2008"
TO_DATE="07/05/2008"
"perl -ne ' print if ( $_ >="$FROM_DATE" && $_ <= "$TO_DATE" ) ' filename"
filename has... (10 Replies)
Hi,
I use AIX (ksh) and Linux (bash) servers. I'm trying to do scripts to will run in both ksh and bash, and most of the time it works. But this time I don't get it in bash (I'm more familar in ksh).
The goal of my script if to read a "config file" (like "ini" file), and make various report.... (2 Replies)
Hello all,
I'm working on a script, and as part of it, I'm trying to create a loop that will run through a stored piece of information a certain number of times pulling out information, and each time create a variable with a unique name that will store that information. I'm sure it's a simple... (3 Replies)
Hello all,
I have this shell script, but do not understand why the variables inside the if block does not keep its value outside. Is it because of the pipe ? How can i fix this problem ?
Thank you for helping.
local alarm=""
local num_alarm=0
local -a alarms
... (3 Replies)
Hello,
I am struggling with using variable made using "eval".
a=4
eval b$a=20
echo $b$a ???
As shown above, I am trying to call back the variable "bX" assuming I do not know the value of "a".
How can I do that?
I tried several combinations but nothing worked.
Thanks (10 Replies)
Hi,
I wrote the following program to understand mutexes. If I run the program , number of threads is shown as zero, even after creating one thread. When running with gdb, it works fine.
The function process is used to update global variable (used to keep track of threads). It looks like the... (2 Replies)
#aa=xxxx
#zz="cc $aa"
#aa=gggg
#echo $zz
out put is
cc xxxx
if I want to get
cc gggg
how should I do, I don't want to write zz="c $aa " after aa=gggg (2 Replies)
Hello,
Can I ask how to expand variable that contains another in bash? I need to loop variable within another one like this:
RD1=testgrp
RD2=testgroup
RD3=testgroupfile
RD4=tstgroup
...
RD40=try2013
DEST=/home/king/finaldir
for i in {1..40}; do
mv ${RD${i}} ${DEST}
done
I do not... (8 Replies)
Hi Team
In the below code, irrespective of the if statement that gets executed, retcd is being assigned a standard value(1) instead of changing as per code. Could you please help to see where is it going wrong.
rval=0
CONF_FILE=/apps/wmroot/scripts/props/UMPath.properties
NOHUP="nohup"... (3 Replies)