06-21-2010
Thanks. I get that since all the special symbols like |,",',; all are evaluated before substitution takes place. Hence, in the case of $cmd, since no specia symbols were initially found, cmd was substituted for "date | wc" and | was taken as an argument to date (as | now has no meaning).
One question though, why do call evaluating the special symbols like | as expanding it? Does it mean that a special code for these symbols is inserted there when they're expanded ?
Also, it seems that there is another function of eval. When there are no substitutions to be made, then eval just removes the special symbols like '' and backslash. Like in the following :
eval echo \\l
eval echo \\\k
eval echo 'h'
I'm using bash shell
Last edited by daudiam; 06-21-2010 at 12:46 AM..
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LEARN ABOUT CENTOS
trace-cmd-reset
TRACE-CMD-RESET(1) TRACE-CMD-RESET(1)
NAME
trace-cmd-reset - turn off all Ftrace tracing to bring back full performance
SYNOPSIS
trace-cmd reset [OPTIONS]
DESCRIPTION
The trace-cmd(1) reset command turns off all tracing of Ftrace. This will bring back the performance of the system before tracing was
enabled. This is necessary since trace-cmd-record(1), trace-cmd-stop(1) and trace-cmd-extract(1) do not disable the tracer, event after the
data has been pulled from the buffers. The rational is that the user may want to manually enable the tracer with the Ftrace pseudo file
system, or examine other parts of Ftrace to see what trace-cmd did. After the reset command happens, the data in the ring buffer, and the
options that were used are all lost.
OPTIONS
-b buffer_size
When the kernel boots, the Ftrace ring buffer is of a minimal size (3 pages per CPU). The first time the tracer is used, the ring
buffer size expands to what it was set for (default 1.4 Megs per CPU).
If no more tracing is to be done, this option allows you to shrink the
ring buffer down to free up available memory.
trace-cmd reset -b 1
SEE ALSO
trace-cmd(1), trace-cmd-record(1), trace-cmd-report(1), trace-cmd-start(1), trace-cmd-stop(1), trace-cmd-extract(1), trace-cmd-split(1),
trace-cmd-list(1), trace-cmd-listen(1)
AUTHOR
Written by Steven Rostedt, <rostedt@goodmis.org[1]>
RESOURCES
git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/trace-cmd.git
COPYING
Copyright (C) 2010 Red Hat, Inc. Free use of this software is granted under the terms of the GNU Public License (GPL).
NOTES
1. rostedt@goodmis.org
mailto:rostedt@goodmis.org
06/11/2014 TRACE-CMD-RESET(1)