UNIX for Dummies Questions & Answers

Don - when I tried Ravichander post#9, print statement only printed the line what I edited

Code:

awk '/kernel.sem/ { if ($3 < "300") { sub($3,"300",$0); print >> "/tmp/test1" } else {print >> "/tmp/test1" }}' ACTUAL_FILE

please find the o/p below

Code:

 
/tmp/test1
kernel.sem = 300   32000   32      128250

So if I move this test1 to ACTUAL file, the file will be replaced with the content of test1.

DO you have any other option to achieve my requirement

First, be sure that you have a back-up copy of sysctl.conf before you try this. Then try something like:

Code:

nawk '
/kernel.sem/ {
	if(($3 + 0) < 300)
		sub($3, "300")
	else	exit(1)
}
1' sysctl.conf > sysctl.conf.$$ && \
	mv sysctl.conf.$$ sysctl.conf || \
	rm sysctl.conf.$$

Thanks Don. I have two doubts in your code

($3 + 0) - why you used + 0 in here
} 1' filename - why used 1 here, is it for prinitng purpose.

Because ($3 < "300") performs a string comparison and would give you the wrong result if in some cases. For example, if the current contents of field 3 was 1000, it would reduce it to 300; and if the current contents of field 3 was 40, it wouldn't increase it to 300.
Yes, the default action in awk when a condition evaluates to true (non-zero) is to print the current line.

Did you try what I suggested? Did it give you the wrong results for any value on the kernel.sem line in your sysctl.conf file(s)?

yes. your code is working perfectly. I am using this sample peice for puppet automation. so for awk part I don't hard code anything. I will write all vlaues to some other yaml file and will ivoke that value variable into puppet awk function.

so my code would be

Code:

awk '/kernel.sem/ { if(($3 + 0) < $value) sub($3, "$value") } 1' /etc/sysctl.conf >> /etc/sysctl.conf.bkp

so what my doubt is, when I use $value parameter should I still use +0 on this

I have no idea what peice, puppet automation, vlaues, nor yaml mean, and there is no puppet() function in this awk script??? The awk variable value is not defined in this script and comparing the number in field 3 of a line to the entire line as a string (containing something like:

Code:

kernel.sem = 250   32000   32      128250

which is what $undefined_variable will expand to in your awk script with your sample input file) is not what you want. If you want to parameterize the threshold value for changes instead of hard coding it to 300, you need something like:

Code:

value=300
awk -v val="$value" '
/^kernel.sem/ {
	if(($3 + 0) < val)
		sub($3, val "")
	else	exit(1)
}
1' sysctl.conf > sysctl.conf.mod && \
	mv sysctl.conf.mod sysctl.conf || \
	rm sysctl.conf.mod

Please DO NOT use the name sysctl.conf.bkp in this script. It is not a backup file; it is a temp file used as a work area for this script. And, if the number found in the 3rd field on the "kernel.sem" line is greater than or equal to your value variable, it will only contain a subset of your file (since this awk script returns an error status and stops processing input if it is not going to change that line. This file will NEVER contain the original contents of your input file unless your input file does not contain a line that contains the string "kernel.sem"! This leaves the current sysctl.conf in place (instead of replacing it with a copy of itself) if no changes are being made, and runs faster than processing the rest of the file and replacing an unmodified file.

Note that I added an anchor to the ERE selecting the line to be changed in case your configuration file might contain a comment that explains why kernel.conf is being set.

I am okay with your code, but my concern is move command. shall we use

Code:

cp sysctl.conf.mod sysctl.conf

rather than

Code:

mv sysctl.conf.mod sysctl.conf