Shell Programming and Scripting

To explain how this works, this doesn't avoid the subshell problem, as much as put the entire relevant program inside the subshell so nothing loses context.

I did not get this part of code

Code:

read REST; while read FS SZ AV US PC MT REST

The read REST; takes care of disregarding the header (first line or unwanted information):

Code:

Filesystem      Size  Used Avail Use% Mounted on

while read FS SZ AV US PC MT REST takes care of dividing the disk information for the mount point.

so these ones are FS (filesystem) SZ (size) AV (avalaible) US (used) PC (????) MT (mounted)

From the heading you used in post #1 in this thread:

Code:

Filesystem      Size  Used Avail Use% Mounted on

It looks to me like the reads should be changed from:

Code:

 read REST; while read FS SZ AV US PC MT REST

to:

Code:

 read REST; while read FS SZ US AV PC REST

where $US will expand to the data in the "Used" field and $PC will expand to the data in the "Use%" (i.e., used percent) field. Note that the standards don't specify the output format used in df -h output (although -h is a common option; it is an extension not required in the standards); on my system the heading used with it is:

Code:

Filesystem          Size   Used  Avail Capacity  iused     ifree %iused  Mounted on

where "Capacity" corresponds to the "Use%" heading you listed and in either case "$REST" will expand to the contents of the remaining fields (whether that is just the "Mount on" (directory on which the filesystem is mounted) as on your system, or the "iused" (number of i-nodes used), "ifree" (number of i-nodes free), "%iused" (percentage of i-nodes used), and "Mounted on" (directory on which the filesystem is mounted) fields produced by OS X df -h. If anyone else is trying to use code similar to yours, they need to adapt the arguments passed to read to assign needed fields to variables with names corresponding to the output format produced by df -h, df, or df -P (the last one being the only one with the output format specified by the standards) on their system for fields that they care about.

But, of course, since the code RudiC suggested doesn't use $US or $AV the order in which those are specified in the read command doesn't really matter in the way the script functions; the only thing that is crucial is the number of fields specified before PC (to get the percentage data needed by your script) and having at least one variable listed after PC so the remaining data on each line is not mixed in with the desired data when $PC is expanded.

So these letters (FS SZ US AV PC REST) are standardised !!!

In the script:

Code:

df -h | {
	read REST
	while read FS SZ AV US PC MT REST
	do	useSpPer=${useSpPer}'-'$PC
		echo 'use '$useSpPer
		echo 'res '$PC
	done
	echo final $useSpPer
}

there is nothing standardized about the chosen variable names RudiC used. The only thing that matters is that the position in the list of values printed by the command df -h corresponds to the variable names used later in the script. In this specific script with the output produced by dh -h on your system, the following script would produce identical output:

Code:

df -h | {
	read junk
	while read junk junk junk junk needed junk
	do	useSpPer=${useSpPer}'-'$needed
		echo 'use '$useSpPer
		echo 'res '$needed
	done
	echo final $useSpPer
}

If there is a possibility that useSpPer might be an exported variable inherited into the environment of these scripts, it would be safer to define an initial value before the loop as in:

Code:

df -h | {
	read junk
	useSpPer=''
	while read junk junk junk junk needed junk
	do	useSpPer=${useSpPer}'-'$needed
		echo 'use '$useSpPer
		echo 'res '$needed
	done
	echo final $useSpPer
}