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Df -h results in a loop

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# 1  
Old 05-19-2016
Df -h results in a loop
Hello everyone,

I am doing a check of the disk space using df -h, I want to combine the result in break line; but the result after while/done is empty:

Code:
# df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs           20G   14G  4.6G  75% /
/dev/root        20G   14G  4.6G  75% /
devtmpfs        993M  4.0K  993M   1% /dev
none            199M  2.7M  196M   2% /run
none            5.0M     0  5.0M   0% /run/lock
none            994M   72K  994M   1% /run/shm
/dev/sda2       894G  840G  9.2G  99% /home

disk.sh
Code:
useSpPer=''

df -h | awk 'NR>2 { print $1 " " $5 }' | while read output;
do

#echo $output
res=$(echo $output | awk '{ print $2}')

useSpPer=${useSpPer}'-'$res

echo 'use '$useSpPer
echo 'res '$res

done
echo 'final '$useSpPer

final prints empty result

Code:
# sh disk.sh 
use -75%
res 75%
use -75%-1%
res 1%
use -75%-1%-2%
res 2%
use -75%-1%-2%-0%
res 0%
use -75%-1%-2%-0%-1%
res 1%
use -75%-1%-2%-0%-1%-99%
res 99%
final

I am expecting a final result like in the last 'use':
Code:
use -75%-1%-2%-0%-1%-99%

to be
Code:
final 75%-1%-2%-0%-1%-99%

Thanks in advance
Last edited by RudiC; 05-19-2016 at 03:54 AM.. Reason: Added code tags.
# 2  
Old 05-19-2016
Hi,
the effect you see is caused by the way you feed your loop. The pipe creates a subshell which has its own copies of the variables, the ones in the parent shell are not affected by actions in the subshell.
Process substitution is one way to fix this problem:
Code:
useSpPer=''

while read output; do
   res=$(echo $output | awk '{ print $2}')
   useSpPer=${useSpPer}'-'$res
   echo 'use '$useSpPer
   echo 'res '$res
done < <(df -h | awk 'NR>2 { print $1 " " $5 }')
echo 'final '$useSpPer

This User Gave Thanks to cero For This Post:
Abu Rayane
# 3  
Old 05-19-2016
That behaviour is due to the loop being executed in a subshell, and the variable's contents is not given back to the parent shell.
# 4  
Old 05-19-2016
Try this pure shell:
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; }
use -28%
res 28%
use -28%-28%
res 28%
use -28%-28%-42%
res 42%
use -28%-28%-42%-3%
res 3%
final -28%-28%-42%-3%

This User Gave Thanks to RudiC For This Post:
Abu Rayane
# 5  
Old 05-19-2016
When you pipe something to a block, the block runs in a sub-shell.
Variables in the block are not copied back to the main shell.

A while loop is such a block.
In RudiC example the { } is the block.
In cero example the pipe is avoided; the <( ) needs bash or zsh.
Last edited by MadeInGermany; 05-19-2016 at 04:48 AM..
This User Gave Thanks to MadeInGermany For This Post:
Abu Rayane
# 6  
Old 05-19-2016
Thank you guys for your effort, but I found a little mistake in cero code:

Quote:
Originally Posted by cero
Code:
useSpPer=''

while read output; do
   res=$(echo $output | awk '{ print $2}')
   useSpPer=${useSpPer}'-'$res
   echo 'use '$useSpPer
   echo 'res '$res
done < <(df -h | awk 'NR>2 { print $1 " " $5 }')
echo 'final '$useSpPer


This is the mistake

Code:
str.sh: line 8: syntax error near unexpected token `<'
str.sh: line 8: `done < <(df -h | awk 'NR>2 { print $1 " " $5 }')'

# 7  
Old 05-19-2016
Quote:
Originally Posted by Abu Rayane
Thank you guys for your effort, but I found a little mistake in cero code:




This is the mistake

Code:
str.sh: line 8: syntax error near unexpected token `<'
str.sh: line 8: `done < <(df -h | awk 'NR>2 { print $1 " " $5 }')'

Only Bash, Zsh and Ksh93 supports process substitution, which is what the <(...) means. Since you are invoking it with sh, judging by this # sh disk.sh , I suggest you use the command grouping

Here's an example using your original post #1

Code:
df -h | awk 'NR>2 { print $1 " " $5 }' |
{
    while read output
    do
        res=$(echo $output | awk '{ print $2}')
        useSpPer=${useSpPer}'-'$res

        echo 'use '$useSpPer
        echo 'res '$res
    done
    echo 'final '$useSpPer
}

This User Gave Thanks to Aia For This Post:
Abu Rayane
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