I have a requirement in which I want to get the iteration count from a WHILE LOOP.
The below mentioned simple script test.sh works fine. In the below script, the WHILE loop will iterate every 5 seconds infinitely until it greps the string BASKETBALL from /tmp/somestring.txt file. Once it greps the string BASKETBALL, it prints the string in the echo command and the loop is terminated using the break command.
Code:
# cat test.sh
export counter=1
while true
do
if grep -iq BASKETBALL /tmp/somestring.txt
then
echo "This loop iterated $counter times before exiting.."
break
fi
sleep 5
counter=`expr $counter + 1`
done
#
# echo GOLF > /tmp/somestring.txt
# Now, execute the script and it will loop indefinitely as /tmp/somestring.txt file has the string GOLF
# ./test.sh &
[1] 43208
# #### after few seconds populate /tmp/somestring.txt with the string basketball and the loop will stop iterating
# echo basketball > /tmp/somestring.txt
# This loop iterated 14 times before exiting..
[1]+ Done ./test.sh
#
The above script works fine. But, in my real-life script, there is a minor problem. The script itself is executed by root user and then the above portion of the script is executed by another user called appuser after a switch using su command. So, the script will look like below. The double quotes in red mark the beginning and end of commands to be executed by appuser. For this reason, I had to escape the double quotes used in the echo command.
Code:
# cat realLife.sh
sudo su - appuser -c "
export counter=1
while true
do
if grep -iq BASKETBALL /tmp/somestring.txt
then
echo \"This loop iterated \$counter times before exiting..\"
break
fi
sleep 5
counter=`expr $counter + 1`
done
"
#
Although the WHILE LOOP runs fine with the above script, the echo command is always printing "This loop iterated 1 times before exiting.."
even if the loop iterated multiple times.
Demo:
Code:
# echo POLO > /tmp/somestring.txt
# Now, execute the script as root
# ./realLife.sh &
[1] 53717
# #### after few seconds populate /tmp/somestring.txt with the string basketball and the loop will stop iterating
# echo basketball > /tmp/somestring.txt
# This loop iterated 1 times before exiting..
[1]+ Done ./realLife.sh
#
I guess the double qutotes for the su command is confusing the shell. I wish there was some special variable which provides the loop count. Is there a workaround to fix this issue with realLife.sh script?
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Discussion started by: prvnrk
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LEARN ABOUT SUSE
while
while(n) Tcl Built-In Commands while(n)
__________________________________________________________________________________________________________________________________________________NAME
while - Execute script repeatedly as long as a condition is met
SYNOPSIS
while test body
_________________________________________________________________DESCRIPTION
The while command evaluates test as an expression (in the same way that expr evaluates its argument). The value of the expression must a
proper boolean value; if it is a true value then body is executed by passing it to the Tcl interpreter. Once body has been executed then
test is evaluated again, and the process repeats until eventually test evaluates to a false boolean value. Continue commands may be exe-
cuted inside body to terminate the current iteration of the loop, and break commands may be executed inside body to cause immediate termi-
nation of the while command. The while command always returns an empty string.
Note: test should almost always be enclosed in braces. If not, variable substitutions will be made before the while command starts execut-
ing, which means that variable changes made by the loop body will not be considered in the expression. This is likely to result in an
infinite loop. If test is enclosed in braces, variable substitutions are delayed until the expression is evaluated (before each loop iter-
ation), so changes in the variables will be visible. For an example, try the following script with and without the braces around $x<10:
set x 0
while {$x<10} {
puts "x is $x"
incr x
}
EXAMPLE
Read lines from a channel until we get to the end of the stream, and print them out with a line-number prepended:
set lineCount 0
while {[gets $chan line] >= 0} {
puts "[incr lineCount]: $line"
}
SEE ALSO
break(n), continue(n), for(n), foreach(n)
KEYWORDS
boolean value, loop, test, while
Tcl while(n)