Let's say we had a loop that we want to have run as quickly as possible. Let's say something was being done to a list of hosts inside that loop; just for the sake of argument, let's say it was a redis query. Let's say that the list of hosts may change occasionally due to hosts being added/removed from a pool (not load balanced); however, the list is predictable (e.g., they all start with “foo” and end with 2 digits. So we want to run this occasionally; say, once every 15 minutes:
listOfHosts=$(dig +noall +ans foo{00..99}.domain | while read -r n rest; do printf '%s\n' ${n%.}; done)
to get the list of hosts. Let's say our loop looked something like this:
while :; do
for i in $listOfHosts; do
redis-cli -h $i llen something
done
(( ( $(date +%s) % 60 * 15) == 0 )) && callFunctionThatSetslistOfHosts
done
(now obviously there's some things missing, like testing to see if we've already run callFunctionThatSetslistOfHosts in the current minute and only running it once, and doing something with the redis output, and maybe the list of hosts should be an array, but basically this is it.)
How can we run callFunctionThatSetslistOfHosts asynchronously so that it doesn't slow down the loop. I.e., have it running in the background setting listOfHosts occasionally (e.g. once every 15 minutes), so that the next time the inner loop is run it gets a potentially different set of hosts to run the redis query on?
My major problem seems to be that in order to set listOfHosts in a loop, that loop has to be a subshell, and listOfHosts is local to that subshell, and setting it doesn't affect the global listOfHosts.
I may resort to pipes, but will have to poll the reader before generating a new list — not that that's terribly bad if I poll slowly, but I thought I'd present this as a problem.
Thanks.
Related
I am trying to create my first bash script. The goal of this script is to check at what rate my public IP changes. It is a fairly straight forward script. First it checks if the new address is different from the old one. If so then it should update the old one to the new one and print out the date along with the new IP address.
At this point I have created a simple script in order to accomplish this. But I have two main problems.
First the script keeps on printing out the IP even tough it hasn't changed and I have updated the PREV_IP with the CUR_IP.
My second problem is that I want the output to direct to a file instead of outputting it into the terminal.
The interval is currently set to 1 second for test purposes. This will change to a higher interval in the final product.
#!/bin/bash
while true
PREV_IP=00
do
CUR_IP=$(curl https://ipinfo.io/ip)
if [ $PREV_IP != "$CUR_IP" ]; then
PREV_IP=$CUR_IP
"$(date)"
echo "$CUR_IP"
sleep 1
fi
done
I also get a really weird output. I have edited my public IP to xx.xxx.xxx.xxx:
Sat 20 Mar 09:45:29 CET 2021
xx.xxx.xxx.xxx
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:--
while true
PREV_IP=00
do
is the reason you are seeing ip each loop. It's the same as while true; PREV_IP=00; do. The exit status of true; PREV_IP=00 is the exit status of last command - the exit status of assignment is 0 (success) - so the loop will always execute. But PREV_IP will be reset to 00 each loop... This is a typo and you meant to set prev_ip once, before the loop starts.
"$(date)"
will try execute the output of date command, as a next command. So it will print:
$ "$(date)"
bash: sob, 20 mar 2021, 10:57:02 CET: command not found
And finally, to silence curl, read man curl first and then find out about -s. I use -sS so errors are also visible.
Do not use uppercase variables in your scripts. Prefer lower case variables. Check you scripts with http://shellcheck.net . Quote variable expansions.
I would sleep each loop. Your script could look like this:
#!/bin/bash
prev=""
while true; do
cur=$(curl -sS https://ipinfo.io/ip)
if [ "$prev" != "$cur" ]; then
prev="$cur"
echo "$(date) $cur"
fi
sleep 1
done
that I want the output to direct to a file instead of outputting it into the terminal.
Then research how redirection works in shell and how to use it. The simplest would be to redirect echo output.
echo "$(date) $cur" >> "a_file.txt"
The interval is currently set to 1 second for test purposes. This will change to a higher interval in the final product.
You are still limited with the time it takes to connect to https://ipinfo.io/ip. And from ipinfo.io documentation:
Free usage of our API is limited to 50,000 API requests per month.
And finally, I wrote a script where I tried to use many public services as I found ,get_ip_external for getting external ip address. You may take multiple public services for getting ipv4 address and choose a random/round-robin one so that rate-limiting don't kick that fast.
I have an infinite loop which uses aws cli to get the microservice names, it's parameters like desired tasks,number of running task etc for an environment.
There are 100's of microservices running in an environment. I have a requirement to compare the value of aws ecs metric running task for a particular microservice in the current loop and with that of the previous loop.
Say name a microservice X has the metric running task 5. As it is an infinite loop, after some time, again the loop come for the microservice X. Now, let's assume the value of running task is 4. I want to compare the running task for currnet loop, which is 4 with the value of the running task for the previous run, which is 5.
If you are asking a generic question of how to keep a previous value around so it can be compared to the current value, just store it in a variable. You can use the following as a starting point:
#!/bin/bash
previousValue=0
while read v; do
echo "Previous value=${previousValue}; Current value=${v}"
previousValue=${v}
done
exit 0
If the above script is called testval.sh. And you have an input file called test.in with the following values:
2
1
4
6
3
0
5
Then running
./testval.sh <test.in
will generate the following output:
Previous value=0; Current value=2
Previous value=2; Current value=1
Previous value=1; Current value=4
Previous value=4; Current value=6
Previous value=6; Current value=3
Previous value=3; Current value=0
Previous value=0; Current value=5
If the skeleton script works for you, feel free to modify it for however you need to do comparisons.
Hope this helps.
I dont know how your input looks exactly, but something like this might be useful for you :
The script
#!/bin/bash
declare -A app_stats
while read app tasks
do
if [[ ${app_stats[$app]} -ne $tasks && ! -z ${app_stats[$app]} ]]
then
echo "Number of tasks for $app has changed from ${app_stats[$app]} to $tasks"
app_stats[$app]=$tasks
else
app_stats[$app]=$tasks
fi
done <<< "$( cat input.txt)"
The input
App1 2
App2 5
App3 6
App1 6
The output
Number of tasks for App1 has changed from 2 to 6
Regards!
I have an AWS RDS process that generates 4 different output as Creating, Modifying,Backing-up and Available. This output of the process changes every 4 to 5 minutes and finally when the process completes it generates last output as available. Which is am storing in a variable "dbState".
What I am trying to do is run a spinner until the variable has the available value.
For this, I will have to run two loops 1st one that keeps checking the value of the variable. 2nd one which keeps running the loop and spinner until the variable value becomes available.
while :; do
dbState=(`aws rds describe-db-instances --db-instance-identifier $Instance_Identifier --query DBInstances[*].DBInstanceStatus --output text`)
sp='/-\|'
printf ' '
sleep 0.1
while [ "$dbState" != "available" ]; do
printf '\b%.1s' "Please wait.....$sp"
sp=${sp#?}${sp%???}
sleep 0.1
done
sleep 120
done
But for some reason it gets stuck in the 2nd loop and spinner keeps running even until the vale of variable becomes available.
Please help me here i can not think of any logic to achieve that.
All i want to to show spinner until the variable vale becomes available.
First, let's focus on the inner loop:
while [ "dbState" != "available" ]; do
printf '\b%.1s' "Please wait.....$sp"
sp=${sp#?}${sp%???}
sleep 0.1
done
Notice how dbState is never updated inside this loop? So there is never an exit condition from the loop. You would have to check the RDS instance state inside each iteration of the loop, so you probably only need the outer loop, and to convert the inner loop into an if statement.
Further, you have a typo in your condition. You are comparing the literal string "dbState" to the string "available". I believe you want to compare the value of the dbState variable, which would be: "$dbState" != "available".
Note that the AWS CLI Tool already has a method for waiting until an RDS instance state is "available":
aws rds wait db-instance-available --db-instance-identifier $Instance_Identifier
Maybe really simple question, but I don't know where to dig.
I have a list of files (random names), and I want to process them using some command
processing_command $i ${i%.*}.txt
I want to speed up by using all processors. How to make such the script occupy the 10 processors simultaneously (by processing 10 files)? processing_command is not parallel by default. Thank you!
the trivial approach would be to use:
for i in $items
do
processing_command $i ${i%.*}.txt &
done
which will start a new (parallel instance of) processing_command for each $i (the trick is the trailing & which will background the process)
the drawback is, that if you have e.g. 1000 items, then this will start 1000 parallel processes, which (while occupying all 10 cores) will be busy doing context switching rather than doing the actual processing.
if you have as many (or less) items as cores, than this is a good and simple solution.
usually you don't want to start more processes than cores.
a simplistic approach (assuming that all items take about the same time when processing), is to split the the original "items" list into number_of_cores equally long lists. the following is slightly modified version of an example taken from an article in the german linux-magazin:
#!/bin/bash
## number of processors
PMAX=$(ls -1d /sys/devices/system/cpu/cpu[0-9]* | wc -l)
## call processing_command on each argument:
doSequential() {
local i
for i in "$#"; do
processing_command $i ${i%.*}.txt
done
}
## run PMAX parallel processes
doParallel() {
# split the arguments into PMAX equally sized lists
local items item currentProcess=0
for item in "$#"; do
items[$currentProcess]="${items[$currentProcess]} "$item""
shift
let currentProcess=$(( (currentProcess+1)%PMAX ))
done
# run PMAX processes, each with the shorter list of items
currentProcess=0
while [ $currentProcess -lt $PMAX ]; do
[ -n "${items[$currentProcess]}" ] &&
eval doSequential ${items[$currentProcess]} &
currentProcess=$((currentProcess+1))
done
wait
}
doParallel $ITEMS
I need to examine the output of a certain script 1000s of times on a unix platform and check if any of it has changed from before.
I've been doing this:
(script_stuff) | md5sum
and storing this value. I actually don't really need "md5", JUST a simple hash function which I can compare against a stored value to see if its changed. Its okay if there are an occassional false positive.
Is there anything better than md5sum that works faster and generates a fairly usable hash value? The script itself generates a few lines of text - maybe 10-20 on average to max 100 or so.
I had a look at fast md5sum on millions of strings in bash/ubuntu - that's wonderful, but I can't compile a new program. Need a system utility... :(
Additional "background" details:
I've been asked to monitor the DNS record of a set of 1000 or so domains and immediately call certain other scripts if there has been any change. I intend to do a dig xyz +short statement and hash its output and store that, and then check it against a previously stored value. Any change will trigger the other script, otherwise it just goes on. Right now, we're planning on using cron for a set of these 1000, but can think completely diffeerently for "seriously heavy" usage - ~20,000 or so.
I have no idea what the use of such a system would be, I'm just doing this as a job for someone else...
The cksum utility calculates a non-cryptographic CRC checksum.
How big is the output you're checking? A hundred lines max. I'd just save the entire original file then use cmp to see if it's changed. Given that a hash calculation will have to read every byte anyway, the only way you'll get an advantage from a checksum type calculation is if the cost of doing it is less than reading two files of that size.
And cmp won't give you any false positives or negatives :-)
pax> echo hello >qq1.txt
pax> echo goodbye >qq2.txt
pax> cp qq1.txt qq3.txt
pax> cmp qq1.txt qq2.txt >/dev/null
pax> echo $?
1
pax> cmp qq1.txt qq3.txt >/dev/null
pax> echo $?
0
Based on your question update:
I've been asked to monitor the DNS record of a set of 1000 or so domains and immediately call certain other scripts if there has been any change. I intend to do a dig xyz +short statement and hash its output and store that, and then check it against a previously stored value. Any change will trigger the other script, otherwise it just goes on. Right now, we're planning on using cron for a set of these 1000, but can think completely diffeerently for "seriously heavy" usage - ~20,000 or so.
I'm not sure you need to worry too much about the file I/O. The following script executed dig microsoft.com +short 5000 times first with file I/O then with output to /dev/null (by changing the comments).
#!/bin/bash
rm -rf qqtemp
mkdir qqtemp
((i = 0))
while [[ $i -ne 5000 ]] ; do
#dig microsoft.com +short >qqtemp/microsoft.com.$i
dig microsoft.com +short >/dev/null
((i = i + 1))
done
The elapsed times at 5 runs each are:
File I/O | /dev/null
----------+-----------
3:09 | 1:52
2:54 | 2:33
2:43 | 3:04
2:49 | 2:38
2:33 | 3:08
After removing the outliers and averaging, the results are 2:49 for the file I/O and 2:45 for the /dev/null. The time difference is four seconds for 5000 iterations, only 1/1250th of a second per item.
However, since an iteration over the 5000 takes up to three minutes, that's how long it will take maximum to detect a problem (a minute and a half on average). If that's not acceptable, you need to move away from bash to another tool.
Given that a single dig only takes about 0.012 seconds, you should theoretically do 5000 in sixty seconds assuming your checking tool takes no time at all. You may be better off doing something like this in Perl and using an associative array to store the output from dig.
Perl's semi-compiled nature means that it will probably run substantially faster than a bash script and Perl's fancy stuff will make the job a lot easier. However, you're unlikely to get that 60-second time much lower just because that's how long it takes to run the dig commands.