Linux Read - Timeout after x seconds *idle* - linux

I have a (bash) script on a server that I have inherited the administration aspect of, and have recently discovered a flaw in the script that nobody has brought to my attention.
After discovering the issue, others have told me that it has been irritating them, but never told me (great...)
So, the script follows this concept
#!/bin/bash
function refreshscreen(){
# This function refreshes a "statistics screen"
...
echo "Enter command to override update"
read -t 10 variable
}
This script refreshes a statistics screen, and allows the user to stall the update in lieu of commands built into a case statement. However, the read times-out (read -t 10) after 10 seconds, regardless of if the user is typing.
Long story short, is there a way to prevent read from timing out if the user is actively typing a command? Best case scenario would be a "Time out of SEC idle/inactive seconds" opposed to just timeout after x seconds.
I have thought about running a background script at the end of the cycle before the read command pauses the screen to check for inactivity, but have not found a way to make that command work.

You can use read in a loop, reading one character at a time, and adding it to a final read string. This would then give the user some timeout amount of time per character rather than per command. Here's a sample function you might be able to incorporate into your script that shows what I'm talking about:
read_with_idle_timeout() {
local input=""
read -t 10 -N 1 variable
while [ ! -z $variable ]
do
input+=$variable
read -t 10 -N 1 variable
done
echo "Read: $input"
}
This will give the user 10 seconds to type each character. If they stop typing, you'll get as much of the command as they had started typing before the timeout occurred, and then your case statement can handle it. Perhaps you can store the final string in a global variable, or just put this code directly into your other function.
If you need more than one word, since read breaks on $IFS, you could call this function multiple times until you get all the input you're expecting.

I have searched for a simple solution that will do the following:
timeout after 10 seconds, if there is no user input at all
the user has infinite time to finish his answer if the first character was typed within the first 10 sec.
This can be implemented in two lines as follows:
read -N 1 -t 10 -p "What is your name? > " a
[ "$a" != "" ] && read b && echo "Your name is $a$b" || echo "(timeout)"
In case the user waits 10 sec before he enters the first character, the output will be:
What is your name? > (timeout)
If the user types the first character within 10 sec, he has unlimited time to finish this task. The output will look like follows:
What is your name? > Oliver
Your name is Oliver
Caveat: the first character is not editable, once it was typed, while all other characters can be edited (backspace and re-type). Any ideas for a simple solution?

Related

Compare A Variable Regularaly Using Linux Scripts and Cron

I'm trying to check if a number differs from what it was last time it was checked, in this case checking a number every minute, using Linux scripts and cron.
eg:
newNum = getNum()
if oldNum != newNum: run some code
oldNum = newNum
(repeat every minute using crontab)
But the problem I am having is that the variables aren't accessible between scripts and using source (eg. source script.sh) runs the script again, hence getting the latest version, not the one from a minute ago.
The best I've got is running a first script which gets the current number, then sleeps for a minute, then runs a second script which is essentially the first two lines of the code above.
eg:
oldNum = getNum()
sleep 60
export oldNum
script2.sh
This seems inefficient to me and I'd like to know if there is a better solution if possible.
You could cache the previous number in a file:
number_cache=/path/to/cache_file
# read the previous number
oldNum=$(< "$number_cache" )
# acquire the new number
newNum=$(getNum)
if [[ "$oldNum" -eq "$newNum" ]]; then
do_something
fi
# cache the new number
printf "%d\n" "$newNum" > "$number_cache"

using awk and bash for monitoring exec output to log

I am looking for some help with awk and bash commands,
my project have an embedded (so very limited) hardware,
i need to run a specific command called "digitalio show"
the command output is:
Input=0x50ff <-- last char only change
Output=0x7f
OR
Input=0x50fd <-- last char only change
Output=0x7f
i need to extract the input parameter and convert it into either Active or Passive and log them to a file with timestamp.
the log file should look like this:
YYMMDDhhmmss;Active
YYMMDDhhmmss;Passive
YYMMDDhhmmss;Active
YYMMDDhhmmss;Passive
while logging only changes
The command "digitalio show" is an embedded specific command that give the I/O state at the time of the execution, so i basically need to log every change in the I/O into a file using a minimal tools i have in the embedded H/W.
i can run the command for every 500msec, but if i will log all the outputs i can finish the flash very quickly, so i need only log changes.
in the end this will run as a background deamon.
Thanks !
Rotem.
As far as I understand, a single run of digitalio show command outputs two lines in the following format:
Input=HEX_NUMBER
Output=0x7f
where HEX_NUMBER is either 0x50ff, or 0x50fd. Suppose, the former stands for "Active", the latter for "Passive".
Running the command once per 500 milliseconds requires keeping the state. The most obvious implementation is a loop with a sleep.
However, sleep implementations vary. Some of them support a floating point argument (fractional seconds), and some don't. For example, the GNU implementation accepts arbitrary floating point numbers, but the standard UNIX implementation guarantees to suspend execution for at least the integral number of seconds. There are many alternatives, though. For instance, usleep from killproc accepts microseconds. Alternatively, you can write your own utility.
Let's pick the usleep command. Then the Bash script may look like the following:
#!/bin/bash -
last_state=
while true ; do
i=$(digitalio show | awk -F= '/Input=0x[a-zA-Z0-9]+/ {print $2}')
if test "$i" = "0x50ff" ; then
state="Active"
else
state="Passive"
fi
if test "$state" != "$last_state" ; then
printf '%s;%s\n' $(date '+%Y%m%d%H%M%S') "$state"
fi
last_state="$state"
usleep 500000
done
Sample output
20161019103534;Active
20161019103555;Passive
The script launches digitalio show command in an infinite loop, then extracts the hex part from Input lines with awk.
The $state variable is assigned to whether "Active", or "Passive" depending on the value of hex string.
The $last_state variable keeps the value of $state in the last iteration. If $state is not equal to $last_state, then the state is printed to the standard output in the specific format.

BASH: Listening for user input without waiting to do other processes

Ok so I have a simple script that has the structure down below. I have this sub process that will run for an indefinite amount of time. As of now, it will only stop when the user kills the process manually. I was wondering if there is a way to listen for input from the user without getting stuck on a read line. In other words, while the sub process is running, in the background, see if the user types "quit\n" in the terminal.
...
if [[ option = "3" ]]; then
while [2 -lt 4]; #THIS IS WHERE I WANT TO CHANGE TO LISTENING FOR INPUT TO QUIT
#Some sub process that continues to run
done
fi
....
Thanks for the help! Sorry if this is badly worded, I couldn't think of a better way to describe the issue. As for my brainstorming attempts, I know can just use a variable in the while condition and say do this until it equals "quit", but setting that variable to the input is where I get lost.
Not sure if I understand correctly, but are you looking for something like this?
while [[ 2 -lt 4 ]] ; do
read -t .1 user_input
[[ ${user_input} == "exit" ]] && exit 0
done
or perhaps simpler:
unset user_input
while [[ "${user_input}" != "exit" ]] ; do
read -t .1 user_input
done
In general you can probably achieve what you want with read -t.
-t timeout
Cause read to time out and return failure if a complete line of input (or a spec‐
ified number of characters) is not read within timeout seconds. timeout may be a
decimal number with a fractional portion following the decimal point. This
option is only effective if read is reading input from a terminal, pipe, or other
special file; it has no effect when reading from regular files. If read times
out, read saves any partial input read into the specified variable name. If
timeout is 0, read returns immediately, without trying to read any data. The
exit status is 0 if input is available on the specified file descriptor, non-zero
otherwise. The exit status is greater than 128 if the timeout is exceeded.

Simulating rolling dice on a dynamic webpage using Bash

I'm currently working on a Bash script that simulates rolling a number of 6-sided dice. This is all taking place within a virtual machine running Debian that's acting as a server. Essentially, my webpage simulates rolling the dice by using the query string to determing the number of dice to be rolled.
For instance, if my URL is http://127.0.0.1/cgi-bin/rolldice.sh?6, I want the webpage to say "You rolled 6 dice" and then, on the next line, print six numbers between 1 and 6 inclusive (that are of course "randomly" generated).
Currently, printing out the "You rolled x dice" header is working fine. However, I'm having trouble with the next part. I'm very new to Bash, so possibly the syntax or something similar is wrong with my loop. Here it is:
for i in {1..$QUERY_STRING }; do
dieRoll = $(( $RANDOM % 6 + 1))
echo $dieRoll
done
Can anyone help me figure out where I'm going wrong? I'll be happy to post the rest of rolldice.sh if needed.
Since .. requires its arguments to be literals, you have to use eval to substitute the variable:
for i in $(eval "echo {1..$QUERY_STRING}"); do
Or if you have the seq command, you can do:
for i in $(seq 1 "$QUERY_STRING")
I recommend the latter -- using eval with input from the user is very dangerous.

Handle "race-condition" between 2 cron tasks. What is the best approach?

I have a cron task that runs periodically. This task depends on a condition to be valid in order to complete its processing. In case it matters this condition is just a SELECT for specific records in the database. If the condition is not satisfied (i.e the SELECT does not return the result set expected) then the script exits immediately.
This is bad as the condition would be valid soon enough (don't know how soon but it will be valid due to the run of another script).
So I would like somehow to make the script more robust. I thought of 2 solutions:
Put a while loop and sleep constantly until the condition is
valid. This should work but it has the downside that once the script
is in the loop, it is out of control. So I though to additionally
after waking up to check is a specific file exists. If it does it
"understands" that the user wants to "force" stop it.
Once the script figures out that the condition is not valid yet it
appends a script in crontab and stops. That seconds script
continually polls for the condition and if the condition is valid
then restart the first script to restart its processing. This solution to me it seems to work but I am not sure if it is a good solution. E.g. perhaps programatically modifying the crontab is a bad idea?
Anyway, I thought that perhaps this problem is common and could have a standard solution, much better than the 2 I came up with. Does anyone have a better proposal? Which from my ideas would be best? I am not very experienced with cron tasks so there could be things/problems I could be overseeing.
instead of programmatically appending the crontab, you might want to consider using at to schedule the job to run again at some time in the future. If the script determines that it cannot do its job now, it can simply schedule itself to run again a few minutes (or a few hours, as it may) later by way of an at command.
Following up from our conversation in comments, you can take advantage of conditional execution in a cron entry. Supposing you want to branch based on time of day, you might use the output from date.
For example: this would always invoke the first command, then invoke the second command only if the clock hour is currently 11:
echo 'ScriptA running' ; [ $(date +%H) == 11 ] && echo 'ScriptB running'
More examples!
To check the return value from the first command:
echo 'ScriptA' ; [ $? == 0 ] echo 'ScriptB'
To instead check the STDOUT, you can use as colon as a noop and branch by capturing output with the same $() construct we used with date:
: ; [ $(echo 'ScriptA') == 'ScriptA' ] && echo 'ScriptB'
One downside on the last example: STDOUT from the first command won't be printed to the console. You could capture it to a variable which you echo out, or write it to a file with tee, if that's important.

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