Friday, October 29, 2021

[SOLVED] How to avoid race conditions in a bash script?

Issue

#!/bin/bash
if [ ! -f numbers ]; then echo 0 > numbers; fi
count=0
touch numbers
echo $count > numbers
while [[ $count != 100 ]]; do
  if ln numbers numbers.lock
  then
    count=`expr $count + 1`
    n=`tail -1 numbers`
    expr $n + 1 >> numbers
    rm numbers.lock
  fi
done

What can I do to avoid the race condition of count=`expr $count + 1` and the n=`tail -1 numbers`, so that when I run two of the scripts at the same time, it only goes to 100, and not 200. I've researched on multiple websites, but there are no concise answers without making a huge function.


Solution

You are already safely avoiding the actual race condition with the lock file. The problem you are describing can be avoided two ways.

(1) Move the lock file outside the main loop, so that two instances of your program cannot run their main loop at the same time. If one is running, the other will have to wait until it's done, then start replacing the output file.

#!/bin/bash

# FIXME: broken, see comments

while true; do
    if ! ln numbers numbers.lock
    then
       sleep 1
    else
        if [ ! -f numbers ]; then echo 0 > numbers; fi
        count=0
        touch numbers
        #echo $count > numbers   # needless, isn't it?
        while [[ $count != 100 ]]; do
            count=`expr $count + 1`
            n=`tail -1 numbers`
            expr $n + 1 >> numbers
            rm numbers.lock
        done
        break
    fi
done

(2) Make the two instances cooperate, by examining what the contents of the file are. In other words, force them to stop looping when the number reaches 100, regardless of how many other processes are writing to this file. (I guess there is an iffy corner case when there are more than 100 instances running.)

#!/bin/bash
# FIXME: should properly lock here, too
if [ ! -f numbers ]; then echo 0 > numbers; fi
n=0
touch numbers
while [[ $n -lt 100 ]]; do
  if ln numbers numbers.lock
  then
    n=$(expr $(tail -1 numbers) + 1 | tee numbers)
    rm numbers.lock
  fi
done

Depending on your requirements, you might actually want the script to clobber any previous value in the file when a new instance of the script is starting, but if not, the echo 0 > numbers should be governed by the lock file, too.

You really want to avoid expr in a Bash script; Bash has built-in arithmetic operators. I have not attempted to refactor that part here, but you probably should. Perhaps prefer Awk so you can factor out the tail too; awk '{ i=$0 } END { print 1+i }' numbers



Answered By - tripleee
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