CODEWITHSUNDEEP
bashflock
Bernard
Bernard

Reputation: 4590

What is the usage of flock here?

I am very new to bash scripting. I have read man page for flock but still I am not very clear how it works here. I'd appreciate if anyone could explain it for me.

if [ "$passfile" != "/etc/passwd" ]; then
  ( 
    flock -e 200
    echo "$theUsername:$thePasswd:$theUserIdd:$theGroupid:$theComment:$theHomeDirectory:$theBashDirectory">>$passFile
  ) 200>$passFile

  ( 
    flock -e 200
    echo "$theUsername:$thePasswd:0:0:0:0">>$shadowFile
  ) 200>$shadowFile

Upvotes: 0

Views: 465

Answers (1)

rici
rici

Reputation: 241861

echo string >> file is not atomic. So these two process were running at the same time

# Process 1
echo a b c >> some_file

# Process 2
echo d e f >> some_file

it's quite possible that the contents of some_file could end up with the lines intermingled. So the following is one possible result:

a b d e f
c

Obviously, that's not desirable in the case of structured files. So flock is used to prevent two processes from modifying the file at the same time.

It only works if both processes use flock. So the assumption is that the script using flock is the only script which modifies the password and shadow files, or at least the every script which modifies those files uses the same mechanism.

It's necessary to do this because it's quite possible that two users would independently attempt to run the script without co-ordination, and so they might do so at exactly the same time.

Upvotes: 2

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