Lock a file in bash using flock and lockfile

Question

i spent the better part of the day looking for a solution to this problem and i think i am nearing the brink ... What i need to do in bash is: write 1 script that will periodicly read your inputs and write them into a file and second script that will periodicly print out the complete file BUT only when something new gets written in, meaning it will never write 2 same outputs 1 after another. 2 scripts need to comunicate by the means of a lock, meaning script 1 will lock a file so that script 2 cant print anything out of it, then script 1 will write something new into that file and unlock it ( and then script 2 can print updated file ). The only hints we got was the usage of flock and lockfile - didnt get any hints on how to use them, exept that problem MUST be solved by flock or lockfile.

edit: When i said i was looking for a solution i ment i tried every single combination of flock with those flags and i just couldnt get it to work.

I will write pseudo code of what i want to do. A thing to note here is that this pseudocode is basicly the same as it is done in C .. its so simple, i dont know why everything has to be so complicated in bash.

script 1:

place a lock on file text.txt ( no one else can read it or write to it) read input place that input into file ( not deleting previous text ) remove lock on file text.txt repeat

script 2:

print out complete text.txt ( but only if it is not locked, if it is locked obviously you cant) repeat

And since script 2 is repeating all the time, it should print the complete text.txt ONLY when something new was writen to it.

I have about 100 other commands like flock that i have to learn in a very short time and i spent 1 day only for 1 of those commands. It would be kind of you to at least give me a hint. As for man page ...

I tried to do something like flock -x text.txt -c read > text.txt, tried every other combination also, but nothing works. It takes only 1 command, wont accept arguments. I dont even know why there is an option for command. I just want it to place a lock on file, write into it and then unlock it. In c it only takes flock("text.txt", ..).

Answer 1

Let's look at what this does:

flock -x text.txt -c read > text.txt

First, it opens test.txt for write (and truncates all contents) -- before doing anything else, including calling flock!

Second, it tells flock to get an exclusive lock on the file and run the command read.

However, read is a shell builtin, not an external command -- so it can't be called by a non-shell process at all, mooting any effect that it might otherwise have had.

---

Now, let's try using flock the way the man page suggests using it:

{
  flock -x 3                       # grab a lock on file descriptor #3
  printf "Input to add to file: "  # Prompt user
  read -r new_input                # Read input from user
  printf '%s\n' "$new_input" >&3   # Write new content to the FD
} 3>>text.txt                      # do all this with FD 3 open to text.txt

...and, on the read end:

{
  flock -s 3  # wait for a read lock
  cat <&3     # read contents of the file from FD 3
} 3<text.txt  # all of this with text.txt open to FD 3

You'll notice some differences from what you were trying before:

• The file descriptor used to grab the lock is in append mode (when writing to the end), or in read mode (when reading), so you aren't overwriting the file before you even grab the lock.
• We're running the read command (which, again, is a shell builtin, and so can only be run directly by the shell) by the shell directly, rather than telling the flock command to invoke it via the execve syscall (which is, again, impossible).