A probably overly complicated way to have multiple shells at the same time access a file, locked for sequential access, in which each shell would receive one line from that file. An example use-case is having 5 shells that are controlled at once to connect to 5 different ips in a file with the one command, this will output a different ip per she…

linegrabber.sh

#!/bin/bash

FILE="ipsToConnectTo"

lineContents() {
  # Checks if a line has a * pointer next to it to denote which is the next line that should be returned.
  LN=$(grep --color=never -n \* $FILE | cut -f1 -d\:)

  # If there is no * pointer it needs a manual reset to the right starting location. A precaution for how I used it but easily customisable.
  if [ "${LN}" == "" ] ; then
    echo '$LN isnt set reset the file'
    exit 1
  fi

  # Current line with the * pointer.
  IP=$(grep --color=never \* $FILE | cut -f1 -d\*)
  echo $IP

  # Set the next line to have the * pointer.
  sed -ie $(expr $LN + 1)'s/$/*&/' $FILE
  # Remove the current lines * pointer.
  sed -ie $LN's:\*::g' $FILE
}

# The below lock system has been adapted from: https://blog.famzah.net/2013/07/31/using-flock-in-bash-without-invoking-a-subshell/
# Used for sequential access from the shells calling the script.
exec {lock_fd}>/tmp/linegrabber || exit 1
flock -w 3 "$lock_fd" || { echo "ERROR: flock() failed." >&2; exit 1; }

# Runs my function, the point of the script.
lineContents

# Unlock file. Unneeded as it's at the end and the process dying will release the lock but a precaution.
flock -u "$lock_fd"