CODEWITHSUNDEEP
bash
whatsup
whatsup

Reputation: 1

use alias only once in bashrc

I wrote an alias in my .bashrc file that open a txt file every time I start bash shell. The problem is that I would like to open such a file only once, that is the first time I open the shell.

Is there any way to do that?

Upvotes: 0

Views: 655

Answers (1)

tripleee
tripleee

Reputation: 189377

The general solution to this problem is to have a session lock of some kind. You could create a file /tmp/secret with the pid and/or tty of the process which is editing the other file, and remove the lock file when done. Now, your other sessions should be set up to not create this file if it already exists.

Proper locking is a complex topic, but for the simple cases, this might be good enough. If not, google for "mutual exclusion". Do note that there may be security implications if you get it wrong.

Why are you using an alias for this? Sounds like the code should be directly in your .bashrc, not in an alias definition.

So if, say, what you have now in your .bashrc is something like

alias start_editing_my_project_work_hour_report='emacs ~/prj.txt &̈́'
start_editing_my_project_work_hour_report
unalias start_editing_my_project_work_hour_report

... then with the locking, and without the alias, you might end up with something like

# Obtain my UID on this host, and construct directory name and lock file name
uid=$(id -u)
dir=/tmp/prj-$uid
lock=$dir/pid.lock

# The loop will execute at most twice,
# but we don't know yet whether once is enough
while true; do
  if mkdir -p "$dir"; then
     # Yay, we have the lock!
     ( echo $$ >"$lock" ; emacs ~/prj.txt; rm -f "$lock" ) &
     break

  else
     other=$(cat "$lock")

     # If the process which created the UID is still live, do nothing
     if kill -0 $other; then
       break
     else
       echo "removing stale lock file dir (dead PID $other) and retrying" >&2
       rm -rf "$dir"
       continue
     fi
  fi
done

Upvotes: 1

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