mjhjmtu

I want to enable my command history across all terminal tabs and windows to be recorded in .bash_history by setting PROMPT_COMMAND in .profile:

1. export PROMPT_COMMAND="history -a; history -c; history -r;$PROMPT_COMMAND"

However, when I check whether this environment variable is already set, I get:

echo $PROMPT_COMMAND
printf "33]0;%s@%s:%s07" "$USER" "$HOSTNAME%%.*" "$PWD/#$HOME/~"

Will exporting PROMPT_COMMAND like this overwrite my existing $PROMPT_COMMAND list or is it necessary to prefix the value of PROMPT_COMMAND with a : before exporting it?

Is $PROMPT_COMMAND a colon separated list?

That's easy enough to test:

$ PROMPT_COMMAND='true:true' bash 
bash: true:true: command not found
$ exit

So, the answer is "no".

But you could take it as a semicolon separated sequence of commands, as any other line of shell code:

$ PROMPT_COMMAND='echo x;echo y' bash 
x
y
$ exit

That's what the assignment in your question has: a number of commands, separated by semicolons, with the earlier value of PROMPT_COMMAND tacked on to the end.

Of course, another way to run multiple commands from PROMPT_COMMAND, would be to make a function and call it from there.

That said, the printf sequence in your PROMPT_COMMAND looks like something that might be better placed in the actual prompt instead, for two reasons. First, it doesn't end in a newline, so it may mess up Bash's idea of where the cursor is, just like other commands that output incomplete lines before exiting. Second, if you have the shell re-print the prompt, through tab-completion, PS1 will be redisplayed, but PROMPT_COMMAND will not run again.