How do I get "previous executed command" in a bash script?

Question

I use multiple bash sessions, and I want to keep track of history from all of them in one file (I don't care that it is multiplexed from multiple sessions, I can always put a session identifier in front of it.) I have tried doing

shopt -s histappend

and also adding

history -a

to the $PROMPT_COMMAND variable. But none of them really work for me, and I don't understand why they don't work (they behave very non-deterministically as far as I can tell... sometimes they multiplex commands from multiple sessions, sometimes they don't).

The goal of this question is to explore an alternate way to keep history from all sessions, where I can control what I write to the history. The idea is to store "previous command" in a shell variable and then echo that variable to a history-log file inside the definition of PS1 variable.

The question is: How do I get the "previous executed command" in a shell variable. I know I can execute echo !! >> logfile.txt in interactive bash session to record it to a log file. But how do I do this in a script file (or .bashrc file)?

I have tried

PROMPT_COMMAND="PC=$_;"
PREVIOUS_COMMAND=$(echo $PC)  # $_ only gives the last argument of previous command
export PS1="[\u@\h \w] [$PREVIOUS_COMMAND $(echo $_) $_] $ "

But none of this works.

Thanks for your time, ~yogi

You can try something with bash's variable $BASH_COMMAND

Carlos Campderrós
– Carlos Campderrós

2013-04-11 10:10:43 +00:00
Commented Apr 11, 2013 at 10:10 — Carlos Campderrós
– Carlos Campderrós, Commented Apr 11, 2013 at 10:10

geekosaur · Accepted Answer · 2011-07-20 02:36:54Z

16

Something like

fc -ln -1

should work. That said, you're probably running into concurrent access issues (read: multiple shells overwriting each others' history) and you may not be able to do any better by hand.

answered Jul 20, 2011 at 2:36

geekosaur

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5 Comments

Yogeshwer Sharma Over a year ago

Wow! That was quite quick, and exactly what I was looking for. I have checked the concurrency issues already. I just asked two bash scripts to blast current timestamps and append them to a text file for 10 seconds. The output lines were in some order, but each line was from either one of the scripts, not a combination of two lines from two different scripts. I hope this is the right behaviour when you append to a text file.

Yogeshwer Sharma Over a year ago

I am now writing export PS1=[`fc -ln -1` : $(fc -ln -1) : `date`] and I don't get what I want. The date prints fine, but fc output does not. Any ideas what I am doing wrong?

geekosaur Over a year ago

Yu would need single quotes in there to prevent the `` ` `` or $() from being expanded during its definition. export PS1='[$(fc -ln -1) : $(date)] ' or something similar — although I have to wonder if some formatting went missing there.

Yogeshwer Sharma Over a year ago

Yeah, I had those quotes in place. Still not doing what it is supposed to do! :-(

LyrePyre Over a year ago

Dunno if anyone else might have my use case (I'm committing dynamic PS1 bullduggery), but modifying this answer to be fc -ln -0 instead of -1 was the ultimate solution. Prints back the command that was just entered, instead of the one before it.

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