Bash – Duplicates in bash history

I have concluded that it is not possible to append the new commands directly and at the same time to remove any duplicates. So my solution is to run the following command ones in a while. or to put it in my crontab.

tac $HOME/.bash_history | awk '!seen[$0]++' | tac > $HOME/.hist_Temp 
mv $HOME/.hist_Temp $HOME/.bash_history

The above command will keep the last occurrence of a command and remove every other repetitions above it from the history file without messing up the order.

Yes, this is using history.  !# is a history event designator that refers to the entire command line typed so far.  :* is a word (range) designator that refers to all of the words, except the 0th.  So, after you have typed echo "This is a sentence. ", then !#:* expands to "This is a sentence.  ".  And x-y (where x and y are integers) is a word (range) designator that refers to word number x through word number y.  If y is omitted (x-), this is interpreted to mean word number x through the second to last word.  So, after your “entire command line typed so far” stands as

echo "This is a sentence. " "This is a sentence. "

then !#:1- expands to "This is a sentence. ", because each of the quoted "This is a sentence. " strings counts as one word, and so !#:1- is equivalent to !#:1 (just word number 1).  So you end up with

echo "This is a sentence. " "This is a sentence. " "This is a sentence. " >text3

The fact that the - and the > appear together in the command is just a confusion; they don’t interact.  And the fact that “This is a sentence.” is quoted obscures what is going on; if you said

echo This is a sentence. !#:* !#:1-

it would expand to

echo This is a sentence. This is a sentence. !#:1-

and thence to

echo This is a sentence. This is a sentence. This is a sentence. This is a

(because !#:1- expands to word number 1 through the second to last word.)