$ history
    1  history
$ a
bash: a: command not found
$ b
bash: b: command not found
$ a
bash: a: command not found
$ history
    1  history
    2  a
    3  b
    4  a
    5  history
$ a
bash: a: command not found
$ a
bash: a: command not found
$ a
bash: a: command not found
$ history
    1  history
    2  a
    3  b
    4  a
    5  history
    6  a
    7  a
    8  a
    9  history