How do I prevent commands from showing up in Bash history?

If you've set the HISTCONTROL environment variable to ignoreboth (which is usually set by default), commands with a leading space character will not be stored in the history (as well as duplicates).

For example:

$ HISTCONTROL=ignoreboth
$ echo test1
$  echo test2
$ history | tail -n2
 1015  echo test1
 1016  history | tail -n2

Here is what man bash says:

HISTCONTROL

A colon-separated list of values controlling how commands are saved on the history list. If the list of values includes ignorespace, lines which begin with a space character are not saved in the history list. A value of ignoredups causes lines matching the previous history entry to not be saved. A value of ignoreboth is shorthand for ignorespace and ignoredups. A value of erasedups causes all previous lines matching the current line to be removed from the history list before that line is saved. Any value not in the above list is ignored. If HISTCONTROL is unset, or does not include a valid value, all lines read by the shell parser are saved on the history list, subject to the value of HISTIGNORE. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTCONTROL.

See also:

• Why is bash not storing commands that start with spaces? at unix SE
• Why does bash have a HISTCONTROL=ignorespace option? at unix SE

In your .bashrc/.bash_profile/wherever you want, put export HISTIGNORE=' *'. Then just begin any command you want to ignore with one space.

$ ls  # goes in history
$  ls # does not