Just a few things I find helpful :)

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Bash history with timestamp.

This is more of an audit thing, but sometimes it's useful to know when you did something.

If you type the command history into your shell.  You get back a list of the last X number of command executed.

$ history | tail -n1
502 history | tail -n1

But by adding the following to your ~/.bashrc (local user) or /etc/bash/bashrc (all users) file, you can inject the time/date.  Please see man date for the options you can use.

HISTTIMEFORMAT='%F %T '

Example

$ history | tail -n1
508 2017-01-14 11:29:58 history | tail -n1

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Bash History Duplicate Removal

It's an annoyance when you execute a command consecutively many time and then have to search further back to get to the last different command.  Behold!  Add this to your .bashrc(local user) or /etc/bash/bashrc (all users) and no matter how many times you execute that command consecutively, it will only store the one time.

HISTCONTROL=ignoreboth

Example

user@server ~ $ vi .bashrc
user@server ~ $ vi .bashrc
user@server ~ $ vi .bashrc
user@server ~ $ vi .bashrc
user@server ~ $ history | tail -n5
508 2017-01-14 11:29:58 history | tail -n1
509 2017-01-14 11:31:36 man date
510 2017-01-14 11:32:43 history
511 2017-01-14 11:40:52 vi .bashrc
512  2017-01-14 11:42:36 history | tail -n5

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Double tap exit/logoff

Accidentally logging out of a shell session or user session can be annoying.  But there is hope :)  Add this to your .bashrc(local user) or /etc/bash/bashrc (all users) and now you have to double tap to get out.

This one ideally needs to go into the /etc/bash/bashrc or else it would only work when closing your own shell session.

IGNOREEOF=1

Example

server ~ # Use "logout" to leave the shell.
server ~ # logout
user@server ~ $

Extract Path or File

$ export VAR=/home/me/mydir/file.c

$ echo "${VAR%/*}"
/home/me/mydir

$ echo "${VAR##*/}"
file.c

Cron Execution Only

Pop this at the top of a bash script.

Eg. The below is for a cronjob that is executed at reboot:

# crontab -l
@reboot /bin/bash /path/to/something/bad.sh

#!/usr/bin/env bash

if [[ ! $(< /proc/${PPID}/comm) == cron* ]]; then
  echo "Do NOT manually execute this script.  ONLY at reboot!!!!"
  exit
fi