Lets say someone sits behind my laptop for a second and runs:
alias cd='Ha Ha, Got You :))'
or we run an unknown software/script/etc and it appends something to ~/.bashrc. like:
alias sort='rm -rf ~'
These are only examples of aliases; As you know, these things also can be done using functions:
cd(){ echo "Removing everything you've got :D"; }
These situations are just imaginary examples, consider anything similar.
What about a small script?
sudo -n ls &>/dev/null
if [ "$?" -eq "0" ]
then
sudo "Some dangerous command"
else
cd $1
fi
Then alias cd="/home/user/.config/gtk/.cd.sh".
For the commands which have been ran in bash we can simply close and reopen our terminal, but what about the ones that been set in startup files, we can't check the files or list of aliases/functions every single time we run a terminal.
2 Answers
Introduction
Bash configuration files
Bash has a bunch of configuration (aka startup) files, it uses these files to setup a specific environment for each user.
Some of these files are located at /etc, one of them that I'm aware of is /etc/profile, it's a global configuration file and its settings will be applied into all sessions, another is /etc/bash.bashrc; We don't need to work around these files because of their location they already are protected and only root has the rights to edit them.
A very important directory which can help us a lot is: /etc/skel; Whenever you create a new user with home directory, the files within this directory will be used as a skeleton for your new user's home directory.
ls -a /etc/skel
.bash_logout .bashrc .profile
We can also use dpkg to find about these files:
$ dpkg -L bash | grep etc
/etc/skel/.bashrc
/etc/skel/.bash_logout
/etc/skel/.profile
/etc/bash.bashrc
we can see that these all are installed by bash.
How thing works in bash
An alias or function can be set in any of these files, so let see how these files will be used by bash.
From bash man page:
When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for
~/.bash_profile,~/.bash_login, and~/.profile, in that order, and reads and exe‐cutes commands from the first one that exists and is readable.
so the order is: ~/.bash_profile > ~/.bash_login > ~/.profile
When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists.
this one will be run every time we exit from a login shell, I can't see how this one can has any effects on our situation.
When an interactive shell that is not a login shell is started, bash reads and executes commands from /etc/bash.bashrc and ~/.bashrc, if these files exist.
so the most important fiel is ~/.bashrc, because almost 90% of bash shells which we run are in interactive and no-login mode. and if we have a look at this file we can see that it will look for another file named ~/.bash_aliases, if it was able to locate it, then it will source that file too.
