Replace grep command with grep -n --colour?

In your $HOME/.bashrc file, add:

export GREP_OPTIONS="-n --color"

This will work with egrep too and in aliases that use grep.

Update: GREP_OPTIONS is deprecated from grep 2.21 on and if you use this solution you will see the following warning:

grep: warning: GREP_OPTIONS is deprecated; please use an alias or script

The best solution is to therefore follow maco's advice in his answer. Or switch to using ag, which outputs in color by default.

Edit ~/.bash_aliases Add a line that says:

alias grep='grep -n --color'

~/.bash_aliases is automatically included by ~/.bashrc if it exists

you can modify the file .bashrc located in your home directory defining an alias, which will override any default setting:

alias grep='grep -n --color'

after the change close and open the terminal again because the file is read only when you open the terminal.

If you take a look on the file .bashrc you will found more default aliases like:

alias ll='ls -l'

alias la='ls -A'

alias l='ls -CF'