reload .bashrc settings without logging out and back in
August 10, 2018 - Reading time: 4 minutes

You can enter the long form command:

source ~/.bashrc

or you can use the shorter version of the command:

. ~/.bashrc

or you could use:
exec bash

To complement and contrast the above commands with . ~/.bashrc and exec bash:

Both solutions effectively reload ~/.bashrc, but there are differences:

  • . ~/.bashrc or source ~/.bashrc will preserve your current shell session:

    • Except for the modifications that reloading ~/.bashrc into the current shell (sourcing) makes, the current shell process and its state are preserved, which includes environment variables, shell variables, shell options, shell functions, and command history.

  • exec bash, or, more robustly, exec "$BASH"[1], will replace your current shell with a new instance, and therefore only preserve your current shell's environment variables (including ones you've defined ad hoc, in-session).

    • In other words: Any ad-hoc changes to the current shell in terms of shell variables, shell functions, shell options, command history are lost.

Depending on your needs, one or the other approach may be preferred.

[1] exec bash could in theory execute a different bash executable than the one that started the current shell, if it happens to exist in a directory listed earlier in the $PATH. Since special variable $BASH always contains the full path of the executable that started the current shell, exec "$BASH" is guaranteed to use the same executable.
A note re "..." around $BASH: double-quoting ensures that the variable value is used as-is, without interpretation by Bash; if the value has no embedded spaces or other shell metacharacters (which is not likely in this case), you don't strictly need double quotes, but using them is a good habit to form.

how to download specific files from some url path with wget
July 11, 2018 - Reading time: ~1 minute

wget -r -l1 --no-parent -A ".deb" http://www.shinken-monitoring.org/pub/debian/

-r recursively
-l1 to a maximum depth of 1
--no-parent ignore links to a higher directory
-A "*.deb" your pattern

generate a list of a site's URLs using wget
June 12, 2018 - Reading time: ~1 minute

You can use wget to generate a list of the URLs on a website.

Spider example.com, writing URLs to urls.txt, filtering out common media files (css, js, etc..):

wget --spider -r http://www.example.com 2>&1 | grep '^--' | awk '{ print $3 }' | grep -v '\.\(css\|js\|png\|gif\|jpg\|JPG\)$' > urls.txt

Note that this gives a list that duplicates URLs.

If you mirror instead of spider you seem to get a more comprehensive list without duplicates:

wget -m http://www.example.com 2>&1 | grep '^--' | awk '{ print $3 }' | grep -v '\.\(css\|js\|png\|gif\|jpg\|JPG\)$' > urls.txt

This will download all pages of the site into a directory with the same name as the domain.

how to see time stamps in bash history
April 9, 2018 - Reading time: 2 minutes

Press Ctrl+Alt+T to open a terminal, then run one of the commands below:

HISTTIMEFORMAT="%d/%m/%y %T "  # for e.g. “29/02/99 23:59:59”
HISTTIMEFORMAT="%F %T "        # for e.g. “1999-02-29 23:59:59”

To make the change permanent for the current user run:

echo 'HISTTIMEFORMAT="%d/%m/%y %T "' >> ~/.bashrc  # or respectively
echo 'HISTTIMEFORMAT="%F %T "' >> ~/.bashrc
source ~/.bashrc

To test the effects run:

history

For commands that were run before HISTTIMEFORMAT was set, the current time will be saved as the timestamp. Commands run after HISTTIMEFORMAT was set will have the proper timestamp saved.

switch monitors from the command line
February 20, 2018 - Reading time: ~1 minute

With the commands

xrandr --output VGA-0 --auto
xrandr --output LVDS --off

The screen automatically transfers to the external display. It doesn't even need sudo powers. To find out the name of the displays just do:

xrandr -q

Which should give something like:

VGA-0 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 338mm x 270mm
...
LVDS connected (normal left inverted right x axis y axis)
...

Extending the displays can probably be achieved in a similar manner.

how to recursively delete empty directories in home directory
December 11, 2015 - Reading time: ~1 minute

The find command is the primary tool for recursive file system operations. Use the -type d expression to tell find you're interested in finding directories only (and not plain files). The GNU version of find supports the -empty test, so

$ find . -type d -empty -print

will print all empty directories below your current directory.

Use find ~ -… or find "$HOME" -… to base the search on your home directory (if it isn't your current directory).

After you've verified that this is selecting the correct directories, use -delete to delete all matches:

$ find . -type d -empty -delete

I would add -mindepth 1 here, to prevent from deleting the starting directory itself, if it would be empty. It's not really probable case for $HOME but if you would use this on any other directory..

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