5 commands that take servers down

Quick reference

1. Don't

$rm -rf $APP_DIR/

If APP_DIR is empty, this becomes rm -rf /.

Do instead

$rm -rf "${APP_DIR:?}"/

${VAR:?} stops the command if the variable is empty.

2. Don't

$crontab -r

One key away from -e. Deletes the whole crontab, no question.

Do instead

$crontab -l > ~/crontab.bak

Back it up first, then edit with crontab -e.

3. Don't

$echo '10.0.0.5 db' > /etc/hosts

\> replaces the whole file. localhost is gone too.

Do instead

$echo '10.0.0.5 db' >> /etc/hosts

\>> appends. Better still, copy the file first.

4. Don't

$systemctl restart sshd

One typo in sshd_config and nobody can log in again.

Do instead

$sshd -t && systemctl reload sshd

Test the config first. Keep a second session open.

5. Don't

$reboot

In the wrong terminal tab, that is production.

Do instead

$apt install molly-guard

Asks for the host name before a reboot over SSH.

Explained

1. rm -rf $APP_DIR/ If APP_DIR is empty or unset, the shell runs rm -rf /. Instead: rm -rf "${APP_DIR:?}"/ — the :? makes the shell stop with an error if the variable is empty.

1. crontab -r r and e sit right next to each other on the keyboard. -e edits. -r deletes the whole crontab without asking. Instead: crontab -l > ~/crontab.bak before you touch it, and keep that habit.

1. echo '10.0.0.5 db' > /etc/hosts A single > replaces the entire file. Your localhost line is gone, and strange things start to break. Instead: >> appends. And for any system file, cp it to a .bak first.

1. systemctl restart sshd After editing sshd_config, one typo means the SSH daemon does not come back, and on a remote server that can mean no way in. Instead: sshd -t to test the config, then reload, with a second session still open.

1. reboot Typed into the wrong terminal tab, that is a production outage. Instead: install molly-guard on Debian or Ubuntu. Over SSH, it asks you to type the host name before a reboot or shutdown goes through.

Before anything risky, three commands: hostname, whoami, pwd. Where am I, who am I, and where exactly will this run?

In one line

Before anything risky

$hostname; whoami; pwd