Bash & Linux Helpers

It seems that many article tutorials on bash are ugly, are suboptimal, and are no straight forward. My goal is for this file to be a reference file whenever I need to do scripting after a long time. One important concept in bash, is that an exit code of 0 indicates True, not False. Which seems counterintuitive if you program in any non-scripting language like C/C++ or Python.

Bash - Argument Parsing and Flags

If you don’t get what’s going on, read my bash quickstart article

Concepts

getopts is overkill, so here is an alternative.
“Booleans” using string comparison
Conditionals (if, elif, else, AND &&, OR ||)
For loop

supplied_arg="default"
# iterate all arguments
for arg in "$@"; do
    # use [[ ]] to avoid double quoting variables
    if [[ $last_arg = "--supply" ]]; then
        supplied_arg="$arg"
        last_arg=""
    # OR ||
    elif [[ $arg = "--flag1" ]] || [[ $arg = "--flag" ]]; then
        flag1=true
    # elif, AND &&
    elif [[ $arg = "--override" ]] && [[ ! $supplied_arg = "default" ]]; then
        # ignore the redundancy, this is to showcase &&
        flag_override=true
    else
        # you can do something with these args here
        # wc -l "$arg"
        # or you can set last_arg in order to parse `--something ARG`
        last_arg="$arg"
    # end of if statement
    fi
# done loop (for or while)
done

# check if flag supplied
if [ flag1 = true]; then
    echo "got --flag1 or --flag"
else
    notGot="got neither --flag1 nor --flag"
fi
# flag not supplied (since variable was never set, we check ! = true rather than = false)
if [ ! flag1 = true ]; then
    echo $notGot
fi

Linux - Add to PATH

A better approach than appending a statement to PATH="$PATH:/new/path">> ~/.bashrc
TODO: add instructions

Linux - Add Cronjob

automated approach
adds the cronjob if not exists
prevents concurrent/overlap runs

add_cronjob() {
    # crontab will not set the working directory
    # create locks directory if it does not exist
    mkdir -p ~/locks
    # my sample job uses flock to prevent overlapping runs
    # minute hour day month day_of_week
    # /X means divisible by X
    cronjob="*/1 * * * * flock -n ~/locks/auto_deploy_$PROJECT.lock $PYTHON $(pwd)/cronjob.py"
    # cronjob.py will set the working directory
    if ! crontab -l &>/dev/null; then
        # crontab file does not exist
        echo "$cronjob" | crontab -
        echo "Created new crontab with job"
    elif ! crontab -l 2>/dev/null | grep -Fq "$cronjob"; then
        # job not found in crontab
        echo $(crontab -l ; echo "$cronjob") | crontab -
        echo "Added $cronjob ... to existing crontab"
    else
        echo "Job already exists in crontab"
    fi
}

Linux - Add Systemd Service

I used lots of echos and tee [-a]
In order to work with spaces in an argument, I created strings with \'$VAR\'
TODO: add code

Linux - Auto Setup Certbot

# Context:
# define $DOMAIN (e.g. lenerva.com)
install_certbot() {
    sudo snap install core; sudo snap refresh core
    sudo snap install --classic certbot
    sudo ln -s /snap/bin/certbot /usr/bin/certbot
    sudo certbot --nginx -d $DOMAIN -d www.$DOMAIN
}

Linux - Auto Configure NGINX - Gunicorn

This works for me but it is possible to fail for you without editing /etc/nginx/nginx.conf manually server name hashes to 128

# Context:
# define $DOMAIN beforehand (e.g. lenerva.com)
# define $PROJECT beforehand (e.g. my-project)
# pwd is currently the project root directory
configure_nginx() {
    # configure nginx
    sudo apt install nginx -y

    sudo mkdir -p /var/www/$DOMAIN/html
    sudo chown -R "$USER":"$USER" /var/www/$DOMAIN/html
    sudo chmod -R 755 /var/www/$DOMAIN

    echo "server {" | sudo tee /etc/nginx/sites-available/$PROJECT >/dev/null
    echo "  listen [::]:80;" | sudo tee -a /etc/nginx/sites-available/$PROJECT >/dev/null
    echo "  listen 80;" | sudo tee -a /etc/nginx/sites-available/$PROJECT >/dev/null
    echo "  location / {" | sudo tee -a /etc/nginx/sites-available/$PROJECT >/dev/null
    echo "    include proxy_params;" | sudo tee -a /etc/nginx/sites-available/$PROJECT >/dev/null
    echo "    proxy_pass http://unix:$(pwd)/gunicorn.sock;" | sudo tee -a /etc/nginx/sites-available/$PROJECT >/dev/null
    echo "  }" | sudo tee -a /etc/nginx/sites-available/$PROJECT >/dev/null
    echo "}" | sudo tee -a /etc/nginx/sites-available/$PROJECT >/dev/null

    sudo ln -s "/etc/nginx/sites-available/$PROJECT" /etc/nginx/sites-enabled

    # TODO: if publishing script, also set max server name hashes to 128 in /etc/nginx/nginx.conf
    # uncomment below if it didn't work
    # nano /etc/nginx/nginx.conf
    sudo systemctl restart nginx
    # Firewall
    sudo ufw allow 'Nginx Full'
}

February 14, 2022

https://blog.elijahlopez.ca/posts/bash-helpers/ Elijah Lopez