Just enough bash to be dangerous

A bash cheat sheet for developers who just want to get by.

Writing setup, CI and deployment flows means a bit of the old bash scripting.

Despite my deep interest in the intricacies of Bash (/sarcasm), I’ve kept hitting up Google and StackOverflow for solutions to the same couple of situations.

To avoid having to do this again myself and for your reading pleasure, here they are.

To be dangerous in terms of setup, CI and depoyment flows we will encounter the following:

• Check if file exists
• Check if a (symbolic) link exists
• Check if environment variable is set
• Switch over an environment variable
• Prompt the user
• Bonus
  • Slap your .env files into your environment

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Check if file exists

if [! -f ./pdfgen/pdfgen]; then
    echo "Building pdfgen binary"
    npm run --prefix pdfgen build:linux
else
    echo "Pdfgen binary already exists, skipping build"
fi

Check if a (symbolic) link exists

if [! -L /usr/local/bin/heroku];
then
    wget https://cli-assets.heroku.com/branches/stable/heroku-linux-amd64.tar.gz
    sudo mkdir -p /usr/local/lib /usr/local/bin
    sudo tar -xvzf heroku-linux-amd64.tar.gz -C /usr/local/lib
    sudo ln -s /usr/local/lib/heroku/bin/heroku /usr/local/bin/heroku
fi

Check if environment variable is set

# long
if [[ -z "${CIRCLE_BRANCH}"] ]; then
    npm run redis-cli flushall
fi

npm run sync

# one-liner
[-z "${CIRCLE_BRANCH}"] && npm run redis-cli flushall; npm run sync

Switch over an environment variable

case $CIRCLE_BRANCH in
    "develop")
        export ENVIRONMENT="dev"
        export HEROKU_APP=dev-app
        ;;
    "staging")
        export ENVIRONMENT="staging"
        export HEROKU_APP=staging-app
        ;;
    "production")
        export ENVIRONMENT="production"
        export HEROKU_APP=production-app
        ;;
esac

Prompt the user

read -p "Are you sure you want to merge 'develop' into 'staging'? (y/N)" -n 1 -r
echo # we like having a new line

if [[$REPLY =~ ^[Yy]$ ]]
then
  git merge develop --ff-only
  git push
fi

A final bit of advice, if it’s more than a couple of lines, try to use something like JavaScript, or Python to write your script.

I’ve got some resources to do that in modern JavaScript/Node:

• ES6 by example: a module/CLI to wait for Postgres in docker-compose
• How to make beautiful, simple CLI apps with Node

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Bonus

Slap your .env files into your environment

We’ve got .env files laying around, Docker Compose deals with this for use usually but say we want to get something running outside of Docker (and without using something like dotenv).

Here’s the snippet for a *NIX shell:

export $(cat .env | xargs)

Guilherme Cunha

Comments

[Ben Sinclair]

You should put spaces between the test brackets and the test conditions like [ -f foo.txt ] otherwise you'll get a command not found error.

The environment variable one is over-complicated - you don't need the else, and you don't need to use false to ensure that the last command is run if you use a semicolon:

# long
if [[ -z "${CIRCLE_BRANCH}"] ]; then
    npm run redis-cli flushall
fi

npm run sync


# one-liner
[-z "${CIRCLE_BRANCH}"] && npm run redis-cli flushall; npm run sync

[Hugo Di Francesco]

Thanks I've updated it.

[Nick Cinger]

Good stuff! I feel like I know how to get around Bash, but I keep learning new tricks ever day, and I learned a few here, thanks!

Also, seeing that you have to end case block with the reverse esac just makes me giggle!

[Vlastimil Pospichal]

I don't like ask the user. The best answer is the one I will not ask.

[BLVKOBLSK]

BASH Docs are your best friend; especially peeping others legit scripts too of course.

Thanks for this insight too!