Automate the boring work in the shell

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$ als gewone gebruiker# als root (of met sudo)

Vaardigheden in dit hoofdstuk

Shell-automatisering — 8

bash — the glue of every pipeline

bash scripts join the other tools: if, loops, functions, exit codes.

Used for: CI steps, entrypoints of containers, small ops scripts.

$cat deploy.sh
#!/usr/bin/env bash
set -euo pipefail
kubectl apply -k "deploy/overlays/${ENV:?ENV is not set}"
kubectl rollout status deploy/shop --timeout=120s
$bash -n deploy.sh && shellcheck deploy.sh

Good practice:

Start every script with set -euo pipefail.
Quote variables ("$file"); check scripts with shellcheck.

3.2

curl — talk to every API

curl sends HTTP requests: health checks, REST APIs, webhooks, downloads.

Used for: smoke tests after a deploy, API calls in scripts, debugging.

$curl -fsS https://staging.example.com/health
$curl -s -o /dev/null -w '%{http_code} %{time_total}\n' https://staging.example.com/
$curl -fsS -H "Authorization: Bearer $TOKEN" https://api.example.com/v1/items
$curl -fsSL https://get.example.com/install.sh -o install.sh

Good practice:

-f makes curl fail on HTTP errors — needed in scripts.
Read install scripts before you run them; never curl | sudo bash blindly.

3.3

jq — read and change JSON

jq takes one value out of JSON — from APIs, kubectl -o json or cloud CLIs.

$kubectl get pods -A -o json | jq -r '.items[].spec.containers[].image' | sort -u
$terraform output -json | jq -r '.lb_ip.value'
$jq -n --arg v "$TAG" '{version: $v}'

Good practice:

-r for raw strings in shell variables; -e for checks.

3.4

yq — read and change YAML

yq is jq for YAML: set an image tag, read a Helm value, merge files.

$yq '.image.tag' values.yaml
$TAG=v1.4.0 yq -i '.image.tag = strenv(TAG)' values.yaml
$yq -o=json deploy.yaml

Good practice:

Use strenv() for versions so 1.10 stays a string.

3.5

xargs — run a command for many items

xargs turns lines into arguments, in batches or in parallel.

$cat images.txt | xargs -P 4 -n 1 docker pull
$find . -name '*.log' -mtime +14 -print0 | xargs -0 -r rm -f
$cat hosts.txt | xargs -I {} ssh {} uptime

Good practice:

Always -0 with find -print0, and -r before rm.

3.6

envsubst — tiny templates

envsubst fills $VARIABLES in a file with environment values.

$export TAG=v1.4.0
$envsubst < deploy.tpl.yaml | kubectl apply -f -
$envsubst '$SERVER_NAME' < nginx.conf.tpl > shop.conf

Good practice:

Name the variables to replace, so other $ words stay.

3.7

watch — a live view

watch runs a command every few seconds and shows what changed.

$watch -n 1 kubectl get pods
$watch -d 'df -h /var'
$watch -g 'kubectl get job migrate -o jsonpath={.status.succeeded}'

3.8

tmux — sessions that survive

tmux keeps a terminal session alive on the server when SSH drops, and splits the screen.

$tmux new -A -s main
$tmux ls
$tmux attach -t main

Good practice:

Start every long remote job (upgrade, migration, big copy) inside tmux.

Onthoud

A 10-line script with set -euo pipefail, curl -f, jq -e and clear exit codes is often better than a big tool.

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