💻 Those bash commands we love

May 22, 2018

terminal kubernetes docker bash automation

The era of the terminal

Looking back at the time of the mainframe with dummy terminal clients and their black/green screen, it makes me wonder if things really change that much. I guess it’s like in everything, some things change, some don’t. Deal with it.

If you spend a decent amount of time in a terminal, you might have realized that there are some tasks that are tedious and error prone such as copy pasting a super long string (ssh public key) or some pods name from a kubernetes cluster. Those errors are wasting precious time that could be allocated to higher quest like indenting YAML in a manifest.

Variables

The advantages of environment variables over arguments:

order doesn’t matter
can be shared by more than one script
it isn’t stored anywhere (unless you keep your history)
No code needed as opposed to a CLI made with Golang/Node.js/Python/…

Notice that there is no space on each side of the ‘=’

AWS_PROFILE=user2 aws ec2 describe-instances  \\
                  --instance-ids i-0787e4282810ef9cf \\
                  --query 'Reservations[0].Instances[0].PublicIpAddress'
# OUTPUT:
#"54.183.22.255"

## Or just to store things for later use
INTERNAL_IP=$(curl -s -H "Metadata-Flavor: Google" \
  http://metadata.google.internal/computeMetadata/v1/instance/network-interfaces/0/ip)

Looping

# Loop over files in the current dir
for file in *; do echo "File -> $file"; done

# Do something at interval
 while true; do kubectl get pods; sleep 1; done

# Iterate over arguments
for var in "$@"
do
    echo "$var"
done

# same same but different by one
for i in 0 1 2; do
  gcloud compute routes create kubernetes-route-10-200-${i}-0-24 \
    --network kubernetes-the-hard-way \
    --next-hop-address 10.240.0.2${i} \
    --destination-range 10.200.${i}.0/24
done

Or in its own script:

#!/bin/bash
# Randomly delete pods in a Kubernetes namespace.
set -ex

: ${DELAY:=30}
: ${NAMESPACE:=default}

while true; do
  kubectl \
    --namespace "${NAMESPACE}" \
    -o 'jsonpath={.items[*].metadata.name}' \
    get pods | \
      tr " " "\n" | \
      shuf | \
      head -n 1 |
      xargs -t --no-run-if-empty \
        kubectl --namespace "${NAMESPACE}" delete pod
  sleep "${DELAY}"
done

Remove all git branches except master

for i in $(git branch); do echo $i | grep -v -e 'master' -e \* | xargs git branch -D ; done

Evaluating

Sometimes we need to copy-paste the content of a file:

cat <<EOF | ~/newly_created_file
input
on multiple lines
EOF

Sometimes we need to have privileges:

cat <<EOF | sudo tee /etc/systemd/system/foo.service
[Unit]
Description=foo

[Service]
ExecStart=/bin/bash -c "while true; do /bin/inotifywait -qq --event close_write /sys/class/backlight/acpi_video0/brightness; su myusername -c '/bin/xbacklight -display :0 -set $(cat /sys/class/backlight/acpi_video0/brightness)'; done"

[Install]
WantedBy=multi-user.target
EOF

Linting

Install shellcheck
Run it against your scripts
Fix everything

This is the most useful tool when it comes to finding bugs and to guide you to write better scripts.

Useful

This part is just for those commands that we need once in while but can never really remember that they exist

# alias
alias c=clear

# The 20 most used command in your history
history | awk 'BEGIN {FS="[ \t]+|\\|"} {print $3}' | sort | uniq -c | sort -nr | head -n 20

# Remove all node_modules forlder recursively
find . -name "node_modules" -type d -prune -exec rm -rf '{}' +

# Copying file with rsync over ssh, very useful to resume the transfer!
rsync -avz -e ssh remoteuser@remotehost:/remote/dir /this/dir/

# exporting a container
docker export my_container | gzip > my_container.tar.gz

# execute last command with sudo
sudo !!

# get the directory of the script
CURRENT_DIR=$(dirname "$0")