Chun Sing Tsui

Coding, Technology, and other Interests

Commonly Used Commands

Here’s a short cheatsheet for some useful commands when working with Linux/Unix systems.

Get specific column using cut. Tab is the default delimiter in cut

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cat content | cut -d 'delimiter' -f <field number>

Output specific column using awk

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cat content | awk -F/ '{print $1}'

Base64 encode (-n for no newline)

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echo -n 'admin:password' | base64

Create gzipped tar

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tar -czvf archive.tgz file1 dir2

Extract gzipped tar

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# Output dir has to exist
tar -xzvf archive.tgz -C targetdir

Listening ports

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sudo netstat -plnt

# or

sudo lsof -i -P | grep -i 'listen'

Disk usage sorted by size

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du -hs * | sort -h

Xargs to pass arguments from stdin

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xargs brew install < list.txt

Heredoc Syntax to create multi-line file from command line

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cat <<EOT > app-pod.yml
apiVersion: v1
kind: Pod
metadata:
  name: my-app-ui
  labels:
    app: my-app
spec:
  containers:
    - name: nginx-server
      image: nginx
EOT

Working with Docker

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# Remove stopped containers and all unused images
docker system prune -a

# Remove certain images 
docker images -a | grep "pattern" | awk '{print $3}' | xargs docker rmi

# Remove all images
docker rmi $(docker images -a -q)

Working with systemd

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# List all services
systemctl list-units --type=service

# Restart
systemctl restart application.service

# Get logs by the service unit
journalctl -u nginx.service

# Get data from yesterday
journalctl --since yesterday

# Get data from specific timestamps
journalctl --since "2015-01-10" --until "2015-01-11 03:00"

# Get log data from previous boot
journalctl -b -1

Working with Openshift

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# Get list of users with IAM in the name and put them in a group
# and bind the group with cluster-admin. Use space in awk split
oc adm groups new cloudiam

oc get users | grep IAM | awk -F '[[:space:]][[:space:]]+' '{print $1}' | xargs -I '{}' oc adm groups add-users cloudiam '{}'

oc adm policy add-cluster-role-to-group cluster-admin cloudiam