bash-firefighter

Curated list of useful bash techniques

Table of Contents

• Helper
• Generic
• Programming
  • Array
  • Join
  • Variables
  • Split
  • Regex
  • Modules
  • Function in Function
• Config Files
  • Convert YAML to key=value
• Cloud Provider
  • AWS

Helper

• yq, refer to install_yq.sh. Or other ways
• jq, refer to install_jq.sh
• Get local directory of script local_dir.sh
• Get OS platform get_os.sh
• Promote AWS ECR Image promote_ecr.sh

Generic

Check if a port is open

if [ "$(timeout 1 bash -c '</dev/tcp/localhost/9100'; echo $?)" == "0" ]; then
        echo "hi"
fi

get OS Platform

OS_PLATFORM=$(cat /etc/os-release | awk -F= '/^ID=/{print $2}' | tr -d '"')
echo $OS_PLATFORM

Exit on error

#!/bin/bash
set -e

Retry on error

• Refer to retry.sh

User Continue

• Refer to user_continue.sh

Programming

Array

Loop through indices, values

for i in "${!foo[@]}"; do
  echo "${i}: ${foo[$i]}"
done

Loop through values

for i in "${foo[@]}"; do
  echo "${i}"
done

Join method

Join array to string

foo_arr=['a','b','c']
bar=$(IFS=. ; echo "${foo_arr[*]}")

# a.b.c

Variables

Declare local variable

function foo() {
  local bar=""
}

foo
# only work with function

Declare global variable

bar=""

function foo() {
  echo "${bar}"
}

Default value

bar="${1:-default}"

echo "${bar}"
# default

Split method

Split string to array

IFS='.' read -ra arr <<< "a.b.c"

echo "${arr[@]}
# ['a', 'b', 'c']

Regex

Validate IP address

if [[ ! $ip =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
  echo -e "error! $ip is not valid"
  return 1
fi

Function in Function

foo() (
  bar() {
    echo "hello"
  }

  bar
)

Modules

inlude bash file as a module

.
├── foo.sh
└── modules
    └── utils.sh

$ cat modules/utils.sh
#!/bin/bash

function bar() {
  echo "hello"
}

$ cat foo.sh
#!/bin/bash

source modules/utils.sh

bar

Config Files

Yaml

Convert yaml file (work with up to level 2 of keys) to key=value environment variables file

• Detect if the input file's content changes with md5() hash
• allow bash to work with yaml for configuration automation

Requirements

• yq
• jq

Refer to convert_yaml_to_env.sh

Cloud Provider

AWS

Getting VPC CIDR from VPC_ID

vpc_id=${1}
aws ec2 describe-vpcs --vpc-ids $vpc_id | jq -r .Vpcs[0].CidrBlock

Getting available ENIs ID

aws ec2 describe-network-interfaces --region us-west-2 --filters Name=status,Values=available,Name=group-name,Values=<GROUP_NAME> --max-items 2 | jq -r .NetworkInterfaces[].NetworkInterfaceId