Useful shell command or cheatsheet

README.md

Useful resources

• https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html

bash-file-descriptors.md

Bash file descriptors explains

The numbers are file descriptors and only the first three (starting with zero) have a standardized meaning:

0 - stdin
1 - stdout
2 - stderr

So each of these numbers in your command refer to a file descriptor. You can either redirect a file descriptor to a file with > or redirect it to another file descriptor with >&

exec 3>&1
# exec 3>/dev/null

echo >&3 "Hello world"

bash-process-id.md

The variable $$ or $BASHPID contains the PID.

See the [manual][1] for more information, including differences between the two.

TL;DRTFM

• $$ Expands to the process ID of the shell.
  • In a () subshell, it expands to the process ID of the invoking shell, not the subshell.
• $BASHPID Expands to the process ID of the current Bash process (new to bash 4).
  • In a () subshell, it expands to the process ID of the subshell [1]: http://www.gnu.org/software/bash/manual/bashref.html#Bash-Variables

docker-entrypoint.d.md

Added entrypoint hook script to make changes management easier

Credit: https://github.com/nginxinc/docker-nginx/blob/master/entrypoint/docker-entrypoint.sh

set -e

if [ -z "${ENTRYPOINT_QUIET_LOGS:-}" ]; then
    exec 3>&1
else
    exec 3>/dev/null
fi

if /usr/bin/find "/docker-entrypoint.d/" -mindepth 1 -maxdepth 1 -type f -print -quit 2>/dev/null | read v; then
    echo >&3 "$0: /docker-entrypoint.d/ is not empty, will attempt to perform configuration"

    echo >&3 "$0: Looking for shell scripts in /docker-entrypoint.d/"
    find "/docker-entrypoint.d/" -follow -type f -print | sort -V | while read -r f; do
        case "$f" in
            *.sh)
                if [ -x "$f" ]; then
                    echo >&3 "$0: Launching $f";
                    "$f"
                else
                    # warn on shell scripts without exec bit
                    echo >&3 "$0: Ignoring $f, not executable";
                fi
                ;;
            *) echo >&3 "$0: Ignoring $f";;
        esac
    done

    echo >&3 "$0: Configuration complete; ready for start up"
else
    echo >&3 "$0: No files found in /docker-entrypoint.d/, skipping configuration"
fi

docker-service-template.md

Usefull Docker Service templates

See: https://docs.docker.com/engine/reference/commandline/service_create/#create-services-using-templates

environmets:
  DOCKER_SERVICE_ID: "{{.Service.ID}}" # Docker Service ID
  DOCKER_SERVICE_NAME: "{{.Service.Name}}" # Docker Service name
  DOCKER_SERVICE_LABELS: "{{.Service.Labels}}" # Docker Service labels
  DOCKER_NODE_ID: "{{.Node.ID}}" # Docker Node ID
  DOCKER_NODE_HOSTNAME: "{{.Node.Hostname}}" # Docker Node Hostname
  DOCKER_TASK_ID: "{{.Task.ID}}" # Docker Task ID
  DOCKER_TASK_NAME: "{{.Task.Name}}" # Docker Task name
  DOCKER_TASK_SLOT: "{{.Task.Slot}}" # Docker Task slot

file-env.md

Read environment variable from secret file

Example: https://github.com/docker-library/mysql/blob/37cf4047a0544046c9ecce8f2885d4f362a227bc/8.0/docker-entrypoint.sh#L210-L227

Code

# usage: file_env VAR [DEFAULT]
#    ie: file_env 'XYZ_DB_PASSWORD' 'example'
# (will allow for "$XYZ_DB_PASSWORD_FILE" to fill in the value of
#  "$XYZ_DB_PASSWORD" from a file, especially for Docker's secrets feature)
file_env() {
	local var="$1"
	local fileVar="${var}_FILE"
	local def="${2:-}"
	if [ "${!var:-}" ] && [ "${!fileVar:-}" ]; then
		echo "Both $var and $fileVar are set (but are exclusive)"
		exit 1
	fi
	local val="$def"
	if [ "${!var:-}" ]; then
		val="${!var}"
	elif [ "${!fileVar:-}" ]; then
		val="$(< "${!fileVar}")"
	fi
	export "$var"="$val"
	unset "$fileVar"
}

Usage

# Initialize values that might be stored in a file
file_env 'MYSQL_ROOT_HOST' '%'
file_env 'MYSQL_DATABASE'
file_env 'MYSQL_USER'
file_env 'MYSQL_PASSWORD'
file_env 'MYSQL_ROOT_PASSWORD'

get-os-and-arch.sh

#!/bin/bash

OS="$(uname | tr '[:upper:]' '[:lower:]')" &&
ARCH="$(uname -m | sed -e 's/x86_64/amd64/' -e 's/\(arm\)\(64\)\?.*/\1\2/' -e 's/aarch64$/arm64/')"

is-sourced.md

Check to see if this file is being run or sourced from another script

Code

# check to see if this file is being run or sourced from another script
_is_sourced() {
	# https://unix.stackexchange.com/a/215279
	[ "${#FUNCNAME[@]}" -ge 2 ] \
		&& [ "${FUNCNAME[0]}" = '_is_sourced' ] \
		&& [ "${FUNCNAME[1]}" = 'source' ]
}

Usage

# If we are sourced from elsewhere, don't perform any further actions
if ! _is_sourced; then
	_main "$@"
fi

parse-dotenv.md

Parse a .env (dotenv) file directly using BASH

Credit: https://gist.github.com/judy2k/7656bfe3b322d669ef75364a46327836

# Pass the env-vars to MYCOMMAND
eval $(egrep -v '^#' .env | xargs) MYCOMMAND
# … or ...
# Export the vars in .env into your shell:
export $(egrep -v '^#' .env | xargs)

# Export the vars in .env into your shell
_parse_env() {
	local env_file=$1
	if [ -f "$env_file" ]; then
		export $(egrep -v '^#' $env_file | xargs)
	fi
}

Alternative

# Export the vars in .env into your shell
_parse_env() {
	local env_file=$1
	if [ -f "$env_file" ]; then
		set -o allexport
		source $env_file
		set +o allexport
	fi
}