Shell Script

In everyday life of a DevOps engineer you will have to create multiple pieces of code. Some of those will be run once, others … well others will live forever. Although it may be compelling to just put all the commands in a text editor, save the result and execute it, one should always consider the “bigger picture”. What will happen if your script is run on another OS, on another Linux distribution, or even on a different version of the same Linux distribution?! Another point of view is to think what will happen if somehow your neat 10-line-script has to be executed on say 500 servers?! Can you be sure that all the commands will run successfully there? Can you even be sure that all the commands will even be present? Usually … No!

Faced with similar problems on a daily basis we started devising simple solutions and practices to address them. One of those is the process of standardizing the way different utilities behave, the way they take arguments and report errors. Upon further investigation it became clear that a pattern can be extracted and synthesized in a series of template, one can use in daily work to keep common behavior between different utilities and components.

Here is the basic template used in shell scripts:

#!/bin/sh
#
# DESCRIPTION: ... Include functional description ...
#
# Requiresments:
#	awk
#	... 
#	uname
#
# Example usag:
#	$ template.sh -h 
#	$ template.sh -p ARG1 -q ARG2
#

RET_CODE_OK=0
RET_CODE_ERROR=1

# Help / Usage function
print_help() {
	echo "$0: Functional description of the utility"
	echo ""
	echo "$0: Usage"
	echo "    [-h] Print help"
	echo "    [-p] (MANDATORY) First argument"
	echo "    [-q] (OPTIONAL) Second argument"
	exit $RET_CODE_ERROR;
}

# Check for supported operating system
p_uname=`whereis uname | cut -d' ' -f2`
if [ ! -x "$p_uname" ]; then
	echo "$0: No UNAME available in the system"
	exit $RET_CODE_ERROR;
fi
OS=`$p_uname`
if [ "$OS" != "Linux" ]; then
	echo "$0: Unsupported OS!";
	exit $RET_CODE_ERROR;
fi

# Check if awk is available in the system
p_awk=`whereis awk | cut -d' ' -f2`
if [ ! -x "$p_awk" ]; then
	echo "$0: No AWK available in the system!";
	exit $RET_CODE_ERROR;
fi

# Check for other used local utilities
#	bc
#	curl
#	grep 
#	etc ...

# Parse command line arguments
while test -n "$1"; do
	case "$1" in
	--help|-h)
		print_help
		exit 0
		;;
	-p)
		P_ARG=$2
		shift
		;;
	-q)
		Q_ARG=$2
		shift
		;;
	*)
		echo "$0: Unknown Argument: $1"
		print_help
		exit $RET_CODE_ERROR;
		;;
	esac
	
	shift
done

# Check if mandatory argument is present?
if [ -z "$P_ARG" ]; then
	echo "$0: Required parameter not specified!"
	print_help
	exit $RET_CODE_ERROR;
fi

# ... 

# Check if optional argument is present and if not, initialize!
if [ -z "$Q_ARG" ]; then
	Q_ARG="0";
fi

# ... 

# DO THE ACTUAL WORK HERE 

exit $RET_CODE_OK;

#!/bin/sh

# DESCRIPTION: ... Include functional description ...

# Requiresments:

# awk

# ...

# uname

# Example usag:

# $ template.sh -h

# $ template.sh -p ARG1 -q ARG2

RET_CODE_OK=0

RET_CODE_ERROR=1

# Help / Usage function

print_help() {

echo "$0: Functional description of the utility"

echo ""

echo "$0: Usage"

echo " [-h] Print help"

echo " [-p] (MANDATORY) First argument"

echo " [-q] (OPTIONAL) Second argument"

exit $RET_CODE_ERROR;

}

# Check for supported operating system

p_uname=`whereis uname | cut -d' ' -f2`

if [ ! -x "$p_uname" ]; then

echo "$0: No UNAME available in the system"

exit $RET_CODE_ERROR;

OS=`$p_uname`

if [ "$OS" != "Linux" ]; then

echo "$0: Unsupported OS!";

exit $RET_CODE_ERROR;

# Check if awk is available in the system

p_awk=`whereis awk | cut -d' ' -f2`

if [ ! -x "$p_awk" ]; then

echo "$0: No AWK available in the system!";

exit $RET_CODE_ERROR;

# Check for other used local utilities

# bc

# curl

# grep

# etc ...

# Parse command line arguments

while test -n "$1"; do

case "$1" in

--help|-h)

print_help

exit 0

;;

-p)

P_ARG=$2

shift

;;

-q)

Q_ARG=$2

shift

;;

echo "$0: Unknown Argument: $1"

print_help

exit $RET_CODE_ERROR;

;;

esac

shift

done

# Check if mandatory argument is present?

if [ -z "$P_ARG" ]; then

echo "$0: Required parameter not specified!"

print_help

exit $RET_CODE_ERROR;

# ...

# Check if optional argument is present and if not, initialize!

if [ -z "$Q_ARG" ]; then

Q_ARG="0";

# ...

# DO THE ACTUAL WORK HERE

exit $RET_CODE_OK;

Nothing fancy. Basic framework that does the following:

Lines 3 – 13: Make sure basic documentation, dependency list and example usage patterns are provided with the script itself;
Lines 15 – 16: Define meaningful return codes to allow other utils to identify possible execution problems and react accordingly;
Lines 18 – 27: Basic help/usage() function to provide the user with short guidance on how to use the script;
Lines 29 – 52: Dependency checks to make sure all utilities the script needs are available and executable in the system;
Lines 54 – 77: Argument parsing of everything passed on the command line that supports both short and long argument names;
Lines 79 – 91: Validity checks of the argument values that should make sure arguments are passed contextually correct values;
Lines 95 – N: Actual programming logic to be implemented …

This template is successfully used in a various scenarios: command line utilities, Nagios plugins, startup/shutdown scripts, UserData scripts, daemons implemented in shell script with the help of start-stop-daemon, etc. It is also used to allow deployment on multiple operating systems and distribution versions. Resulting utilities and system components are more resilient, include better documentation and dependency sections, provide the user with similar and intuitive way to get help or pass arguments. Error handling is functional enough to go beyond the simple OK / ERROR state. And all of those are important feature when components must be run in highly heterogenous environments such as most cloud deployments!

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