Linux Classroom Series – 01/Sept/2020

Some examples of popular conditions

1. if [[ -f /tmp/file ]]; then do-something => do something if the file /tmp/file exists
2. if [[ ! -f /tmp/file ]]; then do-something => do something if the file /tmp/file does not exists
3. if [[ -n ${variable} ]]; then do-something => do something if the ${variable} is not empty
4. if [[ !-n ${variable} ]]; then do-something => do something if the ${variable} is empty
5. if [[ -z ${variable} ]]; then do-something => do something if the ${variable} is empty

Applying above conditions to make script handle errors

• The ifthenrcdemo.sh file will have a script

#!/bin/bash
mkdir temps
mkdir_rc=$?

# Test if the directory creation is success

if [[ ${mkdir_rc} -ne 0 ]]; then
    echo "mkdir didnot created the directory, so stopping script execution"
    exit 1
fi

touch temps/tempfile.txt

• Lets execute this script with a positive & negative condition
• So now lets adopt if-then-else

#!/bin/bash
FILE=randomfile.txt

# check if the file exists
if [[ ! -f ${FILE} ]]; then
    echo "File mentioned as ${FILE} doesn't exist so exiting"
    exit 1
else
    echo "Printing contents of file at ${FILE}"
    cat ${FILE}
fi

• Lets write one more if-then-else script which accepts parameters (positional parameters)

#!/bin/bash

### Usage ./ifthenelsedemo2.sh <path-of-file>

file_name=$1

# user might enter empty values
if [[ -z ${file_name} ]]; then
    echo "Incorrect usage: ./ifthenelsedemo2.sh <filename>"
    exit 1
fi

if [[ ! -f ${file_name} ]]; then
    echo "Please correct the file path and re-execute."
    exit 1
else
    echo "Contents of the file are"
    cat ${file_name}
fi

Checking my arguments

• Lets try to write a very simple shell scrip which creates a specified file in the specified directory with specified content
• So argument list should be

./createfile.sh <directory_path> <filename> <file contents>

• The script will be as shown below

#!/bin/bash

####################################################################
# Author: Shaik Khaja Ibrahim
# Version: v1.0.0
# Date: 01-Sep-2020
# Description: This script demonstrates basic user inputs
# Usage: ./createfile.sh <directory-name> <file-name> <file-content>
#####################################################################

# We need three arguments, so checking if the arguments passed count
# is 3 or not

if [[ $# -ne 3 ]]; then
    echo "Incorrect number of arguments passed"
    echo "Usage: ./createfile.sh <directory-name> <file-name> <file-content>"
    exit 1
fi

# create parameters with argument values
directory_name=$1
file_name=$2
file_content=$3

# check if the directory exists, if it doesnot exist create directory

if [[ ! -d ${directory_name} ]]; then
    mkdir ${directory_name} || { echo "Cannot create directory"; exit 1; }
fi

# lets create absolute file path
abs_file_path=${directory_name}/${file_name}

# Try to create a file if the file doesnot exist
if [[ ! -f ${abs_file_path} ]]; then
    touch ${abs_file_path} || { echo "Cannot create a file"; exit 1; }
fi

# Since file is created or present add the contents to it
echo ${file_content} > ${abs_file_path}

• Now execute the script
• In command line usage when arguments are in <> they are required arguments and if the arguments are in [] they are optional.

Dealing with y/n options in interactive scripts

• When we ask input from the user, user might enter many possibilities for yes (YES,yes,YeS,y,Y) and same for no.
• How can write a script which is case sensitive to yes value

#!/bin/bash

read -p "Do you like linux? " reply

if [[ ${reply,,} = 'y' ]] || [[ ${reply^^} == 'YES' ]]
    echo "Great, Continue your learning journey"
    exit 0
fi