Linux Classroom Series – 02/Sept/2020

Special parameters

• $0 $1 …
• $# : count of number of arguments passed
• $-:
  • Create a simple script and execute

  #!/bin/bash
  
  echo "My shell used id $0 and the options passed are $-"
  

  • Directly execute the following statement in terminal

  echo "My shell used id $0 and the options passed are $-"
  

  • The following results will be shown
  • The options set are as follows
    • h: This is short for hash all
    • i: Shows its and interactive
    • m: This is short for monitor
    • B: This allows brace expansion. “`mkdir dir{1,2}
    • H: This allows history expansion of running commands

Setting default values to parameters

• Its often a good practice to assign default values for non-critical parameters. See the below example and execute this

#!/bin/bash

#####################################################################
# Author: Shaik Khaja Ibrahim
# Version: v1.0.0
# Date: 02-Sep-2020
# Description: This script demonstrates default values for 
# positional parameters
# Usage: ./defaultparamvalues.sh <name> <course>
#####################################################################


name=$1
course=$2

[ -z $name ] && name="Khaja"
[ -z $course ] && course="Linux"

echo "Hello ${name}, Welcome to world of ${course} "

• There is an alternative syntax to assign default value to parameter

${parameter-default}

• lets apply this syntax to shell script

#!/bin/bash

#####################################################################
# Author: Shaik Khaja Ibrahim
# Version: v1.0.0
# Date: 02-Sep-2020
# Description: This script demonstrates default values for 
# positional parameters
# Usage: ./defaultparamvalues.sh <name> <course>
#####################################################################


name=${1-"Khaja"}
course=${2-"Linux"}

echo "Hello ${name}, Welcome to world of ${course} "

• If the parameter is declared and it has a null value

${parameter:-default}

Looping constructs in bash scripting

• Lets try to understand

  • for loops
  • The internal field separator
  • Counting directories & files
  • Nested Loops
  • Redirecting loop output
  • while and until loops

• Sample for script

#!/bin/bash

# printing multiple courses
echo "DevOps"
echo "AWS"
echo "Azure"
echo "Linux"
echo "Windows"

echo "Now using for"

# with for loop
for course in DevOps AWS Azure Linux Windows ; do
   echo "${course}"
done

# other kind of for loop

echo "Now using for which is c-styled"
courses=(DevOps AWS Azure Linux Windows)

for (( index=0; index<5; index++ )) do
   echo "${courses[$index]}"
done

Internal Field Separator

• By default the IFS value has one of (space, newline or tab)
• Lets assume you want o iterate over

Hello, 
This is Linux
I'm fun to work with

• Now if we write to script to iterate and print over this content as shown below

#!/bin/bash
file="text.txt"

for item in $(cat $file)
do
    echo "$item"
done

• What should be done to this script to iterate over lines. Now add IFS=$’\n’ to the shell script and this will do the trick

#!/bin/bash
file="text.txt"
IFS=$'\n'
for item in $(cat $file)
do
    echo "$item"
done