Shell Scripting

#!/bin/bash
# Check if arguments are given, $0 is the script name #
if [ $# -lt 3 ]
then
        echo "Missing Arguments"
        echo "Usage : $0 arg1 arg2 arg3"
        exit
fi

# $1 is the first argument, expecting a string
if [ -z $1 ]; then
   echo "You must specify a string in first argument"
   exit
fi

# using translate command 'tr', Translate any uppercase characters into lowercase #
test=$( echo "$1" | tr -s  '[:upper:]'  '[:lower:]' )

# Check if the given file exists where arg3 is the given filename to check #
if [ ! -f $3 ]
then
        echo "Filename given \"$3\" doesn't exist"
        exit
fi

For string comparision use,
==  
!=  
<  

For numeric comparision use,
-gt
-lt
-eq
-ne

#!/bin/bash
for variable in {list}
do
    <commands>
done

# Example
for i in {1..10}
do
    echo $i
done

# Bash v4.0+ has inbuilt support for setting up a step value using {START..END..INCREMENT} syntax
#!/bin/bash
echo "Bash version ${BASH_VERSION}..."
for i in {0..10..2}
  do
     echo "Count $i times"
 done

#output
Bash version 4.4.20(1)-release...
Count 0 times
Count 2 times
Count 4 times
Count 6 times
Count 8 times
Count 10 times

# C Style for loop
#!/bin/bash
for (( i=1; i<=5; i++ ))
do  
   echo "Welcome $i times"
done

# Infinite loop
#!/bin/bash
for (( ; ; ))
do
   echo "infinite loops [ hit CTRL+C to stop]"
done

BOOKS=('Title-1' \
     'Title-2' \
     'Title-3' \
     'Title-4')
for book in "${BOOKS[@]}"
do
  echo "Book: $book"
done

# Output
Book: Title-1
Book: Title-2
Book: Title-3
Book: Title-4

for i in {1..10}
do
  statements1      #Executed for all values of ''i'', up to a disaster-condition if any.
  statements2
  if (disaster-condition)
  then
     break            #Abandon the loop.
  fi
  statements3         #While good and, no disaster-condition.
done

# Example
# This shell script will go though all files stored in /etc directory.
# The for loop will be abandon when /etc/resolv.conf file found.

#!/bin/bash
for file in /etc/*
do
     if [ "${file}" == "/etc/resolv.conf" ]
     then
          countNameservers=$(grep -c nameserver /etc/resolv.conf)
          echo "Total  ${countNameservers} nameservers defined in ${file}"
          break
     fi
done

#!/bin/bash
while [[condition]]  
do
    <commands>
done

# Example
num=1
while [ $num -le 5 ]
do
   echo "$num"
   num=$((num+1))
done

for I in 1 2 3 4 5
do
  statements1      #Executed for all values of ''I'', up to a disaster-condition if any.
  statements2
  if (condition)
  then
     continue   #Go to next iteration of I in the loop and skip statements3
  fi
  statements3
done

# Example
# This script will make backup of all file names specified on command line. If .bak file exists, it will skip the cp command.
#!/bin/bash
FILES="$@"
for f in $FILES
do
        # if .bak backup file exists, read next file
     if [ -f ${f}.bak ]
     then
          echo "Skiping $f file..."
          continue  # read next file and skip the cp command
     fi
     /bin/cp $f $f.bak
done

# The CASE statement is the simplest form of the IF-THEN-ELSE statement in BASH.
case $variable in
     pattern-1)
          commands
          ;;
     pattern-2)
          commands
          ;;
     pattern-3|pattern-4|pattern-5)
          commands
          ;;
     pattern-N)
          commands
          ;;
     *)
          commands
          ;;
esac