shell script overview

Shell script best practices:

• Variable name should not have a dashes, use underscore instead.
• define meaningful variable name and name of the file as well.
• Variable should be encapsulated in { }. Change variable to ${file_name}_bkp
• Design your script to be re-usable
• Script should not require to be edited before running
• use command line argument to pass inputs
• try to avoid using backtick in script instead of that use $( )
• always start with a shebang in your script 
• always return appropriate exit codes in your script like if something is fail then use exit 1.
• For more best practices refer this doc

What is Bash(Bourne-Again SHell)

Bash is a command language interpreter. It is widely available on various operating systems and is a default command interpreter on most GNU/Linux systems.

$ which bash
/bin/bash

$ cat hello.sh
#!/bin/bash
echo "Hello World"

$ cat test.sh
echo "Hello World"

$ cat test2.sh
 #!/bin/bash
 # This is comment
 user=$(whoami)
 input=/home/$user
 output=/tmp/${user}_home_$(date +%Y-%m-%d_%H%M%S).tar.gz
 echo "Backup of $input completed! "

• If you want to define your script's interpreter as Bash, try to locate the path of bash and then add the path by add #! prefix.
• Another way to call bash interpreter explictly is to run script with bash i.e bash test.sh
• ${parameter} is called parameter expansion since $user is followed by character which is not part of variable

#!/bin/bash
echo $1 $2 $4  ## positional parameters
echo $#        ## the total number of supplied arguments
echo $*        ## to print all arguments.

Syntax or For loop and While loop:

$ cat for-loop.sh
  #!/bin/bash
  for i in 1 2 3; do
      echo $i
  done


$ cat while-loop.sh
  #!/bin/bash
  counter=0
  while [ $counter -lt 3 ]; do
      let counter+=1
      echo $counter
  done

Read input in shell script from user :

read -p "Enter you name: " name

Note : The value will be store in name varialbe

Arithmetic operations in shell:

##using expression 
exp 6 + 3

Note : please mind the space while using exp, and in shell * is reserved keyworkd so while multiplying you need to escape it using backslash

for operation you could use double parenthesis
echo $(( A + B ))
echo $(( ++A ))

for floating point answer you need to use bc
A = 10
B = 3
echo $A / $B | bc -l

for ((i = 1; i <= MAX_NO; i++)); do echo $i; done;

Conditional logic in shell :

#########String comparison
if [ $rocket_status = "failed" ]

string --> =,!=, >, <

#########Arithmetic comparison
if ! [ $MAX_NO -ge 5 -a $MAX_NO -le 9 ]; then echo "enter b/w 5 & 9" fi

number --> -eq, -ge, -gt, -le, -lt, -ne 

Double bracket in shell(only in bash) :

[[ STRING1 = STRING2 ]]
[[ supports some extra features like following, this is been found in bash extension shell or latest shell

[[ "abcd" = *bc* ]]
if abcd contains bc true | no "" given since it is pattern

[[ "abc" = ab[cd] ]]
if 3rd character of abc is c or d true

[[ "abc" > "bcd" ]]
if abc comes after bcd when sorted in alphabetical lexographical order
------------------------------------------------------------------------------
conditional operations:

[ COND1 ] && [ COND2 ]
[ COND1 ] || [ COND2 ]

enhanced version:
[[ COND1 && COND2 ]]
[[ COND1 || COND2 ]]

Example:
if [ $first -eq 0 ] && [ $second -eq 0 ]
if ! [ $MAX_NO -ge 5 -a $MAX_NO -le 9 ]; then echo "hello"; fi;

[ -e FILE ]  if file exists
[ -d FILE ]  if file exists and is a directory
[ -s FILE ]  if file exists and has size greater than o
[ -x FILE ]  if file is executable
[ -w FILE ]  if file is writable
Use -z $1 to check for command line argument. 

Some good example of above :

for file in $(ls images)
do
     if [[ $file = *.jpeg ]]
         then
         new_name=$(echo $file| sed 's/jpeg/jpg/g')
         mv images/$file images/$new_name
     fi
done

Loop over list of pdfs file name from batch3.txt and see if that exists in CSV file :

#!/bin/bash
i=1
while read p; do
    echo "$p"
    file="s3://test-bucket-images/v2/batch3/$p".
    echo $file
    echo $i
    i=$((i+1))
    line=`cat textract.csv  | grep  "$file" | wc -l`
    echo $line
    if [ $line -eq 0 ];
    then
        echo $file > missing.txt
    fi
done <batch3.txt

Reference:

• https://www.redhat.com/sysadmin/learn-bash-scripting 
• https://www.javacodegeeks.com/2013/10/shell-scripting-best-practices.html