bash-script

This is all about bash scripting. Bash is one of the commonly used Unix shells. Bash stands for "Bourne Again SHell". It is an improvement of the original Bourne shell (sh).

command line arguments

$ ./filename.sh argument1 argument2 ... argumentN

Where, filename.sh is a shell script file and argument1, argument2 ... argumentN are list of arguments.

The $0 variable This holds the name of the script.
The $1 $2 ... $N variables These variables hold the arguments provided to the script.
The $# variable This variable hold the total number of arguments passed to the script.
The $@ and $* variables They both holds the list of arguments provided to the script.
The $? variable This variable holds the exit value of the last run command or return code from a function.
The $! variable This variable holds the PID of the last run background process.

Example 1


# display the file name
echo "The name of the script file is $0"

# display total number of arguments passed to the script
echo "Total number of arguments passed to the script = $#"

# display all the arguments using for loop
if [ $# -gt 0 ]
then
  
  echo "List of arguments:"
  for arg in $@
  do
    echo "$arg"
  done

else
  
  echo "No argument provided to the script."

fi 

Output:

$ sh example.sh 
The name of the script file is example.sh
Total number of arguments passed to the script = 0
No argument provided to the script.


$ sh example.sh Hello World! What's up ?
The name of the script file is example.sh
Total number of arguments passed to the script = 5
List of arguments:
Hello
World
WWhat's
up
?

Example 2


# function to check number of arguments passed to the script
function isArgumentPresent {
  if [ $1 -gt 0 ]
  then
    return 0	# success code
  else
    return 1	# failure code
  fi
}
# calling the function
# and passing number of arguments passed to the script
isArgumentPresent $#

# get the returned code
returnedCode=$?

# check returnedCode
if [ $returnedCode -eq 0 ]
then
  echo "Arguments present!"
else
  echo "Arguments not present!"
fi
Output:

$ ./example.sh hello world
Arguments present!

$ ./example.sh 
Arguments not present!

Example 3


echo "PID of the current file is $$"

Output:

$ sh example03.sh 
PID of the current file is 71084

Example 4


echo "The PID of the last run background process was $!"
Output:

$ ./example.sh 
The PID of the last run background process was 84014

Example 5


-eq
is equal to

if [ "$a" -eq "$b" ]

-ne
is not equal to

if [ "$a" -ne "$b" ]

-gt
is greater than

if [ "$a" -gt "$b" ]

-ge
is greater than or equal to

if [ "$a" -ge "$b" ]

-lt
is less than

if [ "$a" -lt "$b" ]

-le
is less than or equal to

if [ "$a" -le "$b" ]