shell_scripting.txt

man 1 test for file/directory checks

bash -x //for debugging
set -x and set +x for debugging inside the script
set -n and set -o as well

#echo -e "smfkfmslk \c" for no newline in echo

read -p //display prompt to user without a newline
read -t 10  n1 //terminates if input not provided within 10 secs.
#Use unset command to delete the variables during program execution. It can remove both functions and shell variables.

The := syntax // ${var:=defaultValue}
for if var is empty assign a default value //also can be used to assign default values in functions like:
die(){
  local error=${1:=Undefined error} 
}//${1:-        means $1 argument

     All command line parameters or arguments can be accessed via $1, $2, $3,..., $9.
    $* holds all command line parameters or arguments.
    $# holds the number of positional parameters.
    $- holds flags supplied to the shell.
    $? holds the return value set by the previously executed command.
    $$ holds the process number of the shell (current shell).
    $! hold the process number of the last background command.
    $@ holds all command line parameters or arguments.

$@ expanded as "$1" "$2" "$3" ... "$n"
$* expanded as "$1y$2y$3y...$n", where y is the value of $IFS variable i.e. "$*" is one long string and $IFS act as an separator or token delimiters. 

${varName?Error varName is not defined}
${varName:?Error varName is not defined or is empty}