Bash Shell Scripting/Input-Output

The read built-in

From help read:

read: read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

    Read a line from the standard input and split it into fields.

read is great for both user inputs and reading standard inputs/piping.

An example of user input:

# 'readline'  prompt          default    variable name
read -e -p "Do this:" -i "destroy the ship" command

echo "$command"

Or even simpler:

pause() { read -n 1 -p "Press any key to continue..."; }

Hello-world level example of stdout operation:

echo 'Hello, world!' | { read hello
echo $hello }

Just be creative. For example, in many ways read can replace whiptail. Here is an example, extracted from Arthur200000's shell script:

# USAGE
#   yes_or_no "title" "text" height width ["yes text"] ["no text"]
# INPUTS
#   $LINE = (y/n)          - If we should use line-based input style(read)
#   $_DEFAULT = (optional) - The default value for read
yes_or_no() {
  if [ "$LINE" == "y" ]; then
    echo -e "\e[1m$1\e[0m"
    echo '$2' | fold -w $4 -s
    while read -e -n 1 -i "$_DEFAULT" -p "Y for ${5:-Yes}, N for ${6:-No}[Y/N]" _yesno; do
      case $_yesno in
        [yY]|1)
          return 0
          ;;
        [nN]|0)
          return 1
          ;;
        *)
          echo -e "\e[1;31mINVALID INPUT\x21\e[0m"
       esac
  else whiptail --title "${1:-Huh?}" --yesno "${2:-Are you sure?}" ${3:-10} ${4:-80}\
         --yes-button "${5:-Yes}" --no-button "$6{:-No}"; return $?
  fi
}

# USAGE
#   user_input var_name ["title"] ["prompt"] [height] [width]
# INPUTS
#   $LINE = (y/n)          - If we should use line-based input style(read)
#   $_DEFAULT = (optional) - The default value for read; defaults to nothing.
user_input(){
  if [ "$LINE" == "y" ]; then
    echo -e "\e[1m${2:-Please Enter:}\e[0m" | fold -w ${4:-80} -s
    read -e -i "${_DEFAULT}" -p "${3:->}" $1
  else
    eval "$1"=$(whiptail --title "$2" --inputbox "$3" 3>&1 1>&2 2>&3)
  fi
}

Shell redirection

In shells, redirection is used for file I/O. The most common usage of is to redirect standard streams (stdin, stdout and stderr) to accept input from another program via piping, to save program output as a file, and to suppress program output by redirecting a stream to /dev/null.