Casing arrow keys in bash

Question

Is it possible to case arrow keys in a bash script to run a certain set of commands if the up/left arrow keys are pressed, and a certain set if the down/right arrow keys are

Answer 1

None of the above answers have worked for me! I had to grab bits and pieces from many answers in this thread, as well as other searches via google. It took me about an hour to concoct this.

I am running Ubuntu 20.04 LTS and this works for me (although, it may not be perfect, as I had to 'hack' at it):

waitkey() {
  local end=""
  local key=""

  echo
  echo "   Press ESC ... "

  while [ "$end" == "" ]; do
    read -rsn1 key
    case "$key" in
      $'\x1b')
        local k=""
        # I'm not sure why I have to do this if statement,
        # but without it, there are errors.  took forever
        # to figure out why 'read' would dump me outta the script
        if [ "$IFS" ]; then
          read -rsn1 -t 0.1 holder && k="$holder"
        else
          IFS=read -rsn1 -t 0.1 holder && k="$holder"
        fi 

        if [ "$k" == "[" ]; then
          read -rsn1 -t 0.1 holder && kk="$holder"

          ##############################
          # you put your arrow code here
          #
          # eg:
          #  case "$kk" in
          #    "A") echo "up arrow!" ;; # do something ...
          #  esac
          ##############################
        elif [ "$k" == "O" ]; then
          read -rsn1 -t 0.1 holder && kk="$holder"

          # I am honestly not knowing what this is for
        elif [ "$k" == "" ]; then
          end=1
        fi
    esac
  done
}

Answer 2

Not sure if this answer the question directly, but I think it's related - I was wandering where do those codes come from, and I finally found:

• Linux Keycode Table (comptechdoc.org)

It's a bit difficult to read at first; for left arrow, lookup "LEFT 4" in the "Key" column, and for the sequence that bash sees, look up the 5th ("keymap" - "normal") column, where it is written as "[D 1b 5b 44" - which are the three bytes (27, 91, 68) representing this key.

Finding the thread How to read arrow keys on really old bash? - The UNIX and Linux Forums, inspired me to write a short one-liner which dumps the key codes of keys pressed. Basically, you press a key, then Enter (to trigger ending of read), and then use hexdump to output what read has saved (and finally hit Ctrl-C to exit the loop):

$ while true; do read -p?; echo -n $REPLY | hexdump -C; done
?^[[D     
00000000  1b 5b 44                                          |.[D| # left arrow
00000003
?^[[C
00000000  1b 5b 43                                          |.[C| # right arrow
00000003
?^[[1;2D
00000000  1b 5b 31 3b 32 44                                 |.[1;2D| # Shift+left arrow
00000006
?^[[1;2C
00000000  1b 5b 31 3b 32 43                                 |.[1;2C| # Shift+right arrow
00000006
?^C

So, while arrow keys require 3 bytes - Shift+arrow keys require 6! However, seemingly all these sequence start with 0x1b (27), so one could possibly check for this value for read -n1, before reading any more bytes; also 5b remains a second byte in multi-byte sequence for the "normal" and "shift/NUM-Lock" columns of the table above.

---

Edit: much easier and proper way to scan for terminal codes of pressed keys in Linux is via showkey:

$ showkey 
Couldn't get a file descriptor referring to the console

$ showkey -h
showkey version 1.15

usage: showkey [options...]

valid options are:

    -h --help   display this help text
    -a --ascii  display the decimal/octal/hex values of the keys
    -s --scancodes  display only the raw scan-codes
    -k --keycodes   display only the interpreted keycodes (default)

$ sudo showkey -a

Press any keys - Ctrl-D will terminate this program

^[[A     27 0033 0x1b
         91 0133 0x5b
         65 0101 0x41
^[[B     27 0033 0x1b
         91 0133 0x5b
         66 0102 0x42
^[[A     27 0033 0x1b
         91 0133 0x5b
         65 0101 0x41
^[[D     27 0033 0x1b
         91 0133 0x5b
         68 0104 0x44
^[[C     27 0033 0x1b
         91 0133 0x5b
         67 0103 0x43
^C       3 0003 0x03
^M       13 0015 0x0d
^D       4 0004 0x04