Understanding the modifier keys in the terminal
I'm sure everyone reading this has ran a curses-based program in their terminal. And most likely this program has taken use of a ctrl- (⎈), alt- (⎇), or meta- (◆), or super-key. Unfortunately the implementation is pretty quirky.
We can check how these keys are interpreted in the terminal by running tee.
The following is the output of Ctrl - a, Alt - a, Alt + Shift - a, Esc, Enter (to flush) and Ctrl - c (to exit)
$ tee output
^A^[a^[A^[
^C
As we can see, all the key combinations are prefixed by a caret symbol, which is called caret notation; used to denote that the character is an ASCII-encoded control character.
If we check out the hex values of the characters we stored in the output file
$ hexdump -C <output
00000000 01 1b 61 1b 41 1b 0a |..a.A..|
00000007
we see that the first character is 01.
To check which character that is, we can open up the ASCII table;
$ man ascii
ASCII(7) BSD Miscellaneous Information Manual ASCII(7)
[...]
The hexadecimal set:
00 nul 01 soh 02 stx 03 etx 04 eot 05 enq 06 ack 07 bel
08 bs 09 ht 0a nl 0b vt 0c np 0d cr 0e so 0f si
10 dle 11 dc1 12 dc2 13 dc3 14 dc4 15 nak 16 syn 17 etb
18 can 19 em 1a sub 1b esc 1c fs 1d gs 1e rs 1f us
20 sp 21 ! 22 " 23 # 24 $ 25 % 26 & 27 '
28 ( 29 ) 2a * 2b + 2c , 2d - 2e . 2f /
30 0 31 1 32 2 33 3 34 4 35 5 36 6 37 7
38 8 39 9 3a : 3b ; 3c < 3d = 3e > 3f ?
40 @ 41 A 42 B 43 C 44 D 45 E 46 F 47 G
48 H 49 I 4a J 4b K 4c L 4d M 4e N 4f O
50 P 51 Q 52 R 53 S 54 T 55 U 56 V 57 W
58 X 59 Y 5a Z 5b [ 5c \ 5d ] 5e ^ 5f _
60 ` 61 a 62 b 63 c 64 d 65 e 66 f 67 g
68 h 69 i 6a j 6b k 6c l 6d m 6e n 6f o
70 p 71 q 72 r 73 s 74 t 75 u 76 v 77 w
78 x 79 y 7a z 7b { 7c | 7d } 7e ~ 7f del
[...]
As we can see, 01 corresponds with SOH, short for Start Of Header, but is most often written as ^A - Control A.
Pretty straightforward.
Then there's ^[a which was Alt and a.
Instead of being a single character, it's composed of two characters; ^[ and a.
We actually see a lot of ^[/0x1b characters in the remaining key combos, the only difference in the next combo is that the a is an A due to us holding down the Shift key.
The escape key is from us actually pressing Esc.
As you've might noticed, Alt and Esc seem to be very similar.
The short version of it is that you can use Esc instead of Alt, but not the other way round.
After understanding the difference between the way Alt and Control are handled in the terminal makes it apparent why you can't have combos like Control + Shift - a, but Alt + Shift - a works.