file manager written in posix shell

shfm
________________________________________________________________________________

file manager written in posix shell

screenshot: https://user-images.githubusercontent.com/6799467/89270554-2b40ab00-d644-11ea-9f2b-bdabcba61a09.png


features
________________________________________________________________________________

* no dependencies other than a POSIX shell + POSIX printf, dd and stty ***
* tiny
* single file
* no compilation needed
* correctly handles files with funky names (newlines, etc)
* works with very small terminal sizes.

*** SIGWINCH and the size parameter to stty are not /yet/ POSIX but will be.
    - https://austingroupbugs.net/view.php?id=1053
    - https://austingroupbugs.net/view.php?id=1151

*** Raw escape sequences are used in place of tput. These are strictly VT100
    sequences minus \033[?1049[lh], \033[?25[lh] and \033[?7[lh] (?)


keybinds
________________________________________________________________________________

j - down
k - up
l - open file or directory
h - go up level
g - go to top
G - go to bottom
q - quit
: - cd to <input>
/ - search current directory *<input>*
- - go to last directory
~ - go home
! - spawn shell
. - toggle hidden files

Additional keybinds:

down  arrow  - down
up    arrow  - up
right arrow  - open file or directory
left  arrow  - go up level

enter/return - open file or directory
backspace    - go up level


todo
________________________________________________________________________________

- [x] sanitize filenames for display.
- [ ] print directories first (hard).
- [x] fix buggy focus after exit from inline editor.
- [ ] maybe file operations.
- [ ] add / to directories.
- [ ] use clearer variable names.
- [x] going up directories should center entry.
- [ ] abstract over sequences.
- [ ] look into whether tput is feasible.


opener
________________________________________________________________________________

Opening files in different applications (based on mime-type or file extension)
can be achieved via an environment variable (SHFM_OPENER) set to the location of
a small external script. If unset, the default for all files is '$EDITOR' (and
if that is unset, 'vi').

The script receives a single argument, the full path to the selected file.
The opener script is also useful on the command-line. The environment variable
is set as follows.

    export SHFM_OPENER=/path/to/script

Example scripts:

    #!/bin/sh -e
    #
    # open file in application based on file extension

    case $1 in
        *.mp3|*.flac|*.wav)
            mpv --no-video "$1"
        ;;

        *.mp4|*.mkv|*.webm)
            mpv "$1"
        ;;

        *.png|*.gif||*.jpg|*.jpe|*.jpeg)
            gimp "$1"
        ;;

        *.html|*.pdf)
            firefox "$1"
        ;;

        # all other files
        *)
            "${EDITOR:=vi}" "$1"
        ;;
    esac


    #!/bin/sh -e
    #
    # open file in application based on mime-type

    mime_type=$(file -bi)

    case $mime_type in
        audio/*)
           mpv --no-video "$1"
        ;;

        video/*)
            mpv "$1"
        ;;

        image/*)
            gimp "$1"
        ;;

        text/html*|application/pdf*)
            firefox "$1"
        ;;

        text/*|)
            "${EDITOR:=vi}" "$1"
        ;;

        *)
            printf 'unknown mime-type %s\n' "$mime_type"
        ;;
    esac


implementation details
________________________________________________________________________________

* Draws are partial!

  The file manager will only redraw what is necessary. Every line scrolled
  corresponds to three lines being redrawn. The current line (clear highlight),
  the destination line (set highlight) and the status line (update location).


* POSIX shell has no arrays.

  It does however have an argument list (used for passing command-line arguments
  to the script and when calling functions).

  Restrictions:

  - Can only have one list at a time (in the same scope).
  - Can restrict a list's scope but cannot extend it.
  - Cannot grab element by index.

  Things I'm thankful for:

  - Elements can be "popped" off the front of the list (using shift).
  - List size is given to us (via $#).
  - No need to use a string delimited by some character.
  - Can loop over elements.


* Cursor position is tracked manually.

  Grabbing the current cursor position cannot be done reliably from POSIX shell.
  Instead, the cursor starts at 0,0 and each movement modifies the value of a
  variable (relative Y position in screen). This variable is how the file
  manager knows which line of the screen the cursor is on.


* Multi-byte input is handled by using a 2D case statement.

  (I don't really know what to call this, suggestions appreciated)

  Rather than using read timeouts (we can't sleep < 1s in POSIX shell anyway)
  to handle multi-byte input, shfm tracks location within sequences and handles
  this in a really nice way.

  The case statement matches "$char$esc" with "$esc" being an integer holding
  position in sequences. To give an example, down arrow emits '\033[B'.

  - When '\033?' is found, the value of 'esc' is set to '1'.
  - When '[1'    is found, the value of 'esc' is set to '2'.
  - When 'B2'    is found, we know it's '\033[B' and handle down arrow.
  - If input doesn't follow this sequence, 'esc' is reset to '0'.


* There is no usage of '[' or 'test'.

  Despite these being commonly provided as "shell builtins" (part of the shell),
  a lot of shells still use the external utilities from the coreutils. All usage
  of these has been replaced with 'case' as it is always a "shell keyword".

  This is one of the approaches taken to reduce the need for anything external.


* Filename escaping works via looping over a string char by char.

  I didn't think this was possible in POSIX shell until I needed to do this in
  KISS Linux's package manager and found a way to do so.

  I'll let the code speak for itself (comments added for clarity):

      file_escape() {
          # store the argument (file name) in a temporary variable.
          # ensure that 'safe' is empty (we have no access to the local keyword
          # and can't use local variables without also using a sub-shell). This
          # variable will contain its prior value (if it has one) otherwise.
          tmp=$1 safe=

          # loop over string char by char.
          # this takes the approach of infinite loop + inner break condition as
          # we have no access to [ (personal restriction).
          while :; do
              # Remove everything after the first character.
              c=${tmp%"${tmp#?}"*}

              # Construct a new string, replacing anything unprintable with '?'.
              case $c in
                  [[:print:]]) safe=$safe$c ;;
                  '')          return ;; # we have nothing more to do, return.
                  *)           safe=$safe\? ;;
              esac

              # Remove the first character.
              # This shifts our position forward.
              tmp=${tmp#?}
          done
      }

      # Afterwards, the variable 'safe' contains the escaped filename. Using
      # globals here is a must. Printing to the screen and capturing that
      # output is too slow.


* SIGWINCH handler isn't executed until key press is made.

  SIGWINCH doesn't seem to execute asynchronously when the script is also
  waiting for input. This causes resize to require a key press.

  I'm not too bothered by this. It does save me implementing resize logic which
  is utter torture. :)