losecontrol.sh — remove control characters from the logged output of a screen session

losecontrol.sh

#!/bin/sh

# ---------------------------------------------------------------------------- #
# losecontrol.sh
# ---------------------------------------------------------------------------- #
#
# Author:
#   Stephan Casas <[email protected]>
#
# Description:
#   Remove control characters from logged terminal output while maintaining the
#   the on-screen output as plaintext.
#
# Why?
#   Logged terminal output may include redundant data. Simply removing control
#   characters will not yield reliable content. It is necessary to both remove
#   the control characters while also evaluating those which modify the output
#   text's visible (on-screen) plaintext value.
# ---------------------------------------------------------------------------- #

ESC=$(printf "\e")

BUF=""
while read -r LINE; do # read file by line
    LINEBUF=""
    ESCAPING=false
    while read -r -n1 CHAR; do # read line by character
        # unencode ifs entities
        CODE=$(printf %d \'$CHAR)

        # `NUL` -> Whitespace
        [ $CODE -eq 0 ] && CHAR=" "
        # `CR`  -> Skip until output buffer
        [ $CODE -eq 13 ] && CHAR=""

        # buffer control sequence
        $ESCAPING && CONTROL="$CONTROL$CHAR"
        [ "$CHAR" = "$ESC" ] && ESCAPING=true && CONTROL=""

        # buffer printable chars to line
        ! $ESCAPING && LINEBUF="$LINEBUF$CHAR"

        # end control sequence at terminal char
        $ESCAPING &&
            # `A-Z` -> [065:090]
            eval "[ $CODE -gt 64 ] && [ $CODE -lt 91 ]" ||
            # `a-z` -> [097:122]
            eval "[ $CODE -gt 96 ] && [ $CODE -lt 123 ]" ||
            # `=`   -> [061:061]
            eval "[ $CODE -eq 61 ]" && ESCAPING=false &&
            # simulate cursor reset
            [ "$CONTROL" = "[0m" ] && STASH="$LINEBUF" && LINEBUF=""
    done <<EOF
$LINE
EOF
    # restore stash content if line empty
    [ ${#LINEBUF} -eq 1 ] && APPEND="$STASH" || APPEND="$LINEBUF"

    # buffer line to output and clear stash
    BUF="$BUF$APPEND$(printf '\n\r')" && STASH=""
done <<EOF
$(cat $1)
EOF

echo "$BUF"

exit 0