Ext4 inode mode calculator

Bash script to calculate inode modes for the Ext4 filesystem. Originally made for an AskUbuntu answer.

---

Achieving the conversion

Creating a test filesystem

In order to preserve our main filesystem from any possible damage after running this experiment, we're going to create a small filesystem inside a normal file for test purposes.

1. Create a zero-filled file called test with a size of 10 megabytes:

    dd if=/dev/zero of=~/test bs=10M count=1
   

2. Create an Ext4 filesystem inside the file, as if it were a partition:

    mkfs.ext4 ~/test
   

Creating some files and directories

Now we hace a fully functional filesystem inside the test file, so we're going to create some files and directories inside it.

1. Mount the newly created filesystem inside /mnt:

    sudo mount ~/test /mnt
   

2. Create a file and a directory:

    sudo mkdir /mnt/folder
    echo "contents" | sudo tee /mnt/file
   

3. Check the contents of the filesystem:

    ls -l /mnt
   
   Output should be something like this:
   
    total 2
    -rw-r--r-- 1 root root     0 may 21 18:53 file
    drw-r--r-- 2 root root  1024 may 21 18:55 folder
   

4. Unmount the test filesystem:

    sudo umount /mnt
   

Swapping the file and the folder

1. Run debugfs against the test file with write permission (-w flag):

    debugfs -w ~/test
   

2. Convert file into a folder:

   • At the debugfs prompt, type this:

        modify_inode file
     

   • A prompt will appear asking you a mode; type this:

        040644
     

   • Keep pressing return to leave the remaining data as-is until the prompt appears again.

3. Convert folder into a file:

   • At the debugfs prompt, type this:

        modify_inode folder
     

   • A prompt will appear asking you a mode; type this:

        0100644
     

   • Keep pressing return to leave the remaining data as-is until the prompt appears again.

4. To exit debugfs prompt, simply hit q and then return

Checking the success of the operation

1. Mount the test filesystem again:

    sudo mount ~/test /mnt
   

2. Check the filesystem contents:

    ls -l /mnt
   
   Now, it should show the file as if it were a directory and *vice versa*:
   
    total 2
    drw-r--r-- 1 root root     0 may 21 18:53 file
    -rw-r--r-- 2 root root  1024 may 21 18:55 folder
   

---

# Script to calculate inode modes

#!/bin/bash

#### See https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout#Inode_Table

## Terminal measures:
x="$(( $(tput cols) / 2 ))"   # Width of the terminal
y="$(( $(tput lines) /  2 ))" # Height of the terminal

## File descriptors:
declare -A types       # Declare an associative array with file descriptors
types[f]='0x8000'      # File
types[l]='0xA000'      # Link
types[s]='0xC000'      # Socket
types[d]='0x4000'      # Directory
types[p]='0x1000'      # Named pipe
types[b]='0x6000'      # Block device
types[c]='0x2000'      # Character device

## Permissions:
declare -A permission  # Declare an associative array with permissions
permission[user_S]='0x800'  # UID
permission[user_s]='0x840'  # UID and user can execute
permission[user_r]='0x100'  # User can read
permission[user_w]='0x80'   # User can write
permission[user_x]='0x40'   # User can execute
permission[group_S]='0x400' # GID
permission[group_s]='0x408' # GID and group can execute
permission[group_r]='0x20'  # Group can read
permission[group_w]='0x10'  # Group can write
permission[group_x]='0x8'   # Group can execute
permission[other_T]='0x200' # Sticky bit
permission[other_t]='0x201' # Sticky bit and other can execute
permission[other_r]='0x4'   # Other can read
permission[other_w]='0x2'   # Other can write
permission[other_x]='0x1'   # Other can execute

## Cleanup function:
function cleanup() {
    tput cvvis        # Make the cursor visible
    tput rmcup        # Restore saved terminal contents
    stty sane         # Fix problems caused by read -s
    exit 0            # Exit gracefully
}

## Function to print at a specified position:
function pprint() {
    tput cup $1 $2
    printf "${@:3}"
}

## Function to clear the notification area:
function reset() {
    pprint $((y+2)) $((x-40)) ' %.0s' {1..25} # Print 25 spaces
}

## Function to notify something to the user:
function notify() {
    reset                          # Clear the notification area
    pprint $((y+2)) $((x-40)) "$@" # Print the notification text
}

## If the terminal is smaller than 100x8, exit gracefully (self-explainatory):
if [ $x -lt 50 ] || [ $y -lt 5 ]; then
    echo 'Error, I need a minimum of 100x10 lines to run'
    exit 0
fi

## Initialize the terminal:
trap cleanup EXIT SIGHUP SIGINT SIGTERM # Call cleanup function after receiving ^C
stty -echo  cbreak                      # Put terminal in silent mode
tput smcup                              # Save terminal contents
tput civis                              # Make the cursor inisible

## Draw the big box:
printf '\033[1;37m'                            # Color
pprint $((y-3)) $((x-48)) '\u2500%.0s' {1..97} # Upper line
pprint $((y+4)) $((x-48)) '\u2500%.0s' {1..97} # Lower line
for ((i=4;i>-4;i--)); do                       # Sides:
    pprint $((y+i)) $((x-49)) '\u2502'             # Left line
    pprint $((y+i)) $((x+49)) '\u2502'             # Right line
done                                           # End sides
pprint $((y-3)) $((x-49)) '\u256D'             # Upper-left corner
pprint $((y+4)) $((x-49)) '\u2570'             # Lower-left corner
pprint $((y-3)) $((x+49)) '\u256E'             # Upper-right corner
pprint $((y+4)) $((x+49)) '\u256F'             # Lower-right corner

## Draw the small box:
printf '\033[1;35m'                             # Color
pprint $((y+1)) $((x-10)) '\u2501%.0s' {1..10}  # Upper line
pprint $((y+3)) $((x-10)) '\u2501%.0s' {1..10}  # Lower line
pprint $((y+2)) $((x-11)) '\u2503'              # Left line
pprint $((y+2)) $((x+00)) '\u2503'              # Right line
pprint $((y+1)) $((x-11)) '\u250F'              # Upper-left corner
pprint $((y+3)) $((x-11)) '\u2517'              # Lower-left corner
pprint $((y+1)) $((x+00)) '\u2513'              # Upper-right corner
pprint $((y+3)) $((x+00)) '\u251B'              # Lower-right corner

## Print type help:
pprint $((y-2)) $((x-44)) '\033[0;37mInode type: \033[1;37mf\033[0;37mile, \033[1;37md\033[0;37mirectory, \033[1;37ml\033[0;37mink, named \033[1;37mp\033[0;37mipe, \033[1;37ms\033[0;37mocket, \033[1;37mc\033[0;37mharacter device or \033[1;37mb\033[0;37mlock device.'

## Print permission help:
pprint $((y-1)) $((x-40)) '\033[0;36mPermission (\033[1;32mu\033[0;32mser\033[0;36m, \033[1;33mg\033[0;33mroup\033[0;36m or \033[1;31mo\033[0;31mther\033[0;36m): \033[1;36mr\033[0;36mead, \033[1;36mw\033[0;36mrite, e\033[1;36mx\033[0;36mecute, \033[1;36mhyphen\033[0;36m or \033[1;36mspace\033[0;36m to skip.'
pprint $((y+0)) $((x+8)) 's\033[1;36mt\033[0;36micky bit and executable, '
pprint $((y+1)) $((x+8)) 's\033[1;36mT\033[0;36micky bit not executable, '
pprint $((y+2)) $((x+8)) '\033[1;36ms\033[0;36metuid/setgid and executable, '
pprint $((y+3)) $((x+8)) '\033[1;36mS\033[0;36metuid/setgid not executable. '

## Endless loop:
while :; do

    ## Clear the input area:
    pprint $((y+2)) $((x-10)) '% *s\n' 10         # Print 16 spaces

    ## Print mask in the input area:
    printf '\033[1;37m'                           # Color for the type
    pprint $((y+2)) $((x-10)) '\u2588'            # Block for the type
    printf '\033[1;36m'                           # Color for the permision
    pprint $((y+2)) $((x- 9)) '\u2588%.0s' {1..9} # Blocks for the permission

    ## Loop through all variables to make a proper input:
    for var in type {user,group,other}_{r,w,x}; do

	## Assign colors and regex to fields:
	case "$var" in
	    (type)    color='\033[1;37m';     regex='^[fdlpscb]$'    ;;

	    (other_x)                         regex='^[-xtT]$'       ;;&
	    (user_x|group_x)                  regex='^[-xsS]$'       ;;&
	    (user_[rw]|group_[rw]|other_[rw]) regex="^[-${var: -1}]$";;&

	    (user*)   color='\033[1;32m'                             ;;
	    (group*)  color='\033[1;33m'                             ;;
	    (other*)  color='\033[1;31m'                             ;;
	esac

	## Change the pointer position:
	pprint $((y+3)) $(((x-10)+pointer)) "${color}\u2501"           # Print the pointer on its new position
	if (( pointer > 0 )); then                                     # If the pointer is not in the first position:
	    pprint $((y+3)) $(((x-10)+(pointer-1))) '\033[1;35m\u2501'     # Clear the old pointer         
	fi

	## Infinite loop until there is a valid input for the current character:
	while :; do
	    printf "$color"                       # Set the character color
	    IFS= read -rn 1 $var                  # Read a character (even if it's a space)

	    declare $var="${!var// /-}"           # Convert spaces to hyphens.
	    if [[ "$var" == "type" ]]; then       # If the current variable is type:
		declare $var="${!var//-/f}"           # Convert "-" to "f"
	    fi

	    if [[ "${!var}"  =~ $regex ]]; then   # If there is a valid input:
		reset                                 # Clear error notification if any
		break                                 # Exit from this loop
	    else                                  # Else:
		notify "\033[1;31mWrong input!"       # Print the error message
	    fi
	done

	## Print the entered value:
	pprint $((y+2)) $(((x-10)+pointer)) "${!var}"

	## Sum the current permission:
	((mode+=permission[${var%_*}_${!var}]))

	## Increment the pointer:
	((pointer++))
    done

    ## Post-read:
    unset pointer                                 # Reset the pointer
    pprint $((y+3)) $((x-1)) "\033[1;35m\u2501"   # Clear the pointer
    read -n 1                                     # Wait for Return or another character

    ## Sum file descriptor type:
    ((mode+=${types[$type]}))

    ## Final commands:
    mode=$(printf "%o" $mode)                      # Convert mode to octal (before this was decimal)
    notify "\033[1;32mOctal mode:\033[1;34m $mode" # Print the octal mode
    unset mode                                     # Reset the mode
done

View script on GitHub

Handicaps

• The folder doesn't open. You can't open it unless you put on it the "raw folder data" that contained it originally.

Further reading

https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout#Inode_Table

---

Thanks to @tallus. He gave me a great hint:

debugfs has a modify_inode command that allows you to edit an inode directly which would allow you to set the file flag to a dir.