Python script to convert a Linux octal permission number to a text string

perm_to_text.py

def perm_to_text(perm):
    perms = {
        "0": "---",
        "1": "--x",
        "2": "-w-",
        "3": "-wx",
        "4": "r--",
        "5": "r-x",
        "6": "rw-",
        "7": "rwx"
        }
    if len(perm) == 4:
        first = perm[0]
        perm = perm[1:]
    else:
        first = ""

    try:
        outperms = ""
        for p in perm:
            outperms += perms[p]
    except KeyError as e:
        outperms = perm

    if first != "":
        if first == '0':
            pass
        elif first == '1':
            pass
        elif first == '2':
            if outperms[5] == 'x':
                outperms = outperms[:5]+'s'+outperms[6:]
            else:
                outperms = outperms[:5]+'S'+outperms[6:]
        elif first == '4':
            if outperms[2] == 'x':
                outperms = outperms[:2]+'s'+outperms[3:]
            else:
                outperms = outperms[:2]+'S'+outperms[3:]
        else:
            outperms = perm

    return "-"+outperms

From the original discussion c/o hack-tramp:

Warning : the above functions do NOT deal with four digit chmod octals.

When the octal is 4 digits long, the first digit is a setuid, setguid or sticky flag. See here for more info: https://linuxize.com/post/chmod-command-in-linux/ and here: https://en.wikipedia.org/wiki/File_system_permissions#Notation_of_traditional_Unix_permissions

Below is a version of the above function which correctly deals with four digit octals - be warned, it is not elegant!

def perm_to_text(octal):
   result =  ''
   first = 0
   octal = str(octal)
   #if there are more than 4 digits, just take the last 4
   if len(octal)>4:
      #separate initial digit
      octal = octal [-4:]
   #if there are 4 digits, deal with first (setuid, setgid, and sticky flags) separately
   if len(octal)==4:
      if octal[0]!='0': 
         first = int(octal [:1])
      octal = octal [-3:]
   value_letters = [(4, 'r'), (2, 'w'), (1, 'x')]
   # Iterate over each of the digits in octal
   for permission in [int(n) for n in octal]:
      # Check for each of the permissions values
      for value, letter in value_letters:
         if permission >= value:
            result += letter
            permission -= value
         else:
            result += '-'
   if first!=0:
      for value in [4,2,1]:
         if first >= value:
            if value==4:
               if result[2] == 'x':
                  result = result[:2]+'s'+result[3:]
               elif result[2] == '-':
                  result = result[:2]+'S'+result[3:]
            if value==2:
               if result[5] == 'x':
                  result = result[:5]+'s'+result[6:]
               elif result[5] == '-':
                  result = result[:5]+'S'+result[6:]            
            if value==1:
               if result[8] == 'x':
                  result = result[:8]+'t'+result[9:]
               elif result[8] == '-':
                  result = result[:8]+'T'+result[9:]             
            first -= value   
   
   return result

print(perm_to_text('3777'))
#rwxrwsrwt
print(perm_to_text('2775'))
#rwxrwsr-x