linux files and folders are not inheriting parent directory permissions

I created a directory /share and gave chmod 2770 permission and chown root:stock /share.

1) When I create touch a file inside /share, I see the file has rw-rw-r-- and I don’t see rwxrws---

2) When I create a directory in /share/data I see the permission as drwxrwsr-x where are the parent directory is drwxrws---

How can I get parent child files and child directories to inherent parent permissions exactly the same.

Answer

When you create a file or directory

• The owner of the new file or directory will be your effective user id (euid). You can change user id beforehand with the su other_user command (which will prompt you for the password of other_user), or sudo su other_user (which will allow you or not, possibly asking for your password, according to the settings in /etc/sudoers*). After creating the file or directory, you can change its owner with sudo chown other_user file_name.

• The group of the new file or directory will be your effective group id. You can change your group id with the newgrp other_group command beforehand. If your current directory has other_group as group and its setgid bit is set, your effective group id will be other_group. After creating the file or directory, you can change its group with chgrp other_group file_name. newgrp, chgrp and setgid will work if you are a member of other_group. If you are not, they won’t: a group password mechanism is theoretically still in place, but it was deprecated decades ago and I’ve never seen anybody using it. Of course, you can always sudo chgrp other_group file_name, or even sudo chown other_user:other_group file_name if you want to change both.

• The read and write permissions of the new file or directory will depend on your umask, which is normally set by your configuration files at login. The most used umask values are 022 which, for files, will give you -rw-r--r-- and 002 which will give you -rw-rw-r--. The command umask will give you your current value. You can set another value with umask new_value and it will be effective till you change it or exit your shell. Directories will have also all execution permissions set by default, unless you have odd values in umask, which will block the corresponding execution bit. E.g. a umask value of 027 will create files with -rw-r----- and directories with drwxrwx---. Please refer to documentation for a complete explanation. Also, if the parent directory has the setgid bit, the new directory will have it too. There is no way of setting the setuid and sticky bits by default, nor the setgid bit for files.

• After the fact, you can always set the permissions you want with the command chmod.

That said, there is no standard command which will do what you want. However, you can easily write bash functions like the following and use them (write them in a file mycreat_functions and source mycreat_functions when needed). This will do for manually created files and directories. For file created by programs, shell redirections and the like, you will still have to correct the permissions manually.

function mymkdir () {
  local parentperms
  for a in "$@"; do

    mkdir "$a"

    # This copies all permissions of the parent,
    # exactly as they are
    parentperms="$(stat -c%a $(dirname "$a"))"
    chmod "$parentperms" "$a"

    # if I’m root...
    if [ $(id -u) = 0 ]; then
      chown "$(stat -c%u:%g "$a")" "$a"
    fi

  done
}


function mytouch () {
  local parentperms newperms
  for a in "$@"; do

    touch "$a"

    # This inherits all permissions of the parent,
    # but removes the excution and setgid bits, as is 
    # appropriate for files.
    parentperms="$(stat -c%a $(dirname "$a"))"
    newperms="$(printf %o $((8#$parentperms & 8#5666)))"
    chmod "$newperms" "$a"

    # if I’m root...
    if [ $(id -u) = 0 ]; then
      chown "$(stat -c%u:%g "$a")" "$a"
    fi

  done
}

Note: Owner, group and permissions are stored in an inode, where there is also other information on how to retrieve the file contents; the directory entry associates the inode with the file name, and ls -i shows the inode numbers of the listed files. When you copy a file, you create a new directory entry and allocate a new inode, so everything mentioned here applies. When you move a file, you create a new directory entry in the new location, but have it point to the old inode, so that owner, group and permissions are effectively untouched. If you want them to change according to the new directory entry’s parent, you have to create a mymv function along the lines of mytouch and mymkdir above.