Chapter 11. Extended Example: Merging User Databases
By now, we’ve come a long way and seen a number of shell scripts. This chapter aims to tie things together by writing shell programs to solve a moderately challenging task.
The Unix password file,
/etc/passwd, has shown up in several places
throughout the book. System administration tasks often revolve around
manipulation of the password file (and the corresponding group file,
/etc/group). The format is well
There are seven fields: username, encrypted password, user ID
number (UID), group ID number (GID), full name, home directory, and
login shell. It’s a bad idea to leave any field empty: in particular, if
the second field is empty, the user can log in without a password, and
anyone with access to the system or a terminal on
it can log in as that user. If the seventh field (the shell) is left
empty, Unix defaults to the Bourne shell,
As is discussed in detail in Appendix B, it is the user and group
ID numbers that Unix uses for permission checking when accessing files.
If two users have different names but the same UID number, then as far
as Unix knows, they are identical. There are rare
occasions when you want such a situation, but usually having two
accounts with the same UID number is a mistake. In particular, NFS
requires a uniform UID space; user number 2076 on
all systems accessing each other via NFS had better be the same user
tolstoy), or ...