The editmap program must be run with one of three mandatory command-line switches. This -u switch is one of those three.

The -u command-line switch tells editmap to open the database file in read/write mode, to look up the key, and to replace that key's value. If the key is not already in the database, both the key and its value are added to the database:

% editmap -u  dbtype dbfile sought-key new-value  

If the dbfile lacks read/write permission, the following error will print and editmap will exit with an EX_IOERR (EX_IOERR) exit value:

editmap: error opening type dbtype map dbfile: Permission denied

If the sought-key is found, its value is replaced with the new-value. If the sought-key is not found, both the key and value are added to the database.

Be careful updating aliases-style database files. If you omit the -N command-line switch with such files, your key will not be found even if it exists, and thus the key and value will wrongly, and silently, update the database. To illustrate, consider this alias:

bob: bob@another.host.domain

If bob moves, you might want to directly edit this aliases-style database to change his address. Here is the correct way to update:

% editmap -N -q hash offsite-aliases bob
bob@another.host.domain
% editmap -N -u hash offsite-aliases bob bob@new.domain
% editmap -N -q hash offsite-aliases bob
bob@new.domain

Here, because offsite-aliases is an aliases-style database, we use the -N command-line switch with all commands. The first and last commands look up (with -q) bob in the database. The second command changes the address for bob.

In the following we omit the -N from the update command. This is the wrong way to update such a database:

% editmap -N -q hash offsite-aliases bob 
bob@another.host.domain
% editmap -u hash offsite-aliases bob bob@new.domain      note no -N 
% editmap -N -q hash offsite-aliases bob 
bob@another.host.domain                             note no update
% editmap -q hash offsite-aliases bob 
bob@new.domain                                     note bogus update

The key bob is in the database, but with a trailing null byte. The update command (the -u) omits the -N and so omits a trailing null byte from the looked-up key. As a result, the key is not found, so the key and value are wrongly added as a new key/value pair to the database. The last two commands illustrate the problem. The first correctly uses the -N and finds that the value was not updated. The second incorrectly omits the -N and finds that another bob (the one without the trailing null byte) got the update.