Linux Command Directory

Linux in a Nutshell

This directory of Linux commands is from Linux in a Nutshell, 5th Edition.

Click on any of the 687 commands below to get a description and list of available options. All links in the command summaries point to the online version of the book on Safari Bookshelf.

Buy it now, or read it online on Safari Bookshelf.


useradd [options] [user]

System administration command. Create new user accounts or update default account information. Unless invoked with the -D option, user must be given. useradd will create new entries in system files. Home directories and initial files may also be created as needed.


-c comment

Comment field.

-d dir

Home directory. The default is to use user as the directory name under the home directory specified with the -D option.

-e date

Account expiration date. Use the format MM/DD/YYYY. Two-digit year fields are also accepted. The value is stored as the number of days since January 1, 1970. This option requires the use of shadow passwords.

-f days

Permanently disable account this many days after the password has expired. A value of -1 disables this feature. This option requires the use of shadow passwords.

-g group

Initial group name or ID number. If a different default group has not been specified using the -D option, the default group is 1.

-G groups

Supplementary groups given by name or number in a comma-separated list with no whitespace.

-k [dir]

Copy default files to the user's home directory. Meaningful only when used with the -m option. Default files are copied from /etc/skel/ unless an alternate dir is specified.


Make user's home directory if it does not exist. The default is not to make the home directory.


Do not create a home directory for the user, even if the system default in /etc/login.defs is to create one.


Red Hat-specific option. Turn off the Red Hat default that creates a group with the same name as the username and puts the user in that group.


Override. Accept a nonunique uid with the -u option. (Probably a bad idea.)

-p passwd

The encrypted password, as returned by crypt(3).


Red Hat-specific option. Create a system account with a non-expiring password and a UID lower than the minimum defined in /etc/login.defs. Do not create a home directory for the account unless -m is also specified.

-s shell

Login shell.

-u uid

Numerical user ID. The value must be unique unless the -o option is used. The default value is the smallest ID value greater than 99 and greater than every other uid.

-D [options]

Set or display defaults. If options are specified, set them. If no options are specified, display current defaults. The options are:

-b dir

Home directory prefix to be used in creating home directories. If the -d option is not used when creating an account, the user name will be appended to dir.

-e date

Expire date. Requires the use of shadow passwords.

-f days

Number of days after a password expires to disable an account. Requires the use of shadow passwords.

-g group

Initial group name or ID number.

-s shell

Default login shell.

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