| CHPASSWD(8) | System Management Commands | CHPASSWD(8) |
.SH "NAME" chpasswd - update passwords in batch mode
.SH "SYNOPSIS"
.HP 216u
chpasswd
[options]
.SH "DESCRIPTION"
.PP The chpasswd command reads a list of user name and password pairs
from standard input and uses this information to update a group of existing
users. Each line is of the format:
.PP user_name:password
.PP By default the passwords must be supplied in clear-text, and are
encrypted by chpasswd. Also the password age will be updated, if
present.
.PP By default, passwords are encrypted by PAM, but (even if not recommended)
you can select a different encryption method with the -e or -c
options.
.PP Except when PAM is used to encrypt the passwords, chpasswd first
updates all the passwords in memory, and then commits all the changes to
disk if no errors occurred for any user.
.PP When PAM is used to encrypt the passwords (and update the passwords in
the system database) then if a password cannot be updated chpasswd
continues updating the passwords of the next users, and will return an error
code on exit.
.PP This command is intended to be used in a large system environment where
many accounts are created at a single time.
.SH "OPTIONS"
.PP The options which apply to the chpasswd command are:
.PP -c, --crypt-method METHOD
Use the specified method to encrypt the passwords.
The available methods are SHA256, SHA512, YESCRYPT and NONE if your libc supports these methods.
By default, PAM is used to encrypt the passwords.
.RE
.PP -e, --encrypted
Supplied passwords are in encrypted form.
.RE
.PP -h, --help
Display help message and exit.
.RE
.PP -R, --root CHROOT_DIR
Apply changes in the CHROOT_DIR directory and use the configuration files from the CHROOT_DIR directory. Only absolute paths are supported. No SELINUX support.
.RE
.PP -P, --prefix PREFIX_DIR
Apply changes to configuration files under the root filesystem found under the directory PREFIX_DIR. This option does not chroot and is intended for preparing a cross-compilation target. Some limitations: NIS and LDAP users/groups are not verified. PAM authentication is using the host files. No SELINUX support.
.RE
.PP -s, --sha-rounds ROUNDS
Use the specified number of rounds to encrypt the passwords.
You can only use this option with crypt method: SHA256 SHA512 YESCRYPT
By default, the number of rounds for SHA256 or SHA512 is defined by the SHA_CRYPT_MIN_ROUNDS and SHA_CRYPT_MAX_ROUNDS variables in /etc/login.defs.
A minimal value of 1000 and a maximal value of 999,999,999 will be enforced for SHA256 and SHA512. The default number of rounds is 5000.
By default, the number of rounds for YESCRYPT is defined by the YESCRYPT_COST_FACTOR in /etc/login.defs.
A minimal value of 1 and a maximal value of 11 will be enforced for YESCRYPT. The default number of rounds is 5.
.RE
.SH "CAVEATS"
.PP Remember to set permissions or umask to prevent readability of
unencrypted files by other users.
.SH "CONFIGURATION"
.PP The following configuration variables in /etc/login.defs change the
behavior of this tool:
SHA_CRYPT_MIN_ROUNDS (number), SHA_CRYPT_MAX_ROUNDS (number)
When ENCRYPT_METHOD is set to SHA256 or SHA512, this
defines the number of SHA rounds used by the encryption algorithm by default
(when the number of rounds is not specified on the command line).
With a lot of rounds, it is more difficult to brute force the password. But
note also that more CPU resources will be needed to authenticate users.
If not specified, the libc will choose the default number of rounds (5000),
which is orders of magnitude too low for modern hardware.
The values must be inside the 1000-999,999,999 range.
If only one of the SHA_CRYPT_MIN_ROUNDS or SHA_CRYPT_MAX_ROUNDS
values is set, then this value will be used.
If SHA_CRYPT_MIN_ROUNDS > SHA_CRYPT_MAX_ROUNDS, the highest
value will be used.
Note: this only affect the generation of group passwords. The generation of
user passwords is done by PAM and subject to the PAM configuration. It is
recommended to set this variable consistently with the PAM
configuration.
YESCRYPT_COST_FACTOR (number)
When ENCRYPT_METHOD is set to YESCRYPT, this defines the cost
factor used by the encryption algorithm by default (when the cost factor is
not specified on the command line).
With a high cost factor, it is more difficult to brute force the password.
But note also that more CPU resources will be needed to authenticate
users.
The value must be inside the 1-11 range.
Note: this only affect the generation of group passwords. The generation of
user passwords is done by PAM and subject to the PAM configuration. It is
recommended to set this variable consistently with the PAM
configuration.
.SH "FILES"
.PP /etc/passwd
User account information.
.RE
.PP /etc/shadow
Secure user account information.
.RE
.PP /etc/login.defs
Shadow password suite configuration.
.RE
.PP /etc/pam.d/chpasswd
PAM configuration for chpasswd.
.RE
.SH "SEE ALSO"
.PP passwd(1), newusers(8), login.defs(5),
useradd(8).
| 08/29/2026 | shadow-utils 4.20.2 |