Linux user etc passwd

Linux user etc passwd

NAME

passwd - the password file

DESCRIPTION

/etc/passwd contains one line for each user account, with seven fields delimited by colons (“:”). These fields are: • login name • optional encrypted password • numerical user ID • numerical group ID • user name or comment field • user home directory • optional user command interpreter If the password field is a lower-case “x”, then the encrypted password is actually stored in the shadow(5) file instead; there must be a corresponding line in the /etc/shadow file, or else the user account is invalid. The encrypted password field may be empty, in which case no password is required to authenticate as the specified login name. However, some applications which read the /etc/passwd file may decide not to permit any access at all if the password field is blank. A password field which starts with an exclamation mark means that the password is locked. The remaining characters on the line represent the password field before the password was locked. Refer to crypt(3) for details on how this string is interpreted. If the password field contains some string that is not a valid result of crypt(3), for instance ! or *, the user will not be able to use a unix password to log in (but the user may log in the system by other means). The comment field is used by various system utilities, such as finger(1). The home directory field provides the name of the initial working directory. The login program uses this information to set the value of the $HOME environmental variable. The command interpreter field provides the name of the user's command language interpreter, or the name of the initial program to execute. The login program uses this information to set the value of the $SHELL environmental variable. If this field is empty, it defaults to the value /bin/sh.

FILES

/etc/passwd User account information. /etc/shadow optional encrypted password file /etc/passwd- Backup file for /etc/passwd. Note that this file is used by the tools of the shadow toolsuite, but not by all user and password management tools.

SEE ALSO

crypt(3), getent(1), getpwnam(3), login(1), passwd(1), pwck(8), pwconv(8), pwunconv(8), shadow(5), su(1), sulogin(8).

© 2019 Canonical Ltd. Ubuntu and Canonical are registered trademarks of Canonical Ltd.

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Introduction

Linux is truly fascinating. In this blog, we’ll understand about a special file in linux.

Let’s dive straight into it.

What is /etc/passwd file?

/etc/passwd is a configuration file which stores user account information. It is a plain text-based file containing information like username, user ID and group ID.

This file is owned by root and has rw-r—r— permissions(octal 644). Thus, the file can be read by any user but only root user or user with sudo privileges can write to the file.

How can I view that file?

To view the contents of the file, open the terminal and type in:

The output of this command should be similar to the one shown below.

daniel@DVM:~$ cat /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin bin:x:2:2:bin:/bin:/usr/sbin/nologin sys:x:3:3:sys:/dev:/usr/sbin/nologin sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/usr/sbin/nologin man:x:6:12:man:/var/cache/man:/usr/sbin/nologin lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin mail:x:8:8:mail:/var/mail:/usr/sbin/nologin news:x:9:9:news:/var/spool/news:/usr/sbin/nologin uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin proxy:x:13:13:proxy:/bin:/usr/sbin/nologin www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin backup:x:34:34:backup:/var/backups:/usr/sbin/nologin list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin systemd-network:x:100:102:systemd Network Management. /run/systemd:/usr/sbin/nologin systemd-resolve:x:101:103:systemd Resolver. /run/systemd:/usr/sbin/nologin systemd-timesync:x:102:104:systemd Time Synchronization. /run/systemd:/usr/sbin/nologin messagebus:x:103:106::/nonexistent:/usr/sbin/nologin syslog:x:104:110::/home/syslog:/usr/sbin/nologin _apt:x:105:65534::/nonexistent:/usr/sbin/nologin tss:x:106:111:TPM software stack. /var/lib/tpm:/bin/false uuidd:x:107:112::/run/uuidd:/usr/sbin/nologin tcpdump:x:108:113::/nonexistent:/usr/sbin/nologin sshd:x:109:65534::/run/sshd:/usr/sbin/nologin landscape:x:110:115::/var/lib/landscape:/usr/sbin/nologin pollinate:x:111:1::/var/cache/pollinate:/bin/false daniel:x:1000:1000:Daniel Tanzer. /home/daniel:/bin/bash 

Can I modify that file?

Yes, you can modify the file contents using any text editor like vim, nano or emacs but it’s considered to be a bad idea unless you know what you are doing.
You must always use dedicated commands to modify the file. Let’s say for an example, you want to add a new user to the system. For doing so, you must use adduser or useradd command instead of manually editing the /etc/passwd file using a text editor.

Understanding /etc/passwd file format

/etc/passwd file contains many lines, one for each user. The first line contains information about root user followed by system user accounts and normal user accounts.

It has 7 fields separated by colon(:).

1) Username :

This is the first field in a line which represents the login name of the user. It has a length ranging from 1 to 32 characters.

2) Password :

This is the second field in a line. In older linux systems, user’s encrypted password was stored here. Now in the modern systems, this field is replaced by a character x and the encrypted password is stored in a file called /etc/shadow .

If the field is blank, we do not need a password to login to the system.

To change the password of any user, use passwd command which stores the password in encrypted form in /etc/shadow .

3) User ID (UID):

This is the third field in a line. It contains a unique identifier of a user which is used by an operating system to refer to a user.

UID 0 is reserved for root user.
UID 1-99 is reserved for other predefined accounts.
UID 100-999 is reserved for system accounts.
UID above 999 are for normal user accounts.

4) Group ID(GID):

This is the fourth field in a line. It determines the primary group of the user. Users can belong to more than one group in linux. To get a full list of groups a user belongs to, type in the command:

The first group in the output is the primary group and the rest are secondary groups .

5) GECOS :

This is the fifth field in a line. It contains comma-separated information about the user including:

6) Home directory :

This is the sixth field in a line which contains the path to the user’s home directory. By default, this path is under /home directory and is named after the user. For example, for a user having a username daniel , his home directory would be /home/daniel .

7) Login Shell :

This is the seventh and the last field in the line. It contains path to the user’s default login shell. For most of the distributions, it is bash having the path /bin/bash .

It is not necessary to for it to be a shell. For example, system administrators can use nologin shell having path /sbin/nologin . So, if a user tries to login to an account with nologin shell, the nologin shell closes the connection.

This is it for the blog. I hope you understood the format of the file /etc/passwd .

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