Accounts and groups
UID and GID, the seven fields of /etc/passwd, the role of /etc/shadow and /etc/group, primary and secondary groups, people accounts and service accounts, and the id, whoami and groups commands.
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User account: what is it?
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An identity under which processes run and to which files belong, with a name, a number, a primary group, a home directory and a shell.
The kernel only knows numbers: the login name is a convenience for humans, which the account files map to the UID.
In this /etc/passwd line, what are tom's UID and shell?
```text
tom:x:1002:1002:Tom Durand:/home/tom:/bin/zsh`- 1002 and /bin/zsh
- x and /bin/zsh
- 1002 and /home/tom
- Tom Durand and /home/tom
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1002 and /bin/zsh
The fields are: name (tom), x, UID (1002), GID (1002), comment, home directory (/home/tom) and shell (/bin/zsh). The UID is the third, the shell is the last.
A line of /etc/passwd has … fields separated by …: name, x, UID, GID, comment, home directory and shell.
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A line of /etc/passwd has seven fields separated by colons: name, x, UID, GID, comment, home directory and shell.
That layout is what cut -d: -f1 exploits to list login names. The sixth field is the home directory, the seventh the shell.
UID: what is it?
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The integer by which the kernel identifies an account; the administrator account's is 0.
Permissions are checked on the UID, not the name: two accounts with the same UID are the same account to the kernel. Ordinary accounts usually start at 1000.
Which accounts in this /etc/passwd excerpt can open an interactive session?
```text
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
lea:x:1000:1000:Léa Martin:/home/lea:/bin/bash`- daemon and www-data
- root and lea
- root, daemon and lea
- All four: the shell does not prevent logging in
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root and lea
A nologin shell refuses the session as soon as you log in: it is the mark of service accounts. Only root and lea have a real shell, /bin/bash.