su — Start a shell (or one command) as another user. You type the password of that user.

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ماذا يفعل

su means "substitute user". Without a name it switches to root. You must know the password of the target user — only root can switch without a password.

Always use su - (or su -l): it starts a full login shell with the environment and home folder of the new user. Without the dash you keep your own PATH and variables, which can give strange results.

On RHEL, sudo is the better tool for admin work: it uses your own password and logs every command. su is still useful for root itself, to test what another user sees.

طريقة الكتابة

su [OPTION]... [-] [USER [ARGUMENT]...]

الخيارات 12

الأكثر استخدامًا

-, -l, --login: Start a login shell: clean environment, home folder and profile of the new user.
-c, --command=COMMAND: Run only this command as the user, then return.
-s, --shell=SHELL: Use this shell instead of the user's login shell (works only if the shell is in /etc/shells or you are root).
-m, -p, --preserve-environment: Keep your current environment (do not set HOME, SHELL, USER, LOGNAME).
-g, --group=GROUP: Set the primary group (root only).
-G, --supp-group=GROUP: Add a supplementary group (root only).
-P, --pty: Create a new pseudo-terminal for the session (better isolation from the calling terminal).
-w, --whitelist-environment=LIST: With --login, keep these variables (comma list).
-f, --fast: Pass -f to the shell (only csh/tcsh: do not read the start files).
--session-command=COMMAND: Like -c but without a new session (not recommended).
-h, --help: Show help.
-V, --version: Show the version.

أمثلة 11

$ مستخدم عادي# بصلاحية root (أو مع sudo)

1Become root with a full login shell
```
$su -
Password:
#pwd
/root
```
Type the root password. exit brings you back.
2Switch to another user
```
$su - lisa
```
You need lisa's password (root does not).
3Run one command as root
```
$su -c 'dnf -y update' -
```
Runs the command and returns to your normal shell.
4Test what a user can see
```
#su - lisa -c 'ls -l /srv/share'
```
root can check permissions as the user without knowing the password.
5Use a shell for an account with nologin
```
#su -s /bin/bash -c 'whoami; id' apache
apache
```
Service accounts have /sbin/nologin; -s gives a shell for a test.
6See the difference with and without the dash
```
$su
#echo $HOME; pwd
/root
/home/student
```
Without - you stay in your folder with your environment.
7Check which accounts may not log in
```
#su - carol
This account is currently not available.
```
The shell /sbin/nologin refuses the login.
8Keep one variable with a login shell
```
#su - -w HTTPS_PROXY lisa
```
Useful when a proxy setting must be kept.
9Start the session in its own terminal (more secure)
```
#su -P - lisa
```
Protects against tricks where a program writes into your terminal (TIOCSTI).
10Find failed su attempts (security)
```
#grep 'su:' /var/log/secure | grep -i fail
```
Many failures for root can mean somebody guesses passwords.
11Allow su to root only for wheel members
```
#grep pam_wheel /etc/pam.d/su
#auth required pam_wheel.so use_uid
```
Remove the # to allow su only for members of wheel (hardening).

انتبه

Without - the environment is not changed. Use su -.
On many cloud images root has no password, so su - fails. Use sudo -i.

الملفات

/etc/pam.d/su: PAM rules for su (like pam_wheel).
/var/log/secure: Log of su and sudo use.

نصائح للحفظ

su = password of the target user; sudo = your own password.
sudo -i gives the same root login shell as su -.

رموز الخروج

0 :: The command ran (or the exit code of the command run with -c). 1 :: Error (for example a wrong password). 126 :: The command could not be run. 127 :: The command was not found.

`su`

ماذا يفعل

طريقة الكتابة

الخيارات 12

أمثلة 11

1Become root with a full login shell

2Switch to another user

3Run one command as root

4Test what a user can see

5Use a shell for an account with nologin

6See the difference with and without the dash

7Check which accounts may not log in

8Keep one variable with a login shell

9Start the session in its own terminal (more secure)

10Find failed su attempts (security)

11Allow su to root only for wheel members

انتبه

الملفات

نصائح للحفظ

رموز الخروج

الدليل الكامل

su

ماذا يفعل

طريقة الكتابة

الخيارات 12

أمثلة 11

1Become root with a full login shell

2Switch to another user

3Run one command as root

4Test what a user can see

5Use a shell for an account with nologin

6See the difference with and without the dash

7Check which accounts may not log in

8Keep one variable with a login shell

9Start the session in its own terminal (more secure)

10Find failed su attempts (security)

11Allow su to root only for wheel members

انتبه

الملفات

نصائح للحفظ

رموز الخروج

أوامر مرتبطة

مُستخدم في هذه الدروس

مُستخدم في هذه المعامل

الدليل الكامل

`su`