id command examples in linux ?

How do I find out the user and group’s names and numeric IDs of the current user or any users on

my server? How can I display and effective IDs on the system using command line options? In Linux,

how do I find a user’s UID or GID?

To find a user’s UID (user ID) or GID (group ID) and other information in Linux/Unix-like operating
systems, use the id command.

This command is useful to find out the following information:

• Get User name and real user ID
• Find a specific user’s UID
• List out all the groups a user belongs to
• Show the UID and all groups associated with a user
• Display security context of the current user
• Effective Linux or Unix user name and effective user ID (UID)
• Name of effective Linux or Unix user’s group and effective group ID (GID)

Purpose

Displays the system identifications of a specified user.

id command syntax

The basic syntax is:

id

id [UserNameHere]

id [options]

id [options] [UserNameHere]

 id --help

Usage: id [OPTION]... [USERNAME]

Print user and group information for the specified USERNAME,

or (when USERNAME omitted) for the current user.

  -a              ignore, for compatibility with other versions

  -Z, --context   print only the security context of the current user

  -g, --group     print only the effective group ID

  -G, --groups    print all group IDs

  -n, --name      print a name instead of a number, for -ugG

  -r, --real      print the real ID instead of the effective ID, with -ugG

  -u, --user      print only the effective user ID

      --help     display this help and exit

      --version  output version information and exit

Display your own UID and GID

Type the command:

id

Sample outputs:

id

uid=1000(dev) gid=1000(dev) groups=1000(dev),4(adm),24(cdrom),27(sudo)

How do I find a specific user’s UID?

In this example, find a dev user’s UID, type:

id -u {UserNameHere}

id -u dev

Sample output:

id -u dev

1000

How do I find a specific user’s GID?

In this example, find a dev user’s GID, run:

id -g {UserNameHere}

id -g dev

Sample output:

id -g dev

1000

How do I see the UID and all groups associated with a user name?

In this example, find the UID and all groups associated with a user called ‘root’, enter:

id {UserNameHere}

id root

Sample output:

id root

uid=0(root) gid=0(root) groups=0(root)

Find out all the groups a user belongs to  !!

In this example, display the UID and all groups associated (secondary groups) with a user called ‘dev’, run:

id -G {UserNameHere}

id -G dev

Sample output:

id -G dev

1000 4 24 27 30 46 108 124 142

How do I display real ID instead of the effective ID for specified user?

You can show the real ID for the -g, -G and -u options instead of the effective ID by passing the -r option:

id -r -g {UserNameHere}

id -r -u {UserNameHere}

### [NOTE]  -r and -G only works on Linux

id -r -G {UserNameHere}

id -r -u dev

Sample output:

id -r -u dev

1000

Determining root privileges in a script

Linux and Unix sysadmin relates shell scripts must be run by root user. The following shell script

shows how to determining root privileges in a script:

#!/bin/bash

## if root user not running this script, die with a message on screen ##

if [ $(id -u -r) -ne 0 ]

then

        echo "Requires root privileges. Please re-run using sudo."

        exit 1

fi

How to Make File undeletable Even By Root in Linux ?

On Unix-like operating systems including Linux, root is the account or user name that by default can

modify all directories and files on a system. In this article, we’ll show how to make directories or

files unremovable even by the root user in Linux.

To make a file undeletable by any system user, including root, you need to make it unmodifiable

using using the chattr command. This command changes file attributes on a Linux file system.

How to Make File Undeletable in Linux

The command below makes test.txt file immutable (or undeletable). This implies that the file can’t

be modified in any way: it can’t be deleted or renamed. You can’t even create a link to it and no data

can be written to the file as well.

Note that you need superuser privileges to set or remove this attribute, using the sudo command:

chattr - change file attributes on a Linux file system

DESCRIPTION

      chattr changes the file attributes on a Linux file system.

      The format of a symbolic mode is +-=[acdeijstuACDST].

      The operator `+' causes the selected attributes to be added to the existing attributes of the files;

`-' causes them to be removed; and `=' causes

      them to be the only attributes that the files have.

      The letters `acdeijstuACDST' select the new attributes for the files: append only (a), compressed

(c), no dump (d), extent  format (e), immutable (i), data  journalling (j),  secure deletion (s), no

tail-merging (t), undeletable (u), no atime

updates (A), no copy on write (C), synchronous

      directory updates (D), synchronous updates (S), and top of directory hierarchy (T).

      The following attributes are read-only, and may be listed by lsattr(1) but not modified by chattr:

huge file (h), compression  error (E), indexed

      directory (I), compression raw access (X), and compressed dirty file (Z).

OPTIONS

      -R Recursively change attributes of directories and their contents.

      -V Be verbose with chattr's output and print the program version.

      -f Suppress most error messages.

      -v version

             Set the file's version/generation number.

$ sudo chattr +i test.txt

OR

$ sudo chattr +i -V test.txt

Output:

sudo chattr +i -V test.txt

chattr 1.42.9 (4-Feb-2014)

Flags of test.txt set as ----i--------e--

To view attributes of a file, use the lsattr command as shown.

$ lsattr test.txt

Output:

lsattr test.txt

----i--------e-- test.txt

Now try to remove the immutable file, both as a normal user and as a root.

$ rm test.txt

$ sudo rm test.txt

Output:

sudo rm -rf test.txt

rm: cannot remove ‘test.txt’: Operation not permitted

use -i sign to remove the attribute

sudo chattr -i -V test.txt

chattr 1.42.9 (4-Feb-2014)

Flags of test.txt set as -------------e--

Now we can delete the file.