When logged in as user A and you want to login as user A on a remote machine, enter: ssh IP-ADDRESS.
When you logged in as user A and want to login as another user, enter either ssh -l USERNAME IP-ADDRESS or ssh USERNAME@IP-ADDRESS.
The flag -X can be added to run the graphical tools. E.g. ssh -X IP-ADDRESS.
Basic syntax of a command is: command options argument. Options are also known as flags or switches.
The ls, list, command produces a list of files and directories and displays these on the screen.
ls -a
Lists hidden files also. Files and directories starting with a dot are considered hidden.
ls -l
Displays long listing with detailed information like the file type, permissions, link count, owner, group, size, date and time of last modification and the name of the file. An alternative is using ll to show a long list.
ls -lh
Displays a long listing like the one before but now with the size in a more human readable format.
ls -ld
Displays a long listing, but hides its contents.
ls -R
List content of directory and sub directories recursively.
ls -lt
List all files sorted by date and time with the newest file on top.
ls -ltr
List all files sorted by date and time with the oldest file on top.
The pwd, print working directory, shows the absolute path to the current directory on screen.
The cd command is used to change between directories.
To travel directly to a specific path, enter: cd PATH. E.g. cd /var/log.
To go directory to the users home directly enter either cd or cd ~. To enter a specific directory in the home directory you can use cd ~/some-directory.
To go to the home directory of another user, enter: cd ~username. E.g. cd ~user1. This command works only when the execution bit is set on the home directory of the user at the public level.
Usage of the ~ character is called tilde substitution.
Use cd / to go directly to the root directory.
Use cd - to switch between current en previous directory.
use cd .. to travel one directory up.
To display the terminal name we are logged-in to to the screen with the command tty.
The who command reads information from /var/run/utmp and prints information like, username, shell name, date/time and the source, :0 for graphical and IP-address for remote, of the users currently logged-in to the system to the screen.
who am i return the same information but then only for the user executing the command.
The w, what, command displays similar information as the who command, but in more detail. It also tells the length of time the user has been idle for, along with CPU utilization and current activity. Average load is shown for the last 1, 5 and 15 minutes.
The uptime command show the current system time, how long it has been up for, number of users currently logged-in, and the average load for the pas 1, 5 and 15 minutes.
The whoami command shows the name of the user executing the command.
The logname, login name, command shows the name of the real user who originally logged-in to the system. When using the su command, this command still shows the real username instead of whoami.
The id command displays a users UID, username, GID, group name, secondary groups, and SELinux context.
The groups command show all the groups the user is member of. The first groups is the primary group, the rest are secondary groups.
The last command shows the history of successful login attempts and system reboots by reading this information from /var/log/wtmp. This file contains information of all login and logout activities, including login time, duration and terminal name (tty).
Filtering this information can be done by e.g. last reboot, to show all reboot information, or last username to show all activity for a specific user.
The lastb command show all information related to unsuccessful login attempts by reading the file /va/log/btmp.
The lastb command can only be executed by the root user.
The lastlog, last login, command shown information related to the last login attempts of users, but also when a user has never logged-in.
The uname command can be used to show information about a system. Called without any options it is showing the kernel name.
Use uname -a to get more detailed information about system. Use the following flags to get specific information, the kernel [-s], hostname [-n], kernel release [-r], date/time of kernel built [-v], machine hardware name [-m], processor type [-p], hardware platform [-i] and operating system name [-o].
Show hostname and hardware information with hostnamectl or only the hostname with hostname.
Change hostname with hostnamectl set-hostname new-hostname.example.com.
Use shortcut Ctrl + l or type clear.
Show the systems date and time information with timedatectl or date to only show the date.
To change the date to August 12, 2015 use timedatectl set-time 2015-08-12.
To change the time to 11:00 use timedatectl set-time 11:00.
An alternative is date --set "2015-08-12 11:00:00".
Show available timezones with timedatectl list-timezones.
To change the timezone use timedatectl set-timezone Europe/Amsterdam.
The which command is showing the path of the command executed when run without the absolute path.
The wc FILE, word count, command can be used to count words, lines and characters. Numbers shown are in order lines, words, characters (bytes).
Following options can be used to can specific counts. -l for counting lines, -w for counting words, -c for counting bytes and -m for counting characters.
The lspci command displays information about PCI buses and the devices attached to them. Options -v, -vv and -vvv can be used to get more verbose information. Use options -m to get a more legible output.
The lsusb command displays information about USB buses and the devices connected to them.
The lscpu command show information about the processor, incl, architecture, operating modus, count, vendor, family, model, speed, and the presence of virtualization support.
Compression tools are use to compress one or more files or an archive to save space. Once a compressed archive is created, it can be copied to a remote system faster than a non-compressed archive.
Example tools:
- bzip2 (bunzip2)
- gzip (gunzip)
The gzip command is used to compress files which the following command, it created a compressed version of the file which adds the .gz extension.
gzip FILE_1 FILE_2Uncompressing the files can be done with one of the commands below.
gunzip FILE_1 FILE_2
gzip -d FILE_1The bzip2 command is used to compress files which the following command, it created a compressed version of the file which adds the .bz2 extension.
bzip2 FILE_1 FILE_2Uncompressing the files can be done with one of the commands below.
bunzip2 FILE_1 FILE_2
bzip2 -d FILE_1RHEL offers many tools that can be utilized to archive files for storage or distribution. For example tar and star. Both tools have the ability to preserve general file attributes, such as ownership, group membership, and timestamps.
The tar, tape archive, command creates, appends, updates, lists, and extracts file to and from a single file called a tar file (or tarball).
The command structure is:
tar OPTIONS DESTINATION_FILE SOURCE_FILE_OR_DIRBelow a list of options available for the tar command. The dash (-) before each option flag is optional.
-c, create a tarball.
-f, specifiy the name of the tarball.
-j, compress the tarball with bzip2.
-r, appends files to an existing tarball. Does not work for compressed tarballs.
-t, lists the contents of a tarball.
-u, appends files to an existing tarball if the specified files are newer. Does not work for compressed tarballs.
-v, verbose output.
-x, extracts a tarball.
-z, compress the tarball with gzip.
--selinux, --no-selinux, includes or excludes SELinux file context in tarball archive.
--xattrs, no-xattrs, includes or excludes extended file attributes in tarball archive.
The star, standard tar, command is an enhanced version of tar. Star does have the same options as tar, but is not installed by default.