Redirecting output to a file
| | | ------------------------------------------------------------ | -------------------------------------------------------------------- | | > file |
redirect stdout to overwrite a file
| | >> File | redirect stdout to append a file | | 2>file | redirect stderr to overwrite a file | | 2> /dev/null | redirect stderr error message by redirecting to /dev/null | | >file2>&1 | redirect stdout and stderr to overwrite the same file | | &> file |
redirect stdout and stderr to overwrite the same file
| | >>file2>&1 | redirect stdout and stderr to append to the same file | | &>>file | redirect stdout and stderr to append to the same file | | **Managing Local Users and Groups** | | | ----------------------------------- | ----------------------------------------------------------------------------------- | | id | Information about currently logged in user | | Ls -l | Owner of File | | Ls -ld | Owner of directory | | su | switch users | | su - user | switch user | | sudo -i | to get interactive root shell | | Useradd username | Create a user | | userdel username | deletes the user with the specified username and removes him from /etc/passwd | | userdel –r username | deletes the user with the specified username and its home directory is also deleted | | groupadd -r 'group\_name' | | | passwd username | sets initial password or changes the existing password | | cat /etc/passwd | Show users | | cat /etc/group | Show groups | | **Controlling access to a file** | | | -------------------------------- | ----------------------------------------------------------------------------------------------------------------------------- | | ls -la | shows detailed information about permissions and ownership | | ls -ld | shows detailed information about a directory itself and not its contents | | chmod | command used to change permissions, it means “change mode” | | Chmod ### file | Change permission using numeric method | | chmod WhoWhatWich file1 | Set permissions using Symbolic method | | Examples: | | | chmod go-rw file1 | Remove read and write permission for group and other on file1 | | chmod a+x file2 | Add execute permission for everyone on file2 | | Chmod 644 file1 | Set read and write permissions for group and others on file1 | | Chmod 750 file1 | Set read, write and execute permissions for user, read and execute permissions for group and no permission for other on file1 | | chown | change owner = used to change file ownership | | Examples: | | | chown user1 file1 | Grant ownership of the **file1** file to the user1 user | | Chown -R user1 directory1 | Grant ownership of directory1 and all files and subdirectories within it to user1 | | Chown :admins directory1 | change the ownership of directory1 to visitor and the group to guests | | | | | **Controlling services and Daemons** | | | ----------------------------------------------- | ------------------------------------------------------------------------------------------------------ | | systemctl | used to explore the current state of the system | | Examples: | | |
systemctl
list-units type=services
| listing all currently loaded service units paginating the output using **less** | | Systemctl list-units –type=service --all | lists all service units | | systemctl | Systemctl without any argument lists units that are both loaded and active | | Systemctl list-unit-file –type=services | To see the state of all unit files installed | | Systemctl status service\_name | Viewing Services States | | Systemctl is-active service\_name | Verify that a service unit is currently active | | Systemctl is-enabled service\_name | Verify whether a service unit is enabled to start automatically during system boot | | Systemctl is-failed service\_name | Verify whether the unit failed during startup | | systemctl –failed –type=service | List all the failed units | | Systemctl start service\_name | Starting service | | Systemctl stop service\_name | Stopping service | | Systemctl restart service\_name | Restart a service | | Systemctl reload service\_name | Reload a service | | Systemctl reload-or-restart service\_name | In case you are not sure if the service has the functionality to reload the configuration file changes | | Systemctl list-dependencies Unit\_name | displays a hierarchy mapping of dependencies to start the service unit | |Systemctl mask Unit\_name
| Masking a service | | **Configuring and Securing SSH** | | | -------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------- | | ssh remotehost | log you in on the remote server **remotehost** using the same user name as the current local user | | exit | log out of the remote system | | ssh user1\@remotehost | log you in on the remote server remotehost using the user name **user1** | | ssh user1\@remotehost hostname | ssh command would run the hostname command on the remotehost remote system as the user1 user without accessing the remote interactive shell | | w | displays a list of users currently logged into the computer | | ssh-keygen | To create a private key and matching public key for authentication | | ssh-keygen -f .ssh/key\_pass | The -f option with the **ssh-keygen** command determines the files where the keys are saved | | ssh-copy-id -i .ssh/key\_pass.pub uer1\@remotehost | copies the public key of the SSH keypair to the destination system | | ssh -i .ssh/key\_pass uer1\@remotehost | **authenticate** to the remote system using the corresponding private key while logging in to the remote system over SSH | | **Analyzing and storing logs** | | | --------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------ | | tail -f /var/log/secure | Returns date only from a datetime value (Only use if variable is problematic) | | logger | sends messages to the rsyslog service | | Logger -p local7.notice "Log entry created on host" | send a message to the rsyslog service that gets recorded in the /var/log/boot.log file | | journalctl | retrieve log messages from the journal | | journalctl –n | shows the last 10 log entries | | journalctl –f | outputs the last 10 lines of the system journal and continues to output new journal entries as they get written to the journal | | journalctl –p | takes either the name or the number of a priority level and shows the journal entries for entries at that priority an above | | journalctl –p -p err | list journal entries at the err priority or higher | | journalctl –-since today | list all journal entries from today’s records | | Journalctl –since "2019-2-10 20:30:00" –-untill "2019-02-13 12:00:00" | list all journal entries ranging from 2019-02-10 20:30:00 to 2019-02-13 12:00:00 | | journalctl -o verbose | | |
Journalctl \_SYSTEMD\_UNIT=
sshd.service \_PID=1182
| shows all journal entries related to the sshd-service systemd unit from a process with PID 1182 | | Journalctl \| grep -E "Runtime\|System journal' | output shows the journal entries that reflect the current size limits | | journalctl -b 'number' | limit the output to a specific system boot | | timedatectl | shows an overview of the current time related system settings | | **Managing Networking** | | | ------------------------------------- | -------------------------------------------------------------------------------------------------- | | ip link show | list all network interfaces available on your system | | ping 'ip' | test connectivity | | ip route | show routing information | | ip -6 route | show the IPv6 routing table | | traceroute 'host' | trace the path that network traffic takes to reach a remote host through multiple routers | | Traceroute6 'host' | For ipv6 trace the path that network traffic takes to reach a remote host through multiple routers | | ss | display socket statistics | | nmcli | used to create and edit connection files from the command line | | nmcli dev status | displays the status of all network devices | | nmcli con show | displays a list of all the connections | |nmcli con show
--active
| list only active connections | | nmcli con add | used to add new network connections | | nmcli dev disconnect 'device' | disconnects (deactivate) the network interface device and brings it down | | nmcli con show 'name' | list the current settings for a connection | | nmcli con up 'name' | command activates the connection name on the network interface it is bound to | |nmcli con down
'name'
| brings down the connection | | nmcli con mod 'name' | change the settings for a connection | | nmcli con del 'name' | deletes the connection identified by name from the system | | Host name | displays the host name | | **Archiving and transferring files** | | | ------------------------------------ | --------------------------------------------------------------------------------------------------------- | | tar | list the contents of archives or extract their files to the current system | | Tar -cf archive.tar file1 file2 | creates an archive named archive.tar with the contents of file1, file2, in the user’s home directory | | tar -cf /root/etc.tar /etc | create the tar archive named /root/etc.tar with the /etc directory | | Tar -tf /root/etc.tar | List an archive content | | Tar -xf /root/etc.tar |restore files from the /root/etc.tar
archive
| | Tar -xpf /root/myscript.tar | an archive named /root/myscripts.tar is extracted while preserving the permissions of the extracted files | | Tar -czf /root/backup.tar.gz /etc | create a gzip compressed archive named /root/backup.tar.gz with the contents from the /etc directory | | Tar -xzf /root/backup.tar.gz | extract the contents of a gzip compressed archive named /root/backup.tar.gz | | scp 'file' 'destenation' | copies files from a remote system to the local system or from the local system to a remote system | | sftp | upload or download files from a SSH server | | sftp: put 'file' | uploads a file to the remote system | | sftp: get 'file' | downloads a file from the remote system | | sftp: exit | exits the sftp session | | rsync | Synchronize Files and directories | | rsync -av /var/log /tmp | synchronize contents of the /tmp directory to /var/log directory | {% file src="/files/47sf8EyXmfo4Tpot0U2w" %}