Command Line Mastery: Key Linux Commands for Everyday Use
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User Management :
| Command | Info | Syntax |
| useradd | Create a new user | useradd username |
| userdel | Delete a user account | userdel username |
| usermod | Modify a user account. | usermod [option] group_name username |
| useradd -d | Add User by Specifying a home directory path | useradd -d /home/test_user test_user |
| useradd -u | To create a new user with a custom UID | useradd -u 1234 username |
| useradd -e | To set an expiry date for a user account, | useradd -e 2020-05-30 username |
| useradd -g | To create a new user and assign a primary group | useradd -g group_name username |
| useradd -g gid | To create a new user and assign a specific group ID | useradd -g 1000 test_user |
| useradd -G | To create a new user and assign a secondary group | useradd -G group_name username |
| useradd -s /bin/sh | To create a user with a different login shell (nologin/login). | useradd -s /bin/sh test_user |
| getent passwd | Display list of users | getent passwd |
Group Management:
| Command | Info | Syntax |
| groupadd | Create a new group | groupadd group_name |
| groupdel | Delete a group | groupdel group_name |
| groupmod | modify or change the existing group | groupmod [option] GROUP |
| gpasswd | Add password to group with group name | gpasswd group_name |
| gpasswd -r | Delete group password | gpasswd -r group_name |
| getent group | list all the available groups |
Options: There are following option available in groupmod command.
-g, –gid GID: The group ID of the given GROUP will be changed to GID.
-n, –new-name NEW_GROUP: The name of group will change into newname.
-h, –help: This option display help message and exist.
-o, –non-unique: This option used with the -g option that allow to change the group GID to a non-unique value.
-p, –password PASSWORD: This gives the encrypted password.
-R, –root CHROOT_DIR: Apply changes in the CHROOT_DIR directory and use the configuration files from the CHROOT_DIR directory.
Process/System Management:
| Command | Info | Syntax |
| ps | Display information about running processes | ps |
| ps -f | Display full information about running processes | ps -f |
| ps process_id | For single-process information, ps along with process id is used | ps process_id |
| bg | Resumes suspended jobs and runs them in the background. | bg [ job ] |
| fg | It continues a stopped job by running it in the foreground. | fg [ %job_id ] |
| kill | Terminate processes | kill process_id |
| top | Display and update sorted information about processes | top |
| htop | Interactive process viewer | htop |
| vmstat | Report virtual memory statistics | vmstat |
| free | Display amount of free and used memory in the system | free -m |
| df | Display disk space usage | df |
| sar | Collect, report, or save system activity information | sar |
| iostat | Report CPU and I/O statistics | iostat |
| nice | It starts a new process (job) and assigns it a priority (nice) value at the same time. | nice [-nice value] |
| renice | To change the priority of an already running process renice is used. | renice [-nice value] [process id] |
For a running program (named process) Pidof finds the process id’s (pids)
Fields described by ps are described as:
UID: User ID that this process belongs to (the person running it)
PID: Process ID
PPID: Parent process ID (the ID of the process that started it)
C: CPU utilization of process
STIME: Process start time
TTY: Terminal type associated with the process
TIME: CPU time is taken by the process
CMD: The command that started this process
There are other options which can be used along with ps command :
-a: Shows information about all users
-x: Shows information about processes without terminals
-u: Shows additional information like -f option
-e: Displays extended information.
Archive Management:
tar: Create a tar Archive.
Syntax : tar [options] [archive-file] [file or directory to be archived]
| Option | Info |
| -c,--create | Create a new archive. |
| -v,--verbose | Verbosely list files processed. |
| -f,--file | Specify the filename |
| -x,--extract | Extracts files from an archive. |
| -t,--list | Show list of file & directory without extract tar file. |
| -j,--bzip2 | Create with bzip2 compression/big file |
| -z,--gzip | Create with gzip compression . |
| -J,--xz | Create with xz compression. |
| -r | recursive store file in tar file |
System Control :
1) cron: Schedule tasks to run periodically.
| Option | Info |
| crontab -e | Edit the scheduling for current user |
| crontab -l | Show list of scheduling for current user |
| crontab -r | Remove all scheduling for current user |
| crontab -e -u | Edit the scheduling for particular user |
| crontab -l -u | Show list of scheduling for particular user |
| crontab -r -u | Remove all scheduling for particular user |
2) systemctl: Control the systemd system and service manager.
Syntax : systemctl [option] service_name
| Option | Info |
| start | start the service |
| stop | stop the service |
| status | check the status of the service |
| restart | 1st stop service and then start service give downtime |
| reload | without stop, start service not given downtime |
| enable | boot time active |
| disable | boot time disable |
| mask | permanent lock/stop service |
| unmask | to remove mask |
3) journalctl: to query and display logs from the systemd journal, which is a system service that collects and stores logging data
Syntax :journalctl
Package Management
| Command | Info | Syntax |
| yum/dnf | Package manager for RPM-based Linux distributions. | yum install package_name |
| apt-get install | Installing a package | apt-get install package-name |
| apt-get update | Updating the package list | apt-get update |
| apt-get upgrade | Upgrading packages | apt-get upgrade |
| apt-get remove | Removing a package | apt-get remove package-name |
| rpm | Package manager for RPM-based Linux distributions. | bash rpm -i package.rpm |
