Useful Linux Commands – System
These are some really useful commands to know in the Linux terminal.
System Command
Reboot the System
Reboot the system immediately:
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shutdown -r now |
Reboot the system after 10 minutes:
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shutdown -r +10 |
Reboot the system after 10 minutes and send a message to all user:
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shutdown -r +10 "The system is rebooting in 10 minutes. Please, save your work." |
Check last reboot time:
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last reboot |
Display information about running processes
ps command is used to display information about the processes that are running in the system.
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ps |
The output will show:
- PID = the process ID that identify the running process
- TTY = the Terminal type
- TIME
- CMD
You can add the -ef option to get the full list (-e option show all the processes, -f option show full details):
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ps -ef |
or add -eF to view more and more details:
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ps -eF |
Show the top running processes
The command top show the top running process in your system.
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top |
The output show information like the user, memory usage, CPU usage, running time, command lunched… You can sort the list pressing SHIFT+O.
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top - 16:13:47 up 6 days, 18:56, 1 user, load average: 0.00, 0.00, 0.00 Tasks: 130 total, 1 running, 129 sleeping, 0 stopped, 0 zombie %Cpu(s): 0.3 us, 0.3 sy, 0.0 ni, 99.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem : 949572 total, 120876 free, 140968 used, 687728 buff/cache KiB Swap: 102396 total, 101380 free, 1016 used. 729356 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 362 stefano 20 0 399000 73304 9344 S 1.0 7.7 65:40.04 java 6395 stefano 20 0 8100 3180 2704 R 1.0 0.3 0:00.10 top 3070 mongodb 20 0 372996 41672 13452 S 0.3 4.4 37:33.36 mongod 1 root 20 0 28108 6116 4884 S 0.0 0.6 0:16.58 systemd 2 root 20 0 0 0 0 S 0.0 0.0 0:00.16 kthreadd 3 root 20 0 0 0 0 S 0.0 0.0 0:20.76 ksoftirqd/0 5 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 kworker/0:+ 7 root 20 0 0 0 0 S 0.0 0.0 0:45.95 rcu_sched 8 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rcu_bh 9 root rt 0 0 0 0 S 0.0 0.0 0:00.11 migration/0 10 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 lru-add-dr+ 11 root 20 0 0 0 0 S 0.0 0.0 0:00.00 cpuhp/0 12 root 20 0 0 0 0 S 0.0 0.0 0:00.00 cpuhp/1 13 root rt 0 0 0 0 S 0.0 0.0 0:00.09 migration/1 14 root 20 0 0 0 0 S 0.0 0.0 0:13.24 ksoftirqd/1 16 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 kworker/1:+ 17 root 20 0 0 0 0 S 0.0 0.0 0:00.00 cpuhp/2 |
Start or stop a service
To check the current status of a service (with the service name in SERVICE_NAME) run:
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service SERVICE_NAME status |
For example, to check the status of samba service:
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service samba status |
To run a service use:
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service SERVICE_NAME start |
or to stop:
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service SERVICE_NAME stop |
or to restart:
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service SERVICE_NAME restart |
If you want to check set status of all running processes:
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service --status-all |
Know free memory
To know free memory in your system type:
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free |
The output is displayed in bytes:
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total used free shared buff/cache available Mem: 949572 140332 121768 13716 687472 729976 Swap: 102396 1016 101380 |
Search for a given string
The grep (general regular expression print) command allow to search for a given string in a file or output of another command.
To search a string in a given file type:
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grep -I "STRING_TO_SEARCH" FILE_TO_BE_SCANNED |
For example, to search the word “express” (case insensitive, option -i) in the file app.js:
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grep -i "express" app.js |
The output will be:
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var express = require("express"); session = require("express-session"), var app = express(); app.use(express.static(path.join(__dirname, "public"))); |
The grep command is very useful. Please, consider to read the full article here.
Show system environmental variables
To show the system environmental variables use the command export:
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export |
The output can be really confusing. If you need to search for a given string, combine the export command with grep:
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export | grep JAVA |
Export a system environmental variable
Use again the export command, with the variable to export:
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export VARIABLE_NAME=VALUE |
For example to export the JAVA_HOME:
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export JAVA_HOME="/usr/lib/jvm/jdk-8-oracle-arm32-vfp-hflt/jre/ |
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