Basic Linux Commands
Basic linux commands are introduced for non Linux users. Commands are the most staightforward way for a user to interact with the Linux operating system (OS). Linux OS gives the user application access to computer resources (CPU, memory, disks, devices, etc) through a file-system. A Linux Shell is a commonly used interface that enables users to access the OS services. Shells are usually available through a terminal, SSH (secure shell) connection, or else. After opening a terminal, or after establishing a SSH connection, the user can enter commands into the shell prompt line. This quick help provides a list of usage examples of some basic commands. In the following example code, lines starting with the dollar symbol $ (also called shell prompt) indicate commands to be entered.
Topics and commands introduced in this tutorial:
| Get Help | Manage files & directories | Manage processes | Check system resources usage |
| -h, --help, -help | pwd, cd | ps | top, htop |
| man | ls, ls -a, ls -l, ls -lh | top, htop | free -m |
| info | cp, mv, rm, rm -r, rmdir | kill, kill -9 | df, du |
| cat, more, less, head, tail | |||
| nano, vi, emacs |
1. Get Help
Commands are case-sensitive words made of symbols and alphabetical letters (examples: hmac256, ssh-copy-id) that allow the user to run programs or to perform shell operations. Commands may have parameters and arguments that control the operations to be made. A command or a combination of commands with or without parameters is called a command line. There are different methods to obtain information about a given command. Three common methods are
1.1 From the command itself
Depending of the command, usage information may be obtained by using one of the -h, --help, or -help parameters, or by entering the command without parameters. For example help information from the command bash
$ bash --help
1.2 From the manual command man,
Example to show the manual page for the command find
$ man find
1.3 From the info command info
$ info
The methods above can be used to obtain more details and examples about the usage of the commands that will appear in the remaining of the tutorial.
2. Manage files and directories
After logging into the cluster or after opening a terminal the user is located into the working directory. Operations like listing, modification, editing, execution, and so forth can be performed to any file in the filesystem that the user has the permissions to access.
2.1 Navigating into directories
The command pwd may be used to obtain the value of the working directory, For example
$ pwd /home/s/someuser
The output of the pwd command is a directory full pathname. Any file or directory in the filesystem has a full pathname which starts with the backslash symbol /. The command cd is used to change into a different existing directory. For example
$ cd /apps $ pwd /apps
2.2 List files and directories
The command ls is used to print the list of the files and directories in the working directory. Without any parameters the command print a multicolumn list
$ ls
Files or directories names can be added to the command to print list of files and list the contents of directories. For example as following where the contents of the system root directory (/) is listed
$ ls / apps cgroup gridware lib64 mnt root srv usr bin dev home lost+found net sbin sys var boot etc imports media opt scratch tmp work bucket genefs lib misc proc selinux unit
Parameters may be added to produce different printing output with more ore less information. For example, the parameter -F allows to differentiate between files types and directories in a mutlicolumn listing
$ ls -F
the parameter -a shows hidden files (hidden files start with a dot symbol, like .bashrc)
$ ls -a
the parameter -l print one file per line with detailed file information (creation date, permissions, file size, etc)
$ ls -l
Parameter can be concatenated to combine their actions. For example adding the parameter -h to the parameter -l makes the command ls outputs the size of the files using k, M, and G suffixes, for kilobytes, megabytes and gigabytes respectively
$ ls -lh
2.3 Copy, delete, rename and move
A given file, say source file, can be copied to another file, say destination file using the command cp. The operation is a duplication if the destination file does not exist, or a overwriting if the destination file exist. For example source file, fileS, is copied to destination file, fileD, by doing
$ cp fileS fileD
Similarly a file can be renamed (if the destination file does not exist) or moved (if the destination file is a directory name) by using the command mv. For example
$ mv fileS fileD
or
$ mv fileS /tmp
The command rm deletes a given file
$ rm fileS
The same command may be used with the option -r to deletes all the files in a directory, directories, sub-directories and files in the sub-directories. For example
$ rm -r dirA
An empty directory can be deleted using the command rmdir, like
$ rmdir dirA
2.4 Show contents of a file
The whole content of a text file can be output to the terminal screen with the command cat. The user needs to use the mouse to scroll into the text. Apply this command with a binary file may have unpredictable results
$ cat .bashrc
The commands, more, less, head, and tail help to make more or less sense of the text file content. more will output the file content into the terminal screen and will allow you to scroll forward and backward using the space key and key b, respectively, until the end of the file is reached. For example
$ more /usr/share/X11/rgb.txt
more allows for example, to make string search using the backslash /. The command less is similar to more, but it has many more features. On of these features is being faster than more for scrolling into big text files. Example
$ less /usr/share/X11/rgb.txt
The command head displays the first line of a text file or list of text files. Without parameters head outputs only the first 10 lines of the file,
$ head /usr/share/X11/rgb.txt
a parameters can be used to print more or less than 10 lines of the file
$ head -5 /proc/meminfo MemTotal: 49409336 kB MemFree: 14880700 kB Buffers: 379124 kB Cached: 30606776 kB SwapCached: 8500 kB
The command tail is similar to the command head but displays the last lines of a file
$ tail /usr/share/X11/rgb.txt
$ tail -24 /usr/share/X11/rgb.txt
2.5 Edit a file
There exist many text file editors for terminal that can be more or less easy to learn and to use. An easy to use and to learn simple text file editor is nano
$ nano /etc/nanorc
Other more sophisticated but very popular text file editor is vi and its modern improvment vim
$ vi /etc/vimrc
$ vim /etc/vimrc
2.6 Special directories names
The symbol / has two meanings, the system root directory when it is at the beginning of a full pathname, or directory name separator in a directory or file pathname. Beside the symbol / there are the symbols ~, ., and .. that represent the user home directory, working directory, and the parent of the working directory, respectively.
List the current working directory
$ ls .
List the parent directory
$ ls ..
List the user home directory
$ ls ~
List the parent directory of the user directory
$ ls ~/..
3. Manage processes
A process is a running program or application that performs operations using the system resources. Each process is identified by a unique process ID (PID).
3.1 Show running processes
The command top allows to monitor the running processes. Press the key q to quit the monitoring. To have help about how to control the monitoring parameters and output, press the key h.
$ top
Tasks: 601 total, 2 running, 573 sleeping, 26 stopped, 0 zombie
Cpu(s): 0.1%us, 0.2%sy, 0.0%ni, 99.6%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 49409336k total, 40975628k used, 8433708k free, 370372k buffers
Swap: 4194296k total, 106720k used, 4087576k free, 38039904k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
14470 etaillef 20 0 26328 1860 1144 R 1.0 0.0 0:00.08 top
1874 root 20 0 0 0 0 S 0.3 0.0 0:49.98 flush-253:0
1 root 20 0 21436 1056 844 S 0.0 0.0 6:44.42 init
2 root 20 0 0 0 0 S 0.0 0.0 0:22.93 kthreadd
3 root RT 0 0 0 0 S 0.0 0.0 0:39.45 migration/0
.....
The command ps shows the processes running on a given Linux operating system. For example, to show all the running process owned by the current user:
$ ps wux USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND 1370 776 0.0 0.0 124368 7052 pts/43 Ss+ Sep09 0:00 -bash 1370 17167 10.0 0.0 121140 1376 pts/33 R+ 17:59 0:00 ps wux 1370 29903 0.0 0.0 114048 4960 ? S Sep09 0:32 sshd: etaillefer@pts/41,pts/43,pts/33 1370 29904 0.0 0.0 124368 7032 pts/41 Ss Sep09 0:00 -bash 1370 31016 0.0 0.0 124372 7072 pts/33 Ss Sep10 0:01 -bash
and to show all processes running on the system
$ ps waux USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 0.0 0.0 21436 1056 ? Ss Jun22 6:45 /sbin/init root 2 0.0 0.0 0 0 ? S Jun22 0:23 [kthreadd] ..... fujie 30391 0.0 0.0 138152 3996 pts/6 Ss+ Sep08 0:00 -zsh 4538 30418 0.0 0.0 119592 2376 pts/13 Ss+ 09:20 0:01 -bash nrpe 30742 0.0 0.0 43400 912 ? Ss Jun25 7:13 /usr/sbin/nrpe -c /etc/nagios/nrpe.cfg -d 1370 31016 0.0 0.0 124372 7072 pts/33 Ss Sep10 0:01 -bash 1053 31272 0.0 0.0 100940 604 pts/4 T Aug29 0:00 tail -f DCN.joboutputs satoshi 31495 0.0 0.0 69720 3736 pts/31 S+ Sep02 0:02 ssh -Y tombo-login1.oist.jp .....
3.2 Stop a process
A process can be stopped by using the kill command with a list of process PID:
$ kill 29903 29904
A SIGKILL signal (-9) can also be sent to processes that refuse to stop:
$ kill -9 29903
4. Check system resources usage
4.1 CPU usage
The CPU usage can be monitored using the column %CPU from the top command. htop is another CPU usage monitor with nice printing.
4.2 Memory usage
The amount of free and used memory is obtained with free. Usage amount can be output in bytes, kilobytes, megabytes or gigabytes using the parameters-b, -k, -m, and -g, respectively:
$ free -m total used free shared buffers cached Mem: 1033944 554314 479630 0 365 541146 -/+ buffers/cache: 12802 1021142 Swap: 4095 0 4095
The columns VIRT, RES, and %MEM in the top command can also be used to monitor the memory usage of a given process.
4.3 Disk usage
A given file or given directory is located into a filesystem which resides into a partition. Partition can be a part of a segregated storage device, the aggregation of several storage devices, or a remote storage system provided by some network protocol. A given partition is attached to a directory full pathname, this pathname is called mounted point. The command df prints the usage information about all the mounted partitions living in the system. Example
$ df -h Filesystem Size Used Avail Use% Mounted on /dev/mapper/vg_system-slash 30G 7.3G 21G 26% / tmpfs 24G 84K 24G 1% /dev/shm /dev/sda1 2.0G 98M 1.8G 6% /boot /dev/mapper/vg_system-scratch 220G 317M 208G 1% /scratch /dev/mapper/vg_system-var_log 20G 278M 19G 2% /var/log ddnsfa10ke-4:/genefs 599T 492T 107T 83% /genefs homes.oist.jp:/vol/home 8.0T 4.0T 4.1T 50% /imports/home tombo-mds1@tcp:tombo-mds2@tcp:/work 393T 328T 62T 85% /work kago.oist.jp:/vol/hpc_software 1.6T 1.4T 197G 88% /imports/hpc_software bucket.oist.jp:/mabuya/bucket 1.2P 236T 940T 21% /imports/bucket web-home.oist.jp:/vol/web_home 820G 97G 723G 12% /imports/web_home
One can also inspect the usage information of a partition to which a given directory belongs to. For example to obtain the usage information of the partition holding the directory /home/e
$ df -h /home/e Filesystem Size Used Avail Use% Mounted on homes.oist.jp:/vol/home 8.0T 4.0T 4.1T 50% /imports/home
4.4 File/directory space usage
The space taken by directories and files can be obtain using the command du. For example, to obtain the amount of space used by the directory /apps/src
$ du -shc /apps/src 5.5G /apps/src 5.5G total