Basic Commands to learn
Overview
Teaching: 45 min
Exercises: 15 minQuestions
Which are the most basic commands that I need to learn?
Objectives
Learn the most basic commands in Unix/Linux
The top 10 basic commands to learn
When you interact with a HPC cluster your interaction is basically by executing commands on a terminal and editing text files. For newcomers using command lines could be a frustrating experience knowing that there are literally hundreds of commands. Certainly there are manuals for most of those commands, but they are of no use if you do not know which is the command you need to use for each situation. The good news is that you can do a lot of things with just a bunch of them and you can learn others in due time.
This is a selection of the 10 most essential commands you need to learn.
ls
List all the files in a directory. Linux as many Operating Systems organize files in files and directories (also called folders).
$ ls
file0a file0b folder1 folder2 link0a link2a
Some terminal offer color output so you can differentiate normal files from folders. You can make the difference more clear with this
$ ls -aCF
./ ../ file0a file0b folder1/ folder2/ link0a@ link2a@
You will see a two extra directories "." and "..". Those are special folders that refer to the current folder and the folder up in the tree.
Directories have the suffix "/". Symbolic links, kind of shortcuts to other files or directories are indicated with the symbol "@".
Another option to get more information about the files in the system is:
$ ls -al
total 36
drwxrwxr-x. 4 gufranco users 86 May 30 12:16 .
drwxr-xr-x. 82 gufranco users 12288 May 30 12:05 ..
-rw-rw-r--. 1 gufranco users 0 May 30 12:08 file0a
-rw-rw-r--. 1 gufranco users 0 May 30 12:08 file0b
drwxrwxr-x. 2 gufranco users 32 May 30 12:07 folder1
drwxrwxr-x. 2 gufranco users 32 May 30 12:07 folder2
lrwxrwxrwx. 1 gufranco users 6 May 30 12:16 link0a -> file0a
lrwxrwxrwx. 1 gufranco users 14 May 30 12:16 link2a -> folder2/file2a
Those characters on the first column indicate the permissions. The first character will be “d” for directories, “l” for symbolic links and “-“ for normal files. The next 3 characters are the permissions for “read”, “write” and “execute” for the owner. The next 3 are for the group, and the final 3 are for others. The meaning of “execute” for a file indicates that the file could be a script or binary executable. For a directory it means that you can see its contents.
cp
This command copies the contents of one file into another file. For example
$ cp file0b file0c
rm
This command deletes the contents of one file. For example
$ rm file0c
There is no such thing like a trash folder on a HPC system. Deleting a file should be consider an irreversible operation.
Recursive deletes can be done with
$ rm -rf folder_to_delete
Be extremely cautious deleting files recursively. You cannot damage the system as the files that you do not own you cannot delete. However, you can delete all your files forever.
mv
This command moves a files from one directory to another. It also can be used to rename files or directories.
$ mv file0b file0c
pwd
It is easy to get lost when you move in complex directory structures. pwd will tell you the current directory.
$ pwd
/home/gufranco/Dropbox/SummerHandsOn
cd
This command moves you to the directory indicated as an argument, if no argument is given, it returns to your home directory.
$ cd folder1
cat and tac
When you want to see the contents of a text file, the command cat displays the contents on the screen. It is also useful when you want to concatenate the contents of several files.
$ cat INCAR
system = LiAu
PREC = High
NELMIN = 8
NELM = 100
EDIFF = 1E-07
...
To concatenate files you need to use the symbol ">" indicating that you want to redirect the output of a command into a file
$ cat file1 file2 file3 > file_all
The command tac shows the files in reverse starting from the last line back to the first one.
more and less
Sometimes text files, as those created as product of simulations are too large to be seen in one screen, the command “more” shows the files one screen at a time. The command "less" offers more functionality and should be the tool of choice to see large text files.
$ less OUTCAR
ln
This command allow to create links between files. Used wisely could help you save time when traveling frequently to deep directories. By default it creates hard links. Hard links are like copies, but they make references to the same place in disk. Symbolic links are better in many cases because you can cross file systems and partitions. To create a symbolic link
$ ln -s file1 link_to_file1
grep
The grep command extract from its input the lines containing a specified string or regular expression. It is a powerful command for extracting specific information from large files. Consider for example
$ grep TOTEN OUTCAR
free energy TOTEN = 68.29101273 eV
free energy TOTEN = -13.46870926 eV
free energy TOTEN = -18.78141268 eV
...
Regular expressions offers ways to specified text strings that could vary in several ways and allow commands such as grep to extract those strings efficiently. We will see more about regular expressions in third chapter.
More commands
The 10 commands above, will give you enough tools to move files around and travel the directory tree. The GNU Core Utilities are the basic file, shell and text manipulation utilities of the GNU operating system. These are the core utilities which are expected to exist on every operating system.
If you want to know about the whole set of coreutils execute:
info coreutils
Each command has its own manual. You can access those manuals with
man <COMMAND>
Output of entire files
cat Concatenate and write files tac Concatenate and write files in reverse nl Number lines and write files od Write files in octal or other formats base64 Transform data into printable data
Formatting file contents
fmt Reformat paragraph text numfmt Reformat numbers pr Paginate or columnate files for printing fold Wrap input lines to fit in specified width
Output of parts of files
head Output the first part of files tail Output the last part of files split Split a file into fixed-size pieces csplit Split a file into context-determined pieces
Summarizing files
wc Print newline, word, and byte counts sum Print checksum and block counts cksum Print CRC checksum and byte counts md5sum Print or check MD5 digests sha1sum Print or check SHA-1 digests sha2 utilities Print or check SHA-2 digests
Operating on sorted files
sort Sort text files shuf Shuffle text files uniq Uniquify files comm Compare two sorted files line by line ptx Produce a permuted index of file contents tsort Topological sort
Operating on fields
cut Print selected parts of lines paste Merge lines of files join Join lines on a common field
Operating on characters
tr Translate, squeeze, and/or delete characters expand Convert tabs to spaces unexpand Convert spaces to tabs
Directory listing
ls List directory contents dir Briefly list directory contents vdir Verbosely list directory contents dircolors Color setup for 'ls'
Basic operations
cp Copy files and directories dd Convert and copy a file install Copy files and set attributes mv Move (rename) files rm Remove files or directories shred Remove files more securely
Special file types
link Make a hard link via the link syscall ln Make links between files mkdir Make directories mkfifo Make FIFOs (named pipes) mknod Make block or character special files readlink Print value of a symlink or canonical file name rmdir Remove empty directories unlink Remove files via unlink syscall
Changing file attributes
chown Change file owner and group chgrp Change group ownership chmod Change access permissions touch Change file timestamps
Disk usage
df Report file system disk space usage du Estimate file space usage stat Report file or file system status sync Synchronize data on disk with memory truncate Shrink or extend the size of a file
Printing text
echo Print a line of text printf Format and print data yes Print a string until interrupted
Conditions
false Do nothing, unsuccessfully true Do nothing, successfully test Check file types and compare values expr Evaluate expressions tee Redirect output to multiple files or processes
File name manipulation
basename Strip directory and suffix from a file name dirname Strip last file name component pathchk Check file name validity and portability mktemp Create temporary file or directory realpath Print resolved file names
Working context
pwd Print working directory stty Print or change terminal characteristics printenv Print all or some environment variables tty Print file name of terminal on standard input
User information
id Print user identity logname Print current login name whoami Print effective user ID groups Print group names a user is in users Print login names of users currently logged in who Print who is currently logged in
System context
arch Print machine hardware name date Print or set system date and time nproc Print the number of processors uname Print system information hostname Print or set system name hostid Print numeric host identifier uptime Print system uptime and load
Modified command
chroot Run a command with a different root directory env Run a command in a modified environment nice Run a command with modified niceness nohup Run a command immune to hangups stdbuf Run a command with modified I/O buffering timeout Run a command with a time limit
Process control
kill Sending a signal to processes
Delaying
sleep Delay for a specified time
Numeric operations
factor Print prime factors seq Print numeric sequences
Key Points
Learning the basic commands allow you to create files and directories and move around the filesystem
Use man to search for arguments of command line tools