Basic Commands

Objective

Get familiar with the file system structure.
Perform essential commands.

cat cp date file head less ls mkdir more most mv rmdir rm tail touch whoami who

Filenames

Every directory/file has a name.
Filenames are case sensitive.
Filenames cannot include the slash / nor the NUL (\0) characters.
Maximum filename length is 255 bytes.
File extensions are not required but may be needed by certain applications.
Files that start with . are called hidden files and are not normally displayed.

Listing Files

`ls`

Syntax

ls [-option] [file or directory name]

Used to list the contents of a directory.
At its most basic form, ls lists the filenames only.

Example

> ls /etc
abrt                        grub2.cfg                 prelink.conf.d
adjtime                     grub.d                    printcap
aliases                     gshadow                   profile
aliases.db                  gshadow~                  profile.d
...

ls accepts options which can change the way the files are listed, including:

Option	Action
`-a`	list all files including system files
`-F`	show file type (`*` after executables, `/` after directories, nothing after plain files)
`-l`	show extended file information (file size, permissions and owner information)
`-r`	list file in reverse order

Identifying File Types

No rules are enforced when it comes to file contents and filename extensions, making it difficult to identify the file type from the filename.

`file`

file is used to look inside a file and identify its type.

Syntax

file filename

Some common file types:
- empty
- directory
- ASCII text
- Bourne-Again shell script
- C source
- PDF document
- JSON document data
- gzip compressed data

Example

> file /etc
/etc: directory
 
> file /bin/cat
/bin/cat: ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV),
dynamically linked, interpreter /lib/ld-linux-aarch64.so.1,
BuildID[sha1]=0ced1703d50f352986337e10825110c509a35677, for GNU/Linux 3.7.0, stripped

Viewing Files

There are two basic commands used to display the contents of an ASCII text file: cat and more.

`cat`

Displays the contents of a file all at once.
Useful for small files.
Short for concatenate.
Can be used to join files together.
No flow control (must use ^S and ^Q to pause and restart the output).

Syntax

cat file [file ...]

Example

> cat /etc/bashrc
# /etc/bashrc
 
# System wide functions and ...

`more`

Displays a file one screen at a time.
Useful for viewing large files.
More flexible than cat.

Syntax

 more file [file ...]

Example

> more /etc/bashrc
# /etc/bashrc
 
# System wide functions and aliases
# Environment stuff goes in /etc/profile
...
--More--(37%)

Prompt usually indicates the percentage of the file that has been viewed.

Some built-in commands that are similar to vim commands:

Name	Function
`<space>`	display next screen
`<return>`	display next line
`b`	display previous screen (back)
`q` or `Q` or `<interrup>`	exit from `more`
`=`	Display current line number
`/pattern`	search for next occurrence of pattern
`n`	search for next occurrence of previously searched `pattern`
`!<cmd>`	execute `<cmd>` in a subshell
`v`	start up `/usr/bin/vi` at current line
`h`	help, display this message
`:f`	display current filename and line number

There are other utilities that enhance on more, namely less and most.

Exercise 1

At the command prompt, enter: ls. This lists the files in your current directory.
Enter ls --help to display available options for the ls command
To display the contents of the hello.cpp file, enter: cat hello.cpp
Display the contents of the /etc/passwd file.
Enter the following command to view the /etc/passwd file a page at a time: more /etc/passwd
Hit <return> to display the next line.
Hit the <space> to proceed to the next screen.
Press q to quit and return to the prompt.
Read the manual page for the more command by entering: man more The more utility is used to paginate the manual pages. Use the <space> and <return> key to move through the pages.
Identify the option used to display command help while in more.
Quit from the manual page and return to the prompt.
Enter the following commands and identify the differences:
- ls /bin/cat
- ls -F /bin/cat
- ls -l /bin/cat

Listing Active Users

`who`

Shows all active users/sessions on the system.

Syntax

who [-uHb]

Displays username, terminal, login time, and other information.
option -u adds idle time.
whoami displays your login information.

Example

> who
> who -u
> whoami
> who -H
> who -b

Exercise 2

See who is logged in to the system by typing: who.
Show who you are logged in as using: whoami.
Check the man pages and explain what does who -b do?

Reading the Date / Time

`date`

Displays the system date and time.

Syntax

date [+format_string]

Example

> date
Sat Sep 3 11:46:25 AM +03 2023
> date +"Year: %Y, Month: %m, Day: %d"
Year: 2023, Month: 09, Day: 03

Creating Files

touch

Used to create empty files or update the time stamp on existing files.

Syntax

touch filename

Example

> ls .lock
ls : .lock: No such file or directory
> touch .lock
> touch .lock
> ls -l .lock .bashrc
-rw-r--r--. 1 user group 522 Jul 19  2023 .bashrc
-rw-r--r--. 1 user group   0 Jan 28 17:25 .lock
> touch .bashrc

Expansion using Wildcards

The shell interprets special characters as wildcards which represent characters in filenames:

Character	Meaning	Example
`?`	any single character	`file?` `?????`
`*`	0 or more characters	`a` `.doc`
`[...]`	range or class of characters (`!` negates range or class)	`[abc]` `[a-z][0-9]` `*[!A-Za-z0-9]`
`{start..end}`	build a sequence of characters	`{10..0}` `{13..24}` `{b..g}`

Example

> ls
> touch memo{a..z}
> ls memo*
> ls ?.c
> ls ?????
> touch part{00..99}
> ls part[2346]
> ls part[1-46]
> ls *[1-46]
> echo {0..10}

Exericse 3

Show the system date and time by typing: date.
Use touch to create the following zero-length files in your directory:
```
file1 file2 file3 file4 file5 file3a files chapter1 Friends filelist Foes
```
touch accepts multiple filenames as arguments.
Use ls to list the files in your directory.
Use the *, ?, and [] wildcard characters as shown below to list specific files only:
- ls file*
- ls file[1-3]
- ls [A-Z]*
- ls file?
- ls *

Creating Directories

`mkdir`

Directories help you organize your files into logical groups.

Syntax

mkdir dirname

Example

> pwd
/home/tux
> ls -F
> mkdir test_dir
> ls -F
test_dir/
> cd test_dir/
> pwd
/home/tux/test_dir
> ls -aF
./ ../
> cd ..
> pwd
/home/tux
> mkdir /etc/junk
mkdir: cannot create directory `/etc/junk': Permission denied
> mkdir /tmp/junk
> mkdir dir_a dir_b dir_c
> ls -F
dir_a/ dir_b/ dir_c/ test_dir/

Removing Directories

`rmdir`

Used to delete empty directories.

Syntax

rmdir dirname

Cannot be your current directory (.) or parent of your current directory (..).
Must delete the directory contents prior to using rmdir.

Example

> pwd
/home/tux
> ls -F
dir_a/ dir_b/ dir_c/ test_dir/
> touch dir_a/file_a
> touch dir_c/veryimportantfile
> rmdir dir_b
> rmdir dir_c
rmdir: 'dir_c': Directory not empty
> rmdir .
rmdir: failed to remove '.': Invalid argument

`rm`

Used to recursively remove non-necessarily empty directory structures with the -r option.

Syntax

rm -r dirname

Can have catastrophic consequences if used improperly!
As a precaution, use it with the -i option as well.

Example

> pwd
/home/tux
> ls -F
dir_a/ dir_c/ test_dir/
> ls dir_a
file_a
> rm -r dir_a
> ls -F
dir_c/ test_dir/
> ls dir_c
veryimportantfile
> rm -r dir_c
> ls
test_dir

Exercise 4

Enter the following commands to create and remove directories:

> cd
> mkdir Ships
> ls -F
> mkdir Ships/Space
> cd ~/Ships/Space
> touch Enterprise Voyager DS9
> cd
> ls -R Ships
> mkdir ~/Ships/Sea
> cd Ships
> ls Sea
> rmdir Sea
> rmdir Space
> ls Space
> rm -ir Space #(answer Y to all prompts)
> cd ..
> pwd
> rm -r Ships

Copying Files

`cp`

Makes an exact duplicate of a file.

Syntax

cp [-i] old_file new_file
cp [-i] file [file ...] directory

If new_file already exists, cp will overwrite it.
Option -i will prompt before overwriting the file.
If the target is a directory, cp will copy one or more files into that directory with the original names.

Example

> ls
file_a  file_b
> cp file_a file_a.cpy
> ls
file_a  file_a.cpy  file_b
> cp fileb /tmp/$LOGNAME
> ls /tmp
tux
> mkdir backup
> ls -F
backup/  file_a  file_a.cpy  file_b
> cp * backup
cp: backup is a directory
> ls -FR
backup/  file_a  file_a.cpy  file_b
backup:
file_a  file_a.cpy  file_b

Moving and Renaming Files

`mv`

Used to move a file to a new location or rename the file.

Syntax

mv [-i] old_file new_file
mv [-i] file [file ...] directory

If new_file already exists, mv will overwrite it.
Option -i will prompt before overwriting the file.
If the target is a directory, mv will move one or more files into that directory.

Example

> ls -F
personal/ phonelist
> mv phonelist addresses
> ls -F
addresses personal/
> mv addresses personal
> ls -R
personal

Removing Files

`rm`

Deletes a file from a directory.

Syntax

rm [-i] file [file ...]

Option -i will prompt before removing a file.
If wildcards are used to select files, ensure that the pattern matches exactly what is to be deleted.
Can not undo the rm command!

Example

> ls -F
backup/ file_a file_a.cpy file_b
> rm -i file_a.cpy
rm: remove file_a.cpy (y/n)? y
> ls -F
backup/ file_a file_b

Resources

1 Overview 3 File System

Basic Commands

Objective

Summary

Filenames

Listing Files

ls

Syntax

Example

Identifying File Types

file

Syntax

Example

Viewing Files

cat

Syntax

Example

more

Syntax

Example

Exercise 1

Listing Active Users

who

Syntax

Example

Exercise 2

Reading the Date / Time

date

Syntax

Example

Creating Files

touch

Syntax

Example

Expansion using Wildcards

Example

Exericse 3

Creating Directories

mkdir

Syntax

Example

Removing Directories

rmdir

Syntax

Example

rm

Syntax

Example

Exercise 4

Copying Files

cp

Syntax

Example

Moving and Renaming Files

mv

Syntax

Example

Removing Files

rm

Syntax

Example

Resources

`ls`

`file`

`cat`

`more`

`who`

`date`

`mkdir`

`rmdir`

`rm`

`cp`

`mv`

`rm`