Linux Command Fundamentals Flashcards
1
Q
What commands help explain the use of specific commands and their syntax?
A
command –help
man command
pinfo
2
Q
What is the difference between su and sudo?
A
- su is used to open the shell as another user.
- sudo allows commands to be run in escalated mode.
3
Q
What is the wheel group used for in RHEL?
A
The wheel group is used to assign a user root privlidges.
Users in this group can be seen in the group configuration.
$ cat /etc/group | grep wheel wheel:x:10:student
4
Q
What is su - used for?
A
su - can be used to open a shell as another user
su - without any username will open as root (if enabled).
5
Q
What is sudo used for?
A
sudo allows authorised users to run tasks with escalated privileges.
sudo ls -l /root
The user must be a member of the wheel group
6
Q
What two types of options are often used with commands?
A
- Long options; these start with a – (double dash) e.g. –help
- Short options; these start with a - (single dash) e.g. ls -l and mkdir -p
7
Q
What does the whoami command do?
A
The whoami prints the username associated with the current user.
8
Q
What is the ls command used for?
A
The ls command lists directory contents.
9
Q
What command line utility is used to administer network interfaces?
A
The ip utility is used to administer network interfaces.
10
Q
What text user interace is used to administer network interfaces?
A
nmtui
11
Q
What command is used to show addresses assigned to all network interfaces?
A
ip a (ip address)
12
Q
What is the cat command used for?
A
The cat command is used to print files to the standard output.
Usage: cat /etc/hosts
13
Q
What command line utility is used to list directory contents?
A
ls lists directory contents
14
Q
What command is used to locate the current directory?
A
The pwd is used to print name of current/working directory.
15
Q
What is the pwd command used for?
A
The pwd is used to print name of current/working directory.
16
Q
What command is used to reset a password?
A
The passwd command is used to update a user’s authentication token(s).
17
Q
What is the passwd command used for?
A
The passwd command is used to update a user’s authentication token(s).
18
Q
What is the touch command useful for?
A
The touch command can be used to quickly create a file and to test if a user has permissions to modify a file e.g.
$ touch /etc/hosts $ touch: cannot touch '/etc/hosts': Permission denied
19
Q
What command can be used to quickly verify if a user has permission to modify a file?
A
The touch command can be used to quickly create a file and to test if a user has permissions to modify a file e.g.
$ touch /etc/hosts $ touch: cannot touch '/etc/hosts': Permission denied
20
Q
What commands can be used to view the manual page names and descriptions?
A
- apropos
- man -k
Example:
$ man -k directory $ adcli (8) - Tool for performing actions
man -k & apropos both search the mandb based on a keyword
21
Q
What could cause man -k or apropos to return “nothing appropriate”?
A
The database searched by apropos is updated by the mandb program; this is updated periodicallly by a cron job and may require a manul execution on newly installed systems.
To manually update the mandb, execute the command with root privileges.
$ mandb
see man man (-k) or man apropos and man mandb
22
Q
What ls command displays hidden files?
A
The ls -a command does not ignore entries starting with . (dot).
23
Q
What is ls -d option useful for?
A
The ls -d option is used to see the properties of the directory and not its contents.
24
Q
When using the ls utility, what is the -l option used for?
A
The ls -l command returns the results in the long listing format.
25
When using the **ls** utility, what is the **-s** option used for?
The ls -s (--size) option prints the allocated size of each file, in blocks.
26
What is the **ls -lrt** command useful for?
The ls -lrt shows the last modified file last.
* -l - use a long listing format
* -r (--reverse) - reverse order while sorting
* -t - sort by time, newest first; see --time
27
What two commands can be used to list the contents of all directories in a parent folder?
The ls -R and tree commands returns all subfolders and their contents.
```
ls -R
```
```
tree
```
Example
$ **ls -R**
.:
Desktop Documents Downloads Music Pictures Public Templates users Videos
./Desktop:
./Documents:
./Downloads:
./Music:
./Pictures:
./Public:
./Templates:
./Videos:
$ **tree**
.
├── Desktop
├── Documents
├── Downloads
├── Music
├── Pictures
├── Public
├── Templates
├── users
└── Videos
| -R, --recursive list subdirectories recursively
28
# .
What key combination can be used to access to terminal session when in desktop session?
Ctrl + Alt + F3
29
What key combination can be used to access the desktop session when in a terminal session?
Ctrl + Alt + F2
30
What command can be used to create a list of active users and their terminals?
```
$ who
```
```
$ w
```
31
What can the **chvt** command be used for?
The command **chvt *N*** (terminal number from who) makes the /dev/tty***N*** the foreground terminal.
| This command must be run using escalated privildeges.
32
What is the difference between command options and arguments and parameters?
## Footnote
[stackoverflow: Difference between terms: "option", "argument", and "parameter"?](https://stackoverflow.com/questions/36495669/difference-between-terms-option-argument-and-parameter)
33
What are the 9 **man** sections?
1. User command (executable programs or shell commands)
2. System calls (functions provided by the kernel)
3. Library calls (functions within program libraries)
4. Special files (usually found in /dev)
5. File formats and conventions eg /etc/passwd
6. Games
7. Miscellaneous (including macro packages and conventions),
8. System administration commands (usually only for root)
9. Kernel routines [Non standard]
The command below can used to read an introduction of a specific section
```
$ man N intro
```
| Sections 1, 5 and 8 are most important for basic administration (RHCSA)
34
How do you search through a man page?
using / (forward slash) with a string
| use n to skip forward through the search results. shift + n reverse.
35
How do you find a command in a specific section?
If the section is known e.g. it is an adminstrator command (8)
```
man -k user | grep 8
```
If the section is not known, the man -k user command (without grep) will return all references to user and the section.
36
What are the two most common editors?
nano & vim (vi improved)
37
What command outputs the results to a file?
'>' (greater than)
```
ls > directory
```
38
What command appends the results to a text file?
'>>' (double greater than)
```
ls >> directory
```
39
What command is used to view recently used commands?
history
| The history file is found under ~/.bash_history
40
How do you remove a line from the history file?
```
history -d n
```
41
How do you replay a specific command from history?
```
!n
```
| The history command returns all stored commands with a row number.
42
What does the **2> /dev/null** command do?
The **2> /dev/null** is used to redirect/supress errors.
The command broken down:
* 2 refers to a standard error,
* '>' (greater than) is a file redirection operator and
* /dev/null is a null device used to suppress any data written to it.
## Footnote
The 3 file descriptors are:
‘0’ for standard input
‘1’ for standard output
‘2’ for standard error
[What exactly does 2>/dev/null do?](https://linuxhint.com/two-dev-null-command-purpose/)
43
What is globbing?
| [Bash Globbing Tutorial](https://linuxhint.com/bash_globbing_tutorial/)
44
How do you search for files and folders begining with a specific character?
```
ls -d a*
```
45
How do you search for files and folders with charachters begining a, b or c?
```
ls -d [abc]*
```
46
How do you **create** a directory path?
```
mkdir -p /tmp/folder1
```
| -p, --parents no error if existing, make parent directories as needed
47
How do you **remove** a directory?
```
rmdir -p /tmp/folder1
```
48
How do you **create** a directory path with multiple subfolders?
```
mkdir -p /tmp/{folder1, folder2, folder3}
```
| -p, --parents no error if existing, make parent directories as needed
49
How do you **remove** a directory path with multiple specific subfolders?
```
rmdir -p /tmp/{folder1, folder2, folder3}
```
50
How do you **copy** between directories?
`cp /tmp/currenthome /tmp/newhome/`
51
How do you **copy** files and folders recursively?
`cp -R`
| -R, -r, --recursive copy directories recursively
52
How do you **copy** files into the current directory?
`cp /tmp/currenthome .`
| . (dot) is the current directory
53
How do you **move** files between locations?
The **mv** command moves files between locations.
## Footnote
Usage: mv [OPTION]... [-T] SOURCE DEST
or: mv [OPTION]... SOURCE... DIRECTORY
or: mv [OPTION]... -t DIRECTORY SOURCE...
Rename SOURCE to DEST, or move SOURCE(s) to DIRECTORY.
54
What command is used to delete folders that are not empty?
`rm -rf /tmp/somefolder`
## Footnote
Usage: rm [OPTION]... [FILE]...
Remove (unlink) the FILE(s).
55
Where is the sudo configuration managed?
Sudo configuration is manged through /etc/sudoers
56
What is a hard link?
A hard link points to the metadata (the file) on the filesystem (inode). Multiple hard links can point to the same file.
A file is only deleted once all hard links refering to an inode have been deleted; there must be at least one hard link to keep a file on the filesystem.
Hard links cannot point to directories and hardlinks must be on the same filesystem.
Hard links are created using the **ln -P** command; the -P option is not required, as it is the default option when using **ln**.
| see man ln
57
What is a symbolic link?
| AKA soft link
A symbolic is a reference to a specific name e.g. filename 2 refers to filename (hard link > inode).
If the referenced filename changes, the symbolic link is broken.
A symbolic link is most commonly used for directories e.g.
/bin > /usr/bin
58
What is the best practice when creating symbolic links?
Absolute filenames should be used to avoid braking changes.
59
What command is used to create a symbolic link?
ln -s /tmp/filename /tmp/symlink
60
What command is used to create a hard link?
ln /tmp/filename /tmp/symlink
| the option -P is used for physical (hard links); this is option default.
61
What command is used to search for files in a directory hierarchy?
The find command can be used to search for files
```
$ find / -name "hosts"
```
| A wildcard * can be used in the name to broaden the search string.
62
How do you copy the results of the find command?
By executing cp against the results e.g.
`$ find / -name "hosts" -exec cp {} ~/hosts/ 2>/dev/null \;`
63
How do you find files of a specific size?
By using the find command along with the size option e.g.
```
$ find / type -f -size +100M
```
| Type -f (regular file) and -size + (greater than), M for mebibytes
64
How do you search for a specific value within a file?
Using the find and grep commands together can help locate files with specific values/contents e.g.
```
$ find /etc -exec grep -l student {} \;
```
65
What is the backslash and semicolon combination used for?
The backslash (\) is an escape character that instructs the shell not to interpret the next character.
[Multi-Line Code Techniques](https://www.baeldung.com/linux/run-multi-line-shell-code)
66
What are the curley braces used for in the command:
`$ find / -name "hosts" -exec cp {} ~/hosts/ 2>/dev/null \;`
The results of the first command is used in the second command being executed under -exec.
In this example, the files found with the name "hosts" are then copied to a folder under the users directory called hosts.
67
What command displays the environment variables?
The **env** command displays the environment variables.
68
What can the **alias** command be used for?
To simplify repetitve and/or complex command combinations e.g. alias dir='ls -ltr'
69
Where can you create user specific and startup programs?
The .bash_profile can be used to create run bash scripts and create alias e.g. alias dir='ls -ltr'
70
How do you mount new partitions such as a USB device?
First view all mount points to locate the USB drive
```
$ lsblk
```
Create a directory to mount the device agaist
```
$sudo mkdir /mnt/usbdrive
```
Mount the drive to the directory
```
$ sudo mount /dev/sdb1 /mnt/usbdrive
```
71
How do you remove a mount point?
`$ sudo umount /mnt/usbdrive`
72
How do you create an archive file that contains the home and etc directories?
Using the tar command you can create a tar ball file
```
$ tar -czvf archive.tar /home /etc
```
| -z filters through gzip for compression
73
How do you display the contents of a tar file?
Using the tar -tvf command you can create a tar ball file
```
$ tar -tvf archive.tar
```
| -tvf lists all files in archive.tar verbosely.
74
What utility can be used in scripts to convert text from upper to lower case?
The tr (translate) command can convert case
```
$ echo hello | tr [:lower:] [:upper:]
$ HELLO
```
75
What grep option can be used to filter out results?
The grep -v (--invert-match) selects non-matching lines e.g.
```
$ ps aux | grep ssh | grep -v grep
```
Without the last option, the results would include both the ssh process and the executed command by the user.
| man grep
76
What does grep mean?
grep stands for
"global / regular expression search / and print"
| [grep](https://en.wikipedia.org/wiki/Grep)
77
How do you update the sudoers configuration?
the /etc/sudoers configuration must be edited using **visudo**
78
What is the best practice for editing the sudo configuration?
It is best practice to create **drop-in files under /etc/sudoers.d**
79
How do you add users to the wheel group?
usermod -aG wheel username
| see man usermod.
-a (append), -G (groups)
80
How do you increase the authentication timeout for sudo access?
You can increase the authentication token expiration by adding:
`Defaults timestamp_type=global,timestamp_timeout=60`
81
What is the best practice when deleting users?
| what options should be used?
The options remove and force should be used to clearly remove the user
```
$ userdel -rf username
```
82
What 4 commands are used for creating and managing users?
* useradd: create user accounts
* usermod: modify user accounts
* userdel: delete user accounts
* passwd: set passwords
83
Where are user default settings stored?
Default user settings should be stored under /etc/login.defs
| useradd settings are located under /etc/default/useradd (don't use this)
84
How do you create files in the user home directory upon creation?
Using the /etc/skel directory; files in this directory are created in the users home directory upon creation.
85
What are the options to lock and unlock a user account?
usermod -L username will lock the account
usermod -U username will unlock the account
86
What option is used to set an account to expire?
usermod -e 2023-12-31 username is used to expire an account e.g. expire username on 31st December 2023.
87
What option is used to prevent accounts from logining into the system?
usermod -s /sbin/nologin myapp
| man usermod -s (shell)
88
What command shows the status of an account?
passwd -S username outputs a short information about the status of the password for a given account.
The status information consists of 7 fields.
* username
* status e.g. LK (locked password), NP (no password), PS (useable password)
* Last password change
* Minimum age
* Maximum age
* Warning period
89
What command is used to temporarily set primary group membership?
The newgrp
90
What command is used to add a user to an existing group?
The **groupmod -U username groupname** command adds a user to a group
| The usermod -G command replaces the existing groups with a new list.
91
What command is used to list all members of a group?
The **lid -g groupname** lists all members of the group
92
Where are passwords stored?
Passwords are stored (encrypted) in the /etc/shadow file.
93
Other than passwd, what other command can be used to change password settings?
The **chage** command can be used to change password expiry information.
94
What command can be used to display user account settings?
| Example last password change, password experation date etc.
The **chage -l username** lists account aging information.
95
What command is used to set user ownership?
The **chown** command sets user ownership
```
$ chown user[:group] filename
```
| group can be used optionally with and without the user possition.
96
What command is used to set group ownership?
The **chgrp** command sets group ownership
```
$ chgrp groupname filename
```
97
WHat command is used to manage filesystem permissions?
The **chmod** command is used to manage permissions.
| chmod can be used in absolute or relative mode.
98
What are the three values used for file system permissions?
1. read (4)
2. write (2)
3. execute (1)
99
What are the three groups to define permissions?
Owner, Group, Others
1. Owner: rwx = 4+2+1 = 7
2. Group: r-- = 4+0+0 = 4
3. Others: r-- = 4+0+0 = 4
100
What does the first letter of the ls output (permissions) identify?
| example: drwxr-xr-x.
The first mode field is the "special file" designator:
d (directory)
c (character device)
l (symlink)
p (named pipe)
s (socket)
b (block device)
D (door, not common on Linux systems, but has been ported)
["special file" designator](https://unix.stackexchange.com/questions/59132/what-does-the-d-mean-in-ls-al-results-and-what-is-that-slot-called)
101
What is the **less** command used for?
The **less** utility opens a text file in a pager, which allows for easy reading
102
What is the **head** command used for?
The **head** utility displays the first ten lines of the text file
| man head
103
What is the **tail** command used for?
The **tail** utility displays the last ten lines of the text file
| man tail
104
What is the **cut** command used for?
The cut command is used to filter specific columns or characters from a text file
| man cut
105
What is the **sort** command used for?
The **sort** command sorts the contents of a text file
| man sort
106
What is the **wc** command used for?
The **wc** (word count) command counts the number of lines, words, and characters in a text file
| man wc
107
What does the grep option **-i** do?
The **-i** option matches upper- and lowercase letters (i.e., not case sensitive).
108
What does the grep option **-v** do?
The **-v**
shows only lines that do not contain the regular expression.
109
What does the grep option **-r** do?
The **-r** searches files in the current directory and all subdirectories.
110
What does the grep option **-e** do?
The **-e** searches for lines matching more than one regular expression.
Use -e before each regular expression you want to use.
111
What does the grep option **-E** do?
The **-E** interprets the search pattern as an extended regular expression.
112
What does the grep option **`-A `** do?
The **`-A `** shows of lines after the matching regular expression.
113
What does the grep option **`-B `** do?
The **`-B `** shows of lines before the matching regular expression.
114
# ssh
What does the ssh option **`-v`** do?
The **`-v`** (verbose) option shows in detail what is happening while establishing the connection
115
# ssh
What does the ssh option **`-Y`** do?
The **`-Y`** (verbose) option enables support for graphical applications
116
# ssh
What does the ssh option **`-p `** do?
The **`-p `** is used to connect to an SSH service that is not listening on the default port 22
117
# Elevated Permissions
What is a **`PolicyKit`**?
A **`PolicyKit`** enables you to set up graphical utilities to run with administrative privileges
118
# Execute Permissions
What does the **`Read`** permission allow on files and directories?
The **`Read`** permission allows
* **Files**: view file content
* **Directories**: list contents of the directory
119
# Execute Permissions
What does the **`Write`** permission allow on files and directories?
The **`Write`** permission allows
* **Files**: change contents of a file
* **Directories**: create and delete files
120
# Execute Permissions
What does the **`Execute`** permission allow on files and directories?
The **`Execute`** permission allows
* **Files**: run a program file
* **Directories**: change to the directory
121
What is the numeric representation for read, write and execute?
* Read: 4
* Write: 2
* Execute: 1
122
What is the **`df -Th`** intended for?
The **df -Th** command was designed to show available disk space on mounted devices; it includes most of the system mounts.
Because it will look on all mounted file systems, it is a convenient command to use to get an overview of current system mounts.
The **-h option** summarizes the output of the command in a human-readable way, and the **-T option** shows which file system type is used on the different mounts.
123
What is the **findmnt** command used for?
The **findmnt** command shows mounts and the relationship that exists between the different mounts.
Because the output of the mount command is a bit overwhelming, you may like the output of findmnt. Notice that because of width limitations of the book page, the output that belongs in the OPTIONS column appears on the left side of the page.
124
What is the **awk** utility used for?
Awk is a utility that enables a programmer to write tiny but effective programs in the form of **statements that define text patterns that are to be searched for** in each line of a document and the action that is to be taken when a match is found within a line.
| `awk -F : '/user/ { print $4 }' /etc/passwd`
## Footnote
This command searches the /etc/passwd file for the text user and will print the fourth field of any matching line.
125
What is the **sed** utility used for?
**sed is a stream editor**. A stream editor is used to perform basic text transformations on an input stream (a file or input from a pipeline).
sed -i s/old-text/new-text/g ~/myfile
```
| ```
## Footnote
In this example, the sed utility is used to search for the text old-text in ~/myfile and replace all occurrences with the text new-text.
