Bash

Question 1

Shell

Answer 1

A (possibly interactive) command interpreter, acting as a layer between the user and the system.

Question 2

Bash

Answer 2

The Bourne Again Shell, a Bourne compatible shell. It’s important to understand that BASH is only an interface for you to execute statements (using BASH syntax), either at the interactive BASH prompt or via BASH scripts.

Question 3

interactive mode

Answer 3

A mode of operation where a prompt asks you for one command at a time.

Question 4

non-interactive mode

Answer 4

execute scripts (list of commands) that are stored in a file. When a script is executed, all these commands are (generally) executed sequentially, one after another.

Question 5

$ man

Answer 5

stands for “manual”. it opens documentation (so-called “man pages”) on various topics. You use it by running the command man [topic] at the BASH prompt, where [topic] is the name of the “page” you wish to read.

Question 6

$ man bash

Answer 6

if you’re looking for information on BASH built-ins (commands provided by BASH, not by external applications) you should look in man bash instead. BASH’s manual is extensive and detailed.

Question 7

$ help

Answer 7

Bash also offers a help command which contains brief summaries of its built-in commands

Question 8

Bash commands

Answer 8

BASH reads commands from its input (which is usually either a terminal or a file). Each line of input that it reads is treated as a command — an instruction to be carried out.

Question 9

$ ls

Answer 9

# List files in the current directory (no output, no files).
Command ls prints out the names of the files in the current directory.

Question 10

$ touch a b c

Answer 10

Create files ‘a’, ‘b’, and ‘c’.

Question 11

$ mkdir test

Answer 11

makes an empty directory called test

Question 12

$ cd test

Answer 12

enter the test directory

Question 13

#

Answer 13

character at the start of a word indicates a comment. Any words following the comment are ignored by the shell, meant only for reading.

Question 14

touch

Answer 14

an application that changes the Last Modified time of a file. If the filename that it is given does not exist yet, it creates a file of that name as a new and empty file.

Question 15

$ rm *

Answer 15

Remove all files in the current directory. rm is an application that removes all the files that it was given. * is a glob. It basically means all and in this case means all files in the current directory. Takes filenames as arguments (if our filenames have spaces and we do not quote them, Bash thinks each word is a separate argument)

Question 16

$ echo

$ echo This is a test.

Answer 16

a command that prints its arguments to standard output (which in our case is the terminal). In the second line, echo takes the arguments “This”, “is”, “a”, “test” and prints them out one by one with a space in between. Extra spaces make no difference

Question 17

Do spaces matter in Bash commands?

Answer 17

Nope

Question 18

How can you add extra spaces to an echo command?

Answer 18

Using quotes! We can use quotes around a sentence to pass it along as a single argument (with extra spaces).

Question 19

arguments

Answer 19

These are additional words specified after the command (‘ls -l foo’ executes ls with two arguments).

Question 20

Quotes

Answer 20

The two forms of quotes, single and double (‘ and “), are used to group words and can protect special characters.

Question 21

tip about quotes

Answer 21

Always quote sentences or strings that belong together, even if it’s not absolutely necessary. This developed practice will reduce the risk of human error in the scripts. (e.g. quoting a sentence of an echo command).

Question 22

alias

$ alias nmapp=”nmap -Pn -A –osscan-limit”
$ nmapp 192.168.0.1

Answer 22

a way of shortening a command. An alias is a word that is mapped to a certain string. Whenever that word is used as a command name, it is replaced by the string before executing the command. Aliases are limited in power; the replacement only happens in the first word. To have more flexibility, use a function. Aliases are only useful as simple textual shortcuts.

Question 23

Alias

Answer 23

a word that is mapped to a string. Whenever that word is used as a command, it is replaced by the string it has mapped.

Question 24

Function

Answer 24

a name that is mapped to a set of commands. Whenever the function is used as a command, it is called with the arguments following it. Functions are the basic method of making new commands.

Question 25

Builtin

Answer 25

certain commands have been built into Bash. These are handled directly by the Bash executable and do not create a new process.

Question 26

Executable

Answer 26

a program that can be executed by referring to its file path (e.g. /bin/ls), or simply by its name if its location is in the PATH variable.

Question 27

!/bin/bash

alternative:
#!/usr/bin/env bash

Answer 27

hashbang, specifies that /bin/bash is to be used as the interpreter when the file is used as the executable in a command. When the kernel executes a non-binary file, it reads the first line of the file. If the line begins with #!, the kernel uses the line to determine the interpreter to relay the file to. The #! must be at the very start of the file, with no spaces or blank lines before it.

Question 28

$ chmod +x myscript

$ ./myscript

Answer 28

# Mark the file as executable.
# Now, myscript can be executed directly instead of having to pass it to bash.