BASH Quick Guide

This quick guide distills essential tips and tricks to get the most out of BASH, from keyboard shortcuts to task automation. We'll explore how terminal mastery not only drives efficiency but also strengthens digital defenses in the fascinating world of cybersecurity.

Our BASH Quick Guide will help you navigate and control your system efficiently from the command line. From understanding file types and permissions to debugging, from controlling jobs to understanding regular expressions, This Guide covers it all.

The information contained in the BASH Quick Guide is valuable for casual Linux users, system administrators, information security professionals, and students preparing for a wide range of exams, from LPIC to OSCP and more.

However you choose to use it, we hope you've found it to be a useful resource to have on hand. 

What is Bash?

Bash (Bourne Again Shell) is a shell language built on top of the original Bourne Shell, which was distributed with V7 Unix in 1979 and became the standard for writing shell scripts. 

Today it is primary for most Linux distributions, MacOS, and has even recently been enabled to run on Windows through something called WSL (Windows Subsystem for Linux).

File Test Operators

Testing files in scripts is easy. This is where shell scripts start to show their glory! In Bash, you can test for file permissions, size, date, file type, or existence.

FLAG	DESCRIPTION
-and	The file exists
-to	The file exists (identical to -e but is obsolete and out of date)
-F	The file is a normal file (not a directory or device file)
-s	the file is not of zero size
-d	the file is a directory
-b	The file is a block device.
-C	The file is a character device.
-p	the file is a pipe
-h	The file is a symbolic link.
-l	The file is a symbolic link.
-S	the file is a socket
-t	the file (descriptor) is associated with a terminal device; This test option can be used to check whether stdin [ -t 0 ] or stdout [ -t 1 ] in a given script is a terminal
-r	The file has read permission (for the user running the test).
-w	the file has write permission (for the user running the test)
-X	the file has execute permission (for the user running the test)
-g	set-group-id (sgid) flag set on a file or directory
-or	set-user-id (suid) flag set in file.
-k	adhesive tip set.
-EITHER	you own the file
-G	File group ID same as yours
-N	file modified since last read
f1 -nt f2	file f1 is newer than f2
f1 -ot f2	file f1 is older than f2
f1-ef f2	The files f1 and f2 are hard links to the same file.
!	Not - invierte el sentido de las pruebas anteriores (devuelve verdadero si la condición está ausente).

Integer comparison operators

How to compare integers or arithmetic expressions in shell scripts.

FLAG	DESCRIPTION
-eq	is equal to
-ne	is not equal to
-gt	is greater than
-ge	is greater than or equal to
-lt	is less than
-you	es menor or equal to
<	es menor que - colocar entre paréntesis dobles
<=	is less than or equal to (same rule as previous row)
>	es mayor que (misma regla que la fila anterior)
>=	is greater than or equal to (same rule as previous row)

String comparison operators

String comparison in Bash.

FLAG	DESCRIPTION
=	is equal to
==	is equal to
!=	is not equal to
<	is greater than ASCII alphabetical order
>	is greater than ASCII alphabetical order
-z	the string is null (i.e. of length zero)
-north	the string is not null (i.e. of length zero)

Discover: How I use Bash to automate tasks on Linux

Composite operators

Useful for Boolean expressions and is similar to && and ||. Compound operators work with the test command or can appear in single square brackets [<expr>].

FLAG	DESCRIPTION
-to	logical and
-either	logical or

Job identifiers

Job control allows you to selectively stop (suspend) the execution of processes and continue their execution at a later time.

NOTATION	DESCRIPTION
%N	Job number [N]
%S	The invocation (command line) of the job starts with the string S
%?S	Invocation (command-line) of job contains within it string S
%%	“Current” job (last job stopped in foreground or started in background)
%+	“Current” job (last job stopped in foreground or started in background)
%-	Last job
%!	Last background process

Construction List

Provides a means to process commands consecutively and is in fact capable of replacing complex if/then/according structures

BUILD	DESCRIPTION
&&	and
||	o

Reserved exit codes

Useful for debugging a script. The output takes integer arguments in the range 0-255.

EXIT CODE NO.	DESCRIPTION
1	Catchall for general errors
2	Misuse of built-in shell functions
126	El comando invocado no puede ejecutarse
127	Command not found
128	Invalid argument to exit
128+n	Fatal error signal "n"
130	Script finished by Control-C

Signs

UNIX V system signs.

NAME	NUMBER	ACTION	DESCRIPTION
SIGHUP	1	exit	Hanging
SIGINT	2	exit	Interrupts.
SIGQUIT	3	memory dump	In peace.
SIGILL	4	memory dump	Illegal instruction.
SIGTRAP	5	memory dump	Trail trap.
SIGIOT	6	memory dump	IOT instruction.
SIGEMT	7	memory dump	MT Instruction.
SIGFPE	8	memory dump	Floating point exception.
SIGKILL	9	exit	Deaths (cannot be caught or ignored).
SIGBUS	10	core dump	Bus error.
SIGSEGV	eleven	core dump	Segmentation violation.
SIGSYS	12	core dump	Bad argument for the system call.
SIGPIPE	13	exit	He writes on a pipe without anyone being able to read it.
SIGALRM	14	exit	Alarm clock.
SIGTERM	fifteen	exit	Software termination signal.

Sending control signals

You can use these key combinations to send signals.

KEY COMBINATION	DESCRIPTION
Ctrl+C	The interrupt signal sends SIGINT to the job running in the foreground.
Ctrl+Y	The nature of delayed suspension. Causes a running process to stop when it attempts to read input from the terminal. Control is returned to the shell, the user can foreground, background or kill the process. Delayed sleep is only available on operating systems that support this feature.
Ctrl+Z	The sleep signal sends a SIGTSTP to a running program, stopping it and returning control to the shell.

Check your styt settings. Suspend and resume output are generally disabled if you use "modern" terminal emulations. The standard xterm supports Ctrl+S and Ctrl+Q by default.

File types

This is very different from Windows, but it's simple once you understand it. I'll expand this section soon with more context.

SYMBOL	MEANING
–	normal file
d	directory
I	Link (symbolic)
c	Character device
yes	Plug
p	Named Pipe
b	bloqueoofdispositivo

Permissions

Now you can tell what that arcane-looking string rwxrwxrwx is when you invoke ls -l

CODE	DESCRIPTION
yes	setuid when in user column
yes	setgid when in group column
t	sticky bit
0——	The right of access that this place should have is not granted.
4—–r	Read access is granted to the user category defined here.
2—–w	Write permission is granted to the user category defined here.
1—–x	The execute permission is granted to the user category defined here.
or	User permits
g	group permissions
oh	other permissions

Special files

Files that are read by the shell. Listed in order of execution.

ARCHIVE	INFORMATION
/etc/profile	Launched automatically on login
~.bash_profile ——————— ~/.bash_login ——————— ~.profile	Whichever is found first is executed at login.
~/.bashrc	It is read by all non-login shells.

String manipulation

Bash supports a surprisingly large number of string operations! Unfortunately, these tools lack a unified approach. Some are a subset of parameter substitution and others fall within the functionality of the UNIX expr command. This results in inconsistent command syntax and feature overlap. MacOS's built-in bash is from 2007 and doesn't support many of these.

String Manipulation Table

PATTERN	DESCRIPTION
${#var}	Find the length of the rope.
${var%pattern}	Remove from shorter end pattern
${var%%pattern}	Remove from longest end pattern
${var:position}	Extract substring from $var into $position
${var:num1:num2}	Substring
${var#pattern}	Remove from shorter front pattern
${var##pattern}	Remove from longer front pattern
${var/pattern/string}	Find and replace (replace only the first occurrence)
${var//pattern/string}	Find and replace all occurrences
${!prefix*}	Expands to variable names whose names begin with a prefix.
${var,}${var,pattern}	Converts the first character to lowercase.
${var,,}${var,,pattern}	Converts all characters to lowercase.
${var^}${var^pattern}	Converts the first character to uppercase.
${var^^}${var^^pattern}	Converts all characters to uppercase.
${string/substring/replacement}	Replace the first match of $substring with $replacement
${string//substring/replacement}	Replace all matches of $substring with $replacement
${string/#substring/replacement}	If $substring matches the front end of $string, replace $replacement with $substring
${string/%substring/replacement}	If $substring matches the end of $string, replace $replacement with $substring
expr match “$string” '$substring'	Matching $substring* length at start of $string
expr “$string” : '$substring'	Matching $substring* length at start of $string
expr index “$string” $substring	Numeric position in $string of the first character in $substring* that matches [0 if not matched, first character counts as position 1]
expr substr $string $position $length	Extract $length characters from $string starting at $position [0 if no match, first character counts as position 1]
expr match “$string” '($substring)'	Extract $substring*, searching from the beginning of $string
expr “$string” : '($substring)'	Extract $substring*, searching from the beginning of $string
expr match “$string” '.*($substring)'	Extract $substring*, searching from the end of $string
expr “$string” : '.*($substring)'	Extract $substring*, searching from the end of $string

Quoting

The following text shows characters that must be cited if you want to use their literal symbols and not their special meaning.

SYMBOL	LITERAL MEANING
;	Command separator
&	Running in the background
()	Command Grouping
|	Tube
< > &	Redirect symbols
? [ ] ~ + – @ !	File name metacharacters
«'	Used to quote characters.
$	Variable, command or arithmetic substitution
#	Start a command that ends in a line break
space tab newline	word separators

Everything between "..." is taken literally, except $(dollar)` (backquote) and » (double quote).

Everything between '…' is taken literally, except ' (single quote).

The following are taken literally. Use it to escape anything in "..." or '..."

Using $ before '…' or '…' causes special behavior. $»…» is the same as «…» except that local translation is performed. Similarly, $'…' is similar to $'…' except that the quoted string is processed for escape sequences.

Command parameters

Command parameters, also known as arguments, are used when invoking a Bash script.

DOMAIN	DESCRIPTION
$0	Name of the script itself.
$1…$9	Parameter 1…9
${10}	Positional parameter 10
$*	It expands to positional parameters, starting from one. When the expansion occurs between double quotes, it expands to a single word with the value of each parameter separated by the first of the IFS environment variable.
$-	Current options
$_	The underscore variable is set when the shell is started and contains the absolute file name of the shell or script being executed as passed in the argument list. It is then expanded to the last argument of the previous command, after expansion. It is also set to the full path of each command executed and placed in the environment exported to that command. When checking mail, this parameter contains the name of the mail file.
$$	shell process id
$?	Exit status of the most recently executed command
$@	All arguments as separate words.
$#	Number of arguments
$!	PID of the most recent background process

Story Expansion

Allows the use and manipulation of previous commands.

DOMAIN	DESCRIPTION
!	A historical replacement begins.
!!	Refers to the last command.
!n	Refers to the <n>-th command line.
!-n	Refers to the current command line minus <n>.
!string	Refers to the most recent command starting with <string>
!?string?	Refers to the most recent command containing <string> (the ending ? is optional)
^string1^string2^	Quick replacement. Repeat the last command, replacing <string1> with <string2>.
!#	It refers to the entire command line written so far.

Variable operations

Perform operations on variables.

EXPRESSION
${parameter:-defaultValue} Get the default value of shell variables
${parameter:=defaultValue} Set the default value of shell variables
${parameter:?”Error Message”} Displays an error message if the parameter is not set

Bash Globing

Bash can't recognize RegEx but understands globbing. The shell performs globalization of file names, while RegEx is used to search for text.

Symbol	DESCRIPTION
*	Matches zero or more occurrences of a given pattern
?	Matches zero or one occurrence of a given pattern
+	Matches one or more occurrences of a given pattern
!	Negates any pattern matches — reverses the pattern so to speak

Regular expressions

Always use quotes in your RegEx to avoid globbing

OPERATOR	EFFECT
.	Matches any individual character.
?	The previous item is optional and will be matched, at most, once.
*	The previous element will match zero or more times.
+	The previous element will match one or more times
{N}	The previous element matches exactly N times.
{N,}	The previous element matches N or more times.
{N,M}	The previous element matches at least N times, but no more than M times.
–	Represents the range if it is not the first or last in a list or the endpoint of a range in a list.
^	Matches the empty string at the beginning of a line; also represents characters that are not in the range of a list.
$	Matches the empty string at the end of a line.
[aoeiAOEI]	Matches any 1 character in the list.
[^AOEIaoei]	Matches any 1 character, which is not in the list!
[af]	Matches any 1 character in the range af

In basic regular expressions, the metacharacters «?», «+», «{«, «|», «(» and «)» lose their special meaning; instead, use the backslash "?" … «)». Check your system documentation if commands that use regular expressions support extended expressions.

Character classes in BRE

A character class [:CharClass:] is a set of predefined patterns and consists of the following:

CHARACTER CLASS	EQUIVALENT	EXPLANATION
[:lower:]	[az]	Lowercase letters.
[:upper:]	[AZ]	Capital letters
[:alpha:]	[A-Za-z]	Alphabetic letters, both uppercase and lowercase.
[:digit:]	[0-9]	Numbers 0-9.
[:alnum:]	[a-zA-Z0-9]	Alphanumeric: both letters (upper case + lower case) and digits.
[:xdigit:]	[0-9A-Fa-f]	Hexadecimal digits.
[:space:]	[ \t\n\r\f\v]	Blank space. Spaces, tabs, new lines and the like.
[:punct:]		Symbols (less digits and letters).
[:print:]	[[:graph] ]	Printable characters (spaces included).
[:blank:]	[\t]	Space and tab characters only.
[:graph:]	[^ [:cntrl:]]	Graphically printable characters without including spaces.
[:cntrl:]		Control characters. Non-printable characters

Shell additions

Bash's built-in shell builds are typically very (if not extremely) fast compared to external programs. Some of the built-in functions are inherited from Bourne Shell (sh); These legacy commands will also work in the original Bourne Shell.

BUILTIN	DESCRIPTION
:	Equivalent to true.
.	Reads and executes commands from a designated file in the current shell.
[	It is synonymous with proof but requires a final argument of ].
alias	Defines an alias for the specified command.
b.g.	Resume a job in background mode.
bind	Binds a keyboard sequence to a read line function or macro.
break	Exits a for, while, select, or Until loop.
built in	Runs the built-in command for the specified shell.
caller	Returns the context of any active subroutine calls.
case
CD	Changes the current directory to the specified directory.
command	Executes the specified command without the normal shell search.
compgen	Generates possible ending matches for the specified word.
complete	Shows how the specified words would be completed.
comopt
continue	Resume the next iteration of a for, while, select, or Until loop.
declare	Declare a variable or variable type.
you will say	Displays a list of currently remembered directories.
disown	Deletes the specified jobs from the jobs table for the process.
threw out	Displays the string specified in STDOUT.
enable	Enables or disables the specified built-in shell command.
eval	Concatenates the specified arguments into a single command and executes the command.
exec	Replaces the shell process with the specified command.
exit	Forces the shell to exit with the specified exit status.
export	Sets the specified variables to be available to child shell processes.
fc	Select a list of commands from the history list.
fg	Resume a job in close-up mode.
getopts	Parses the specified positional parameters.
hash	Finds and remembers the full path of the specified command.
help	Displays a help file.
history	Shows command history.
if	Used for branching.
jobs	Lists active jobs.
kill	Sends a system signal to the specified process ID (PID).
let	Evaluate each argument in a mathematical expression.
local	Create a limited scope variable in a function.
logout	Exits a login shell.
mapfile
popd	Removes entries from the directory stack.
printf	Displays text using formatted strings.
pushd	Adds a directory to the directory stack.
pwd	Displays the path name of the current working directory.
read	Reads a line of data from STDIN and assigns it to a variable.
readonly	Reads a line of data from STDIN and assigns it to a variable that cannot be changed.
return	Forces a function to exit with a value that can be retrieved by the calling script.
set	Sets and displays values ​​of environment variables and shell attributes.
shift	Turn the positional parameters down one position.
shopt	Toggles the values ​​of variables that control optional shell behavior.
source	Reads and executes commands from a designated file in the current shell.
suspend	Suspends shell execution until a SIGCONT signal is received.
test	Returns an exit status of 0 or 1 depending on the specified condition.
times	Shows the accumulated system and user shell time.
trap	Executes the specified command if the specified system signal is received.
type	Shows how the specified words would be interpreted if used as a command.
typeset	Declare a variable or variable type.
ulimit	Sets a limit on the specific resource for system users.
umask	Set default permissions for newly created files and directories.
unalias	Delete the specified alias.
one set	Deletes the specified environment variable or shell attribute.
until	Loop that is very similar to the while loop except that it runs until the test command is executed successfully. As long as the test command fails, the until loop continues.
wait	Have the shell wait for a job to finish.
while	Waits for the specified process to complete and returns the exit status.

Bash Symbols Overview

Here we have put together a collection of all the arcane syntax along with a brief description. Many of these symbols are repeated from before, but many are new; This is a good starting point if you are new to the language.

SYMBOL	QUICK REFERENCE
#	used for comments
$	It is used for parameters and variables. It has a lot of edge cases.
()	It is used to execute commands in a subshell.
ps	is used to save the output of commands that are sent to be executed in a subshell.
(( ))	It is used for arithmetic.
$()	is used to retrieve the output of arithmetic expressions, either for use with a command or to save the output to a variable.
$[ ]	obsolete integer expansion construct replaced by $(( )). Evaluates whole numbers in square brackets
[ ]	It is used for testing and is integrated. It is useful in some cases for file name expansion and string manipulation.
[[ ]]	It is used for testing. This is the one you should use unless you can think of a reason not to.
<( )	It is used for process replacement and is similar to a pipe. It can be used whenever a command expects a file and can use several at once.
{ }	It is used for sequence expansion.
${ }	It is used for variable interpolation and string manipulation.
|	is a pipe used to chain commands.
<	used to feed input to commands from a file
>	is used to send results to a file and delete any previous content in that file.
|  |	logical or
&&	logical and
–	used for option prefixes
—	used for long option prefixes
&	used to submit a job to the background
<<WORD<<-WORD	used for heredocs
<<'WORD'<<-'WORD'	strings are used   here
<<<	used to add output to a file.
>>	Single quotes are used to preserve the literal value.
' '	Double quotes are used to preserve the literal value of all characters except $, ` ` and
» «	The backslash is used to escape otherwise interpreted symbols/characters that have a special meaning.
\	used to separate the components of a file name
/	similar to a NOP: a do-nothing operation. It is a built-in shell with a true exit status.
:	Used to separate commands intended to be executed sequentially.
;	It is used to link arithmetic operations. They are all evaluated but only the last one is returned.
,	represents the current directory.
.	represents the main directory.
..	expands to the home directory.
~	It is deprecated and should not be used. Read more in their respective section.
` `	It is deprecated and should not be used. Read more in their respective section.

Flow control

The control flow structures in Bash are simple, although Bash is unforgiving if you get the syntax wrong.

See   examples  on how to use control flow in bash.

SYNTAX STRUCTURE	KEY WORDS OR SYMBOLS	DESCRIPTION
If	yes then fi	Test a condition.
If-else	yes then yes no fi	Test a condition and use a fallback if the test fails.
if-elif-else	if then elif if not fi	Provides additional tests and a fallback if all tests fail. You can omit the elif conditions or add as many intermediate conditions as you like. Similarly, you can skip the else resource.
For	to make	Iterate over a sequence, a list or anything as far as the imagination goes.
While	while doing	As long as a condition is true, repeat until that condition is no longer true.
Until	until it finish	The inverse of the while loop: as long as the test command fails, the until loop continues.
Select	select what to do	It is used to easily generate menus. Any declaration inside can be another selection construct, thus allowing the creation of submenus.
case	case ) ;; that C	Alternative if branch. Each case is an expression that matches a given pattern (that is, a case).

Frequent questions

What do $1 and $2 mean in Bash?

These would be "positional parameters" in a bash script. In this context, they would refer to the first and second arguments passed to the script. For example, if you wrote a script called “myTestScript.sh” with two arguments, such as:

./myTestScript.sh arg1 arg2

$1 would refer to the first argument and $2 would refer to the second.

What is $ in Bash?

$ represents a variable, which can be used to store values ​​such as numbers, strings, or arrays. You use $ to call that variable in a script. 

For example, if you set a variable like this: fav_color=blue

You can invoke it in a script with the line: echo “My favorite color is $fav_color” for the response “My favorite color is blue”

Is Bash easier than Python?

While the two languages ​​are similar in many ways, we would say that Bash would probably be easier to learn. 

We say this not because there is a steep learning curve for Python, as you can learn the basics of any of them in about the same amount of time. Python is simply capable of performing more complex operations, so as you progress in your learning, there is more to cover in Python.

How to run the Bash command in Windows Powershell?

To run Bash on a Windows system, you need to install the Windows Subsystem for Linux  . As described on the Microsoft website,

“The Windows Subsystem for Linux allows developers to run a GNU/Linux environment (including most tools, utilities, and command-line applications) directly on Windows, without modification, without the overhead of a traditional virtual machine or configuration. dual boot.  «

Conclusion

As we close this BASH Quick Guide, we've broken down the keys to optimizing the terminal experience. From customizing the interface to efficiently executing commands, these tips and tricks not only save time but also boost cybersecurity skills. The BASH terminal, when mastered, becomes a powerful tool for professionals looking to hone their art in digital defense.

We hope you found this cheat sheet helpful. The bash terminal is a powerful tool for automating tasks and managing settings. It's useful for everyone from system administrators, developers, and cybersecurity personnel to the average user who chooses Linux as their daily driver.

We recommend that you take a look at our Chronological list of resources to learn Bash from complete beginner to advanced level to fully develop your knowledge base and get the most out of the command line.