BASH acronym for The GNU Bourne-Again SHell is an interface for you to execute statements, either at the interactive BASH prompt or via BASH scripts. It can run in interactive mode and also run in non-interactive mode when executing scripts. Scripts are lists of commands stored in a file. When you execute a script, all commands are executed one after another.
This Bash cheat sheet will show you all useful bash commands that any network or system admin can use as a quick reference.
Bash Scripting Basics
Here, we list some basic bash syntax with a brief explanation. It is a good starting point if you are a beginner.
Syntax
Explanation
#!/bin/bash
Used to tell the operating system the path it should use to interpret the file.
bash file.sh
Used to execute the script in the terminal.
./file.sh
Used to execute the script if it is executable.
#
Used to make comments in the script.
&&
logical AND operator.
| |
logical OR operator.
$#
Used to expands the number of arguments passed to the script.
$0
Used to expands to the name of the shell.
$1, $2
Used as an input parameter that you can add when running script.
exit [0-255]
Used to exit the script and return the number from 0 to 255.
$
Used for parameters and variables.
()
Used for running commands in a subshell.
$()
Used to save the output of commands.
(())
Used for arithmetic.
$(())
Used to retrieve the output of arithmetic expressions.
[]
Used in filename expansion and string manipulation.
<( )
It is very similar to a pipe and used for process substitution.
{ }
Used to expand sequences.
${ }
Used for string manipulation and variable interpolation.
|
Used to run multiple commands together.|
>
Used to send output to a file.
>>
Used to append output to a file.
;
Used to separate multiple commands.
<
Used to get input from a file.
~
Expands to the home directory.
~/.bashrc
Read by every non-login shell.
/etc/profile
Executed automatically at login.
File Test Operators
Here, we will list some helping testing operators for permissions, size, date, file type, or existence in the bash script.
Operators
Explanation
-e
To test if a file exists.
-f
To test if a given file is a regular file.
-d
To test if the file is a directory.
-b
To test if the file is a block device.
-s
To test if the file is not zero sizes.
-L
To test if the file is a symbolic link.
-S
To test if the file is a socket.
-r
To test if the file has read permission.
-w
To test if the file has write permission.
-x
To test if the file has execute permission.
-g
Set group id on file or directory.
-u
Set user id on file or directory.
-k
Set a sticky bit.
-O
You are the owner of the file.
f1 -nt f2
file f1 is newer than f2.
f1 -ot f2
file f1 is older than f2
Comparison Operators
Comparison operators are used in bash to compare two strings to check if they are equal or not. Here, we will list some comparison operators including, string, and integer operators.
Integer Operators
Operators
Explanation
-eq
is equal to
-ne
is not equal to
-gt
is greater than
-ge
is greater than or equal to
-lt
is less than
-le
is less than or equal to
String Operators
Operators
Explanation
=
is equal to
==
is equal to
!=
is not equal to
<
less than
<=
is less than or equal to
>
greater than
>=
is greater than or equal to
-z
string is null
-n
string is not null
Regular Expressions
Regular expressions are shortened as ‘regexp' or ‘regex'. They are strings of characters that define a search pattern. It can be used as a search or search & replace operation.
Expressions
.
Matches any single character.
?
The preceding item is optional and will be matched, at most, once.
*
The preceding item will be matched zero or more times.
+
The preceding item will be matched one or more times.
{N}
The preceding item is matched exactly N times.
{N,}
The preceding item is matched N or more times.
^
Matches the empty string at the beginning of a line.
$
Matches the empty string at the end of a line.
[a-d]
Matches any one character in the range a-d.
Loops and Conditions
A loop is a statement in a bash programming language that allows code to be repeatedly executed. You can set specific conditions during the script execution.
Loops
if then fi
Used to test a condition.
if then else fi
Used to test a condition and use a fallback if the test fails.
if then elif else fi
Used to test a condition and use a fallback if all tests fail.
for do done
Iterate over a list of values.
while do done
Used to performs a given set of commands an unknown number of times as long as the given condition evaluates to true.
until do done
Used to execute a given set of commands as long as the given condition evaluates to false.
sleep time
Wait for a specified amount of time before continuing through the script.
break
Used to exit from the while or for loop but continue the rest of the script.
continue
Used to skip the current iteration of a loop and continue to the next iteration of the loop.
Bash Arrays and Functions
Array
array=("elements of array")
Used to create an array of strings.
${array[0]}
Used to get the first element of the array.
${array[*]}
Used to get all values in the array.
${array[-1]}
Get the last value in the array.
${array[@]}
Expand all of the array elements.
shift
Move argument from $2 to $1.
function() { content-of-function }
Used to define a function.
alias
Used to list all aliases defined in the current session.
alias alias='any command'
Used to define an alias.
Common Utilities and Switches
This cheat sheet will show you the most useful commands and switches to help you in your network and system administration.
Commands
ls -l
List files by type and permission.
ls -a
List all files, including hidden files.
pwd
Display current working directory.
whoami
Who you are logged in as.
last
Display last user logins information.
find /home -name *.txt
Search all text files in /home directory.
find . -size 10k -print
Find all files greater than 10k in the current directory.
egrep "(foo|bar)" file.txt
Find the words foo and bar in file.txt.
sed s/foo/bar/g file.txt
Find the word foo and replace it with a bar in file.txt.
locate file.txt
Find the location of the file.txt quickly.
grep foo file.txt
Searches the word foo in file.txt.
ps -ef
To check all running services.
netstat -ant
To check all network connections.
netstat -ent
To check established network connections.
ifconfig
To check all network interfaces, IPs, and Mac addresses.
ping
Used to check host reachability.
nslookup
Used for DNS query.
ssh
Used to login remote Linux system.
scp -r dir user@remote-ip:/opt/
Copy all files and directories recursively from the local system to a remote system.
scp -r user@remote-ip:/opt/ dir/
Copy all files and directories recursively from the remote system to a local system.
rsync -avz localdir user@remote-ip:/backup
Synchronize files/directories between the local and remote systems.
df -h
Shows free and used space on mounted filesystems.
du -sh
Shows total disk usage of the current directory.
free -m
Show free and used memory and swap space.
lsof
Lists files opened by running processes.
chown user:group filename
Change the owner of the file and directory.
chmod ugo file.txt
Change the user, group, and other permissions for file.txt.
kill pid
Kill any running process.
passwd username
Used to set or reset the user password.
top
Display all running processes, memory usage, cpu usage in real-time.
Shell Builtins
Builtin commands are contained within the shell itself. They called from a shell, that is executed directly in the shell instead of an external executable program.
Builtins
.
Used to reads and runs commands from a designated file in the current shell.
alias
Used to define an alias for a specific command.
bg
Run a job in background mode.
bind
Used to bind a keyboard sequence.
break
Used to exit from a running loop in script.
cd
Change the directory to another directory.
command
Run a specific command without the normal shell lookup.
continue
Resumes the next iteration of the loop in script.
declare
Used to declare a variable.
dirs
Shows a list of all remembered directories.
disown
Remove a job from the job table.
enable
Used to enable or disable built-in command.
exec
Replace the shell process with the specified command.
exit
Used to exit the shell with an exit status.
export
Used to set a variable available for sub-process.
fc
Select a list of commands from the history list.
fg
Run a job in foreground mode.
hash
Used to find and remember the full path of the specified command.
help
Used to display the help file.
history
List the history of all commands.
jobs
List all active jobs.
logout
Used to exit from the current shell.
pwd
Display the path of the current working directory.
read
Read one line from STDIN and assigns it to a variable.
popd
Removes entries from the directory stack.
pushd
Add a directory to the directory stack.
printf
Displays text in a formatted string.
source
Read and executes commands from a specified file in the current shell.
times
Displays the accumulated user and system shell time.
Là một người thích vọc vạch và tò mò với tất cả các lĩnh vực từ khoa học tự nhiên, lập trình, thiết kế đến ... triết học. Luôn mong muốn chia sẻ những điều thú vị mà bản thân khám phá được.
Đăng ký nhận bản tin của chúng tôi
Nhận bài viết mới nhất gửi vào ngay inbox của bạn.
