Bash cheat sheet I¶

Here are the bash scripts you frequently use to accomplish your tasks.

User Information¶

who command¶

It is used to retrieve information about the currently logged-in user on the system.

who

It provides the following information:

Login name of the user 
User terminal
Date & Time of login
Remote host name of the user

whoami command¶

It displays the system’s username:

whoami

id command¶

It displays the user identification, including the user ID and group ID:

id 

System information¶

To display system and hardware information, you can use uname command with various options.

File and directory commands¶

pwd command¶

This command displays the current working directory. We often use it with the -P flag, which shows the physical directory without any symbolic links:

pwd -P

ls command¶

It displays a list of files and directories.

ls

-a Show all files
-R list subdirectories recursively
-r Reverse order
-t Sort by last modified
-S Sort by file size, largest first
-l Use a long listing format
-1 One file per line
-m Comma-­sep­arated output
-Q Quoted output

The following command displays all files, including hidden ones.

ls -a 

mkdir command¶

mkdir creates the directories

mkdir ./folder 

mkdir -p ./folder/folder2/folder3 

rm command¶

It removes directories and files. To remove a file,

rm file.txt

To remove files forcefully without prompting for confirmation, use the -f flag.

rm -f file.txt

To remove a directory, use the -r flag.

rm -r folder

To move files or folders forcefully, use the -rf flag.

rm -rf folder

touch command¶

It can be used to create, change, and modify timestamps. The following commands create a file and multiple files, respectively:

touch file1.txt
touch file2.txt  file3.txt

Change only the modification time.

touch -m file2.txt

Change only the access time.

touch -a file2.txt

Use the timestamps of other files.

touch -r file1.txt file2.txt

cat command¶

The cat command is used to create single or multiple files, view the contents of a file, concatenate files, and redirect output to the terminal or to files.

How create file¶

By using the following command, you can create a file and add content. Once you’re done, press Ctrl+D.

cat > file1.txt
You care creating a file

How view content¶

View the content

cat  file1.txt file2.txt

How view content of large file¶

If you have a large file that does not fit in the terminal, use more and less

cat file.txt | more #show page by page
cat file.txt | less #show line by line

head and tail¶

You can use the head command to print the first few lines of its input. The following example shows how to print the first 10 lines.

head -10  file.txt 

The tail command is the opposite of head and prints the last few lines of its input.

head -10  file.txt 

wc command¶

The wc command, which stands for 'word count,' has different flags: -l gives the number of lines, -w gives the number of words, and -m gives the number of characters. These flags can be combined to get all the information at once.

wc  -l  file.txt 

sort command¶

The sort command will sort it's input. By default it will sort alphabetically but there are many options available to modify the sorting mechanism. The following code sorts the file based on the second column.

sort -t' ' -k2,2 file

The flags used are:

• -t: specifies the delimiter (in this case, a space).
• -k: specifies the column for sorting. For example:
  • -k2: means sorting based on the second column
  • -k1,3: means sorting from column 1 through column 3.
  • -k1: means sorting from column 1 through the end.

echo command¶

It can be used to display the text or message,

echo "First message"

It has -e flag that enable interpretation of backslash escapes. In the below, \n make new line.

echo -e "First message \n Second message "

echo -e "First message \n Second message " > out.txt
cat out.txt

Append vs overwrite¶

We know > can be used to create a new file, and by reruning it create a new file.

echo -e "First message \n Second message " > out.txt
cat out.txt
echo -e "First message \n Second message " > out.txt
cat out.txt

If you want to append, use >>

echo -e "First message \n Second message " > out.txt
cat out.txt
echo -e "First message \n Second message " >> out.txt
cat out.txt

Variable¶

Instead the file, you can store the value or message in a variable

var1=Welcome
var2=20

echo $var1
echo $var2

You can call variable inside the echo

echo "$var1, I guess your age is $var2"

What we created are user variable, linux has many variables

echo $USER

here we had single wor, but if you assign comlex value, you need single quotes ( ' ) or double quotes ( " ).

var3="Welcome to cheat sheet" 
echo $var3

You use declare -i to set the type integer,

declare -i var4=20
echo $var4

declare -i var5=Welcome
echo $var5

You define it as read only.

declare -r var6="WEll"
var6="WEll2"

declare +r var6="WEll"
var6="WEll2"

To determine the type of a variable

declare -p $var5

bash Scripts¶

Instead of running a series of Bash commands interactively, we often place them in a file with a .sh file extension. Typically, we include #!/bin/bash at the top of the file, indicating that /bin/bash should be used as the interpreter to execute the script. If you place your script in the ~/bin directory, it will be executable, provided ~/bin is included in your $PATH. To check if ~/bin is in your $PATH, run echo $PATH. If it is not included, you can add it to your ~/.bash_profile.

PATH=$PATH:$HOME/bin
export PATH

PATH is an environment variable that contains a list of directories where executable programs are located. When you type a command in the command line, the system searches these directories, as defined by the PATH variable, to find the appropriate program interpreter. To make your script executable, run chmod u+x file.sh or chmod 755 file.sh.

> test.sh
#!/bin/bash
echo "WELCOME TO FIRST SHELL SCRIPT "
ls
echo "End of FIRST SHELL SCRIPT "

bash test.sh

Users can modify the environment using the set command. By default, Bash does not handle errors automatically and leaves error handling up to the user. For example, the following commands will run without failing, even if an error occurs:

#!/bin/bash
echo $TEMP
echo Hello World

However, by adding set -u, Bash will treat the use of undefined variables as an error, causing the script to fail. For instance, if $TEMP is not defined, the script will exit with an error.

#!/bin/bash
set -u
echo $TEMP
echo Hello World

The following is a list of set options that can be used to control error handling in Bash:

If you create a bash file, paset the following code in it and run it If you create a Bash script, paste the following code into it, and run it

#!/bin/bash
myfunc | echo Hello World
echo Hi

you get the following

Hello World
aa.sh: line 2: myfunc: command not found
Hi

Now, add set -eo pipefail to observe the difference

#!/bin/bash
set -eo pipefail
myfunc | echo Hello World
echo Hi

aa.sh: line 3: myfunc: command not found
Hello World

Comments¶

A comment in a script is a note meant for reference and clarity, and it is not executed. To add a comment, simply use a hash symbol (#); everything following it on the same line will be treated as a comment. Comments can occupy an entire line or be placed at the end of a line of code.

# First comment
echo "Welcome" # Second comment

Command Subsitution¶

You can store the result of a command in a variable by placing the command within backquotes (`) and assigning it to the variable.

myvars=`ls /etc`
echo $myvars

You can also use $() to store the result of a command in a variable.

myvars=$(ls /etc)
echo $myvars

Statue¶

To check the result of the last executed command, use echo $?. If the output is 0, it means the command ran successfully.

> AAA
bash: AAA: command not found
> echo $?
127
> ls 
> echo $?
0 

It's important to note that even when an error is encountered, the script will continue running by default. To prevent this and stop execution on errors, use set -e.

Global and Local Environment Variables¶

To view global environment variables, use the env command. To make environment variables persistent, add them to ~/.bashrc. For example, to set a global variable VAR="VALUE", open .bashrc ( sudo nano ~/.bashrc) and add the line export VAR="VALUE". To remove an environment variable, use the unset command. The following command deletes MYVARIABLE:

unset MYVARIABLE

File permissions¶

You can define the access levels for files and folders to prevent people from accessing other users’ data without permission.

Ownership¶

Each file\folder has three parts:
u: user\owner of file who created it.
g: group of user who has access permissions of files\directories.
o: other users.

The following command shows the permissions of files and folders.

mkdir folder1
touch ./folder1/file.sh
touch ./folder1/file1.sh
ls -l ./folder1
ls -l file.sh

You can see it has four parts.
1- first one digit: "-" or "d"
2- second three digits: it shows the permission of owner.
3- third three digits: designate permissions for the group.
4- fourth three digits: designate permissions for the group.

Permissions¶

Each file or folder has three types of owners:
- Read: It gives permission to open
- Write: Permission to modify the contents of files\folders.
- Execute: Give permission to run it.

The following shows the indicators of the permissions:

  r = read permission = 4
  w = write permission = 2
  x = execute permission = 1
  - = no permission = 0 

The following diagram shows the permissions.

Change access¶

chmod¶

It can be used to change the access mode. This command sets permissions (read, write, execute) on a file or directory for the owner, group, and others.

chmod [reference][operator][mode] file\folder

Examples
chmod a+r files: readable by all
chmod a-r files: cancels the ability for all to read the file
chmod a-rwx cancels all access for all
chmod g+rw files give the group read and write permission
chmod u+rwx files give the owner all permissions
chmod og+rw files give the world and the group read and write permission

The following command is a common command:
chmod 755 file.txt:
Owner can read, write, execute files
Group can read and execute (use) but not change files.
Other can read and execute (use) but not change.

chown command¶

You can change the user and group.

chmod user:group file\folder
chmod -R user:group file\folder

With the -R flag, you can change the ownership of the directory and all its contents recursively.

chgrp command¶

It can be used to change the group owner. The chgrp and chown commands use the same system call and are functionally identical.

chgrp -R group file\folder

Disk usage¶

You can use du [option] [file/folder] to check disk usage. The command du ./folder shows the disk usage summary of the /folder directory tree and each of its subdirectories.
du -h ./folder: to o determine the disk usage in a human-readable format.
du -sh ./folder: to find out the total disk usage.
du -ah /home/: To determine the total disk usage of files and directories.
du -ah --max-depth 2 /home/: show total disk usage of all files and directories up to a certain depth.
du -ah --exclude="*.txt" /home/: to find the total disk usage of files and directories while excluding files that match a given pattern.

Networking¶

Display all network information.

ipconfig -a 

Test the connection to a remote machine:

ping <ip-address> or hostname

Displays active or listening ports.

netstat -pnltu

find command¶

You can use find to search for files and directories. The following command searches for .txt files:

find ./directory -type f -name '*.txt'

find ./directory -type f -empty

Search for files with the specified permissions.

find ./directory -type f -perm 755

If you are looking for the path of command, use whereis,

whereis python3

Wildcards¶

Wildcards are special characters used to create patterns that match sets of files or directories. Here are the common wildcards:

• *: Matches zero or more characters.
• ?: Matches a single character.
• []: Matches any character within the specified range.
• !: Excludes characters inside the brackets.
• #: Matches any single numeric character.

Let see some useful example.

The following command will match any file with a name that contains an 'a' followed by zero or more characters, then a character that is not 'a', 'b', or 'c', and ending with the '.txt' extension.

ls a*[!abc].txt

Search pattern in File¶

You can use grep command to search pattern in file. grep pattern files. The following search for "hello" in the file.

grep "hello" file.txt 

If you want the command to be case-sensitive, use the -i flag.

grep -i "hello" file.txt

pwd command¶

It stands for 'print working directory' and is used to display the current working directory.

Process management¶

ps command¶

The ps command displays information about a selection of active processes.

ps

You can terminate active processes as follows:

kill <process_id>

curl Command¶

This command can be used to transfer data to or from a server. It supports various protocols, including HTTP, HTTPS, FTP, SFTP, etc. You can easily request an HTML page or a file.

curl -o aa.htlm  https://saeidamiri1.github.io/

Downloads a file and saves it with the same name as in the URL:

curl -O   https://saeidamiri1.github.io/

Handling HTTP Requests¶

It allows you to send custom HTTP requests using various methods such as GET, POST, PUT, DELETE, etc. For instance, to send a GET request:

curl -X GET https://api.example.com/resource

Similarly, to send a POST request with data:

curl -X POST -d "key1=value1&key2=value2" https://api.example.com/resource

Includes custom headers in the request.

curl -H "Content-Type: application/json" http://example.com

Uploading Files¶

In addition to downloading, you can also upload files using the -T flag. In the following example, the file is uploaded to the server.

curl -T uploadfile.txt ftp://example.com/upload/

Authentication¶

By using the -u flag, you can specify the password.

curl -u username:password https://example.com/api

Interrupted¶

If the download is interrupted for some reason, you can use the -C - flag:

curl -C - -O ftp://speedtest.tele2.net/1MB.zip

history Command¶

The history command displays a list of previously issued commands. There are a couple of options that can be used to modify the history.

history -c # clears the entire command history.
history -a # appends the current session's history to the history file.

Useful references¶

-[ref]: https://swcarpentry.github.io/shell-novice/