CodeAlchemy

Jotting one man's journey through software development, programming, and technology

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CLI Essentials

🛠 Text-based interface used to interact with a computer by entering commands. It provides direct access to system functions and scripting capabilities, often used for automation and troubleshooting.

Table of Contents

~

The ~ (tilde) represents the home directory of the current user in Unix-based systems like macOS and Linux.

If your username is pablogarcia, then ~ is equivalent to /Users/pablogarcia/ on macOS. If you were on Linux, ~ would typically be /home/pablogarcia/.

;

The ; in the command line is used to separate multiple commands so they run sequentially on the same line.

cd ~/bookshelf; ~/.local/bin/gunicorn -b :8080 main:app

${...}

This syntax is used for referencing a shell variable.

WORKFLOW_TRIGGER_SA="my-service-account@project.iam.gserviceaccount.com"
echo ${WORKFLOW_TRIGGER_SA}
# Output: my-service-account@project.iam.gserviceaccount.com

&& (Logical AND operator)

&& is a logical AND operator in Bash that runs the second command only if the first command succeeds.

mkdir my_folder && cd my_folder

Creates my_folder and changes into it only if mkdir succeeds.

$_

$_ is a special variable in Bash that holds the last argument of the previous command.

mkdir my_folder && cd $_

$_ expands to my_folder, so this is equivalent to cd my_folder.

cat

The cat command in Unix-based systems (like macOS and Linux) is used to concatenate and display the contents of files. It is mostly used to quickly view or combine files.

Common Uses

  1. View a file’s contents 
    cat filename.txt

    Prints the content of filename.txt to the terminal.

  2. Concatenate multiple files 
    cat file1.txt file2.txt > combined.txt

    Merges file1.txt and file2.txt into combined.txt.

  3. Create a new file 
    cat > newfile.txt

    Starts writing into newfile.txt (Press Ctrl + D to save).

cat > ~/bookshelf/oauth.py <<EOF
# (Python code here)
EOF
  1. Creates (or overwrites) a file named oauth.py inside the bookshelf directory.
  2. Writes the Python code inside the file (between «EOF and EOF).
  3. Saves and closes the file automatically.
  1. Append content to an existing file 
    cat file1.txt >> file2.txt

    Appends the content of file1.txt to file2.txt.

  2. Number the lines in a file 
    cat -n filename.txt

    Displays filename.txt with line numbers.

Conditional Blocks

if [[ -f myfile.txt ]]; then
    echo "File exists"
else
    echo "File does not exist"
fi  # End of if statement

cp

The cp (copy) command in Unix-like systems is used to copy files and directories. The syntax varies slightly depending on whether you’re using it for local files (cp) or cloud storage (gcloud storage cp).

Local File Copy

cp [options] source destination

Examples

Copy a file:

cp file1.txt file2.txt  # Copies file1.txt to file2.txt

Copy a file to a directory:

cp file1.txt ~/Documents/  # Copies file1.txt into Documents

Copy a directory (recursively):

cp -r my_folder backup_folder  # Copies my_folder and its contents

Common Options:

curl

The curl command is a command-line tool used to transfer data to or from a server using various network protocols, including HTTP, HTTPS, FTP, and others. It stands for Client URL and is commonly used for interacting with APIs, downloading or uploading files, and testing network connections.

Common Use Cases

  1. Fetch a webpage or file:
curl http://example.com

This will retrieve the content of http://example.com.

  1. Download a file:
curl -O https://example.com/file.zip

This downloads a file and saves it with the same name as on the server (file.zip).

  1. Send data with POST request:
curl -X POST -d "name=John&age=30" http://example.com/api

Sends a POST request with form data (name=John&age=30).

  1. Include headers in the request:
curl -H "Authorization: Bearer YOUR_TOKEN" http://example.com/api

Sends an HTTP request with a custom header (e.g., an Authorization token).

  1. Save the output to a file:
curl -o filename.txt http://example.com

This downloads content from the URL and saves it as filename.txt.

  1. Show only the HTTP response headers:
curl -I http://example.com

This fetches only the HTTP headers (e.g., status code, content type) without the body.

Key Options:

echo

The echo command prints text to the terminal or outputs it to a file. It’s commonly used to display messages or write text into files.

echo "Hello, world"       # prints "Hello, world" to the terminal
echo "export VAR=value" >> ~/.bashrc   # appends the line to the end of the ~/.bashrc file

The >> operator in echo commands is used to append text to a file.

If you used a single > instead of >>, it would overwrite the entire file — which is typically not what you want when modifying config files.

export

Makes the variable available to child processes (subprocesses) started from that shell session.

export IMAGE_NAME=neon.jpg

Defines IMAGE_NAME as neon.jpg, which is the filename of the image being uploaded.

Flags


Why use –user?

- Avoids needing admin/root privileges (no sudo required).
- Installs packages in ~/.local/ (Linux/macOS) or %APPDATA%\Python (Windows).
- Prevents conflicts with system Python packages.

When to use it?

- When you don’t have admin rights.
- When you want to keep system Python clean.
- If using a single-user setup without a virtual environment. > Using virtual environments (venv or conda) is usually preferred over --user for better package management.

htop

The htop command is an interactive process viewer for Unix-based systems. It allows users to monitor system resources (CPU, memory, etc.) and manage processes in real time, with features like sorting, filtering, and killing processes directly from the interface.

htop

kill

Use the kill command followed by the process ID (PID)

kill -9 <PID>

ps aux | grep <[app.py](http://app.py/)>

Use the ps command to locate the process associated with your application or Python script

Example

(quality_checker) pablogarcia@Pablos-MacBook-Pro cap_windows % ps aux | grep [app.py](http://app.py/)
pablogarcia        468   0.1  0.5 413903088 179056   ??  S     1:05PM   0:17.97 /opt/homebrew/Caskroom/miniconda/base/envs/quality_checker/bin/python [app.py](http://app.py/)
pablogarcia        455   0.0  0.2 412976304  69216   ??  S     1:05PM   0:01.41 python [app.py](http://app.py/)
pablogarcia       2405   0.0  0.0 410063504    528 s014  R+    1:29PM   0:00.00 grep [app.py](http://app.py/)

The PID (Process ID) is the number in the second column of the output from the ps aux command. It’s a unique identifier assigned to each running process.

kill -9 468 455

To stop the Python script, we want to terminate the processes with PIDs 468 and 455 (related to app.py)

ln

Creates links between files or directories.

Feature Soft Link (Symbolic Link) Hard Link
Definition A pointer to the original file. A duplicate reference to the same file data on disk.
Inode [^1] Has a different inode number than the original file. Shares the same inode as the original file.
Works Across Filesystems? Yes, can link to a file on a different filesystem. No, must be on the same filesystem.
Works for Directories? Yes, can link to directories. No, hard links cannot be created for directories.
If Original File is Deleted? The soft link breaks (becomes useless). The hard link still works because the file data remains.
Command to Create ln -s target link_name ln target link_name

[^1]: An inode (index node) is a data structure used by Unix-like file systems to store metadata about a file. It contains information such as: File size, File type (regular file, directory, etc.), Permissions (read, write, execute), Owner and group ID, Timestamps (creation, modification, access), Pointers to data blocks (where the file’s actual content is stored). Each file has a unique inode number, except hard links, which share the same inode. Soft (symbolic) links, however, have different inodes. To check a file’s inode number, use: ls -i filename.

What is the difference between a hard link and copying the file?
A hard link is just another name for the same file, pointing to the same data on disk. A copied file is a completely independent version, taking up extra space.

The following command creates a symbolic link (~/code) that points to ~/training-data-analyst/courses/orchestration-and-choreography/lab1. After running this command, instead of navigating to the full path, you can just use ~/code to access the same location.

ln -s ~/training-data-analyst/courses/orchestration-and-choreography/lab1 ~/code

mkdir (Make Directory)

Command used to create new directories (folders) in a filesystem.

mkdir my_folder  # Creates a directory named "my_folder"

Use -p to create parent directories if they don’t exist:

mkdir -p parent/child  # Creates "parent" and "child" inside it if they don't exist

mv

The mv command in Unix/Linux is used to move or rename files and directories.

Move a file:

mv file.txt /path/to/destination/

Rename a file:

mv oldname.txt newname.txt

Move and rename at the same time:

mv file.txt /new/path/newname.txt

nano

The nano command is used to open the Nano text editor, a simple and user-friendly command-line text editor available on Unix-based systems like Linux and macOS.

nano myfile.txt

Opens (or creates) myfile.txt for editing in the terminal.

Common Nano Commands

| Shortcut | Action | |———–|—————-| | CTRL + O | Save (Write Out) | | CTRL + X | Exit | | CTRL + K | Cut a line | | CTRL + U | Paste a line | | CTRL + W | Search | | CTRL + G | Show help |

After CTRL + X, you will have to type Y and press Enter to save the file.

ping

The ping command is used to check the reachability of a host on a network and measure round-trip time for data packets.

ping [options] <hostname or IP address>

For example:

ping google.com

Common Options

Output

  • Reply from [IP address]: The destination is reachable.
  • time: Round-trip time in milliseconds (ms).
  • TTL (Time to Live): Number of hops the packet made.

touch

Creates an empty file or updates the timestamp of an existing file.

touch file.txt

Creates file.txt if it doesn’t exist or updates its last modified time if it does.

unzip

Extracts files from a .zip archive.

unzip archive.zip

Extracts archive.zip contents into the current directory.