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landing/docs/Bash-Scripts/023-bash-redirection.md

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+# Redirection
+
+Master Bash redirection and pipes. These essential Linux features are critical for efficient system administration. Every command handles input, output, and errors through **File Descriptors (FDs)**:
+
+* **STDIN** (0): Standard Input – receives data.
+* **STDOUT** (1): Standard Output – sends regular data.
+* **STDERR** (2): Standard Error – sends error messages.
+
+---
+
+### Pipes vs. Redirection
+
+Both manage data streams, but with a key difference:
+* **Redirection** routes a command's input/output to or from a **file**.
+* **Pipes** (`|`) connect the **output of one command directly to the input of another command**.
+
+---
+
+### STDIN (Standard Input)
+
+Commands often expect input. By default, this is your keyboard, but you can redirect it from other sources.
+
+**Redirecting from a file (`<`)**:
+Use `<` to feed a file's content as input to a command. Create `input.txt`:
+
+```bash
+touch input.txt
+```
+Open `input.txt` and add:
+```
+Line 1
+Line 2
+```
+Save `input.txt`.
+Then run:
+
+```bash
+cat < input.txt
+```
+This prints `input.txt` content, similar to `cat input.txt`.
+
+**Here-Documents (`<< DELIMITER`)**:
+For multi-line input directly in your script or terminal, use a here-document with a custom delimiter (e.g., `EOF`).
+
+```bash
+cat << EOF
+Hello World!
+How are you?
+EOF
+```
+This will print:
+```
+Hello World!
+How are you?
+```
+Similarly, with `wc -l` to count lines:
+```bash
+wc -l << EOF
+Hello World!
+How are you?
+EOF
+```
+Output:
+```
+2
+```
+
+---
+
+### STDOUT (Standard Output)
+
+Regular command output typically goes to your terminal. Redirect it to a file.
+
+**Overwrite (`>`)**:
+Use `>` to send output to a file. If the file exists, its content is overwritten. Create `file.txt` (or ensure it's empty):
+
+```bash
+echo "Hello World!" > file.txt
+cat file.txt # Output: Hello World!
+```
+Running again overwrites:
+
+```bash
+echo "How are you?" > file.txt
+cat file.txt # Output: How are you? (Previous content is gone)
+```
+
+**Append (`>>`)**:
+Use `>>` to add output to the end of a file without overwriting existing content.
+
+```bash
+echo "Hello World!" > file.txt # Start fresh
+echo "How are you?" >> file.txt
+cat file.txt
+```
+Output:
+```
+Hello World!
+How are you?
+```
+
+You can also explicitly specify the STDOUT file descriptor (1): `echo "Hello" 1> file.txt`.
+
+---
+
+### STDERR (Standard Error)
+
+Error messages, distinct from standard output, are sent to **STDERR**. Redirect them using file descriptor 2.
+
+**Redirecting Errors (`2>`)**:
+Use `2>` to redirect error messages. For example, `ls --invalid-flag` generates an error.
+
+```bash
+ls --invalid-flag 2> error.txt
+cat error.txt # Contains the error message
+```
+Use `2>>` to append error messages to a file.
+
+**Discarding Errors (`2> /dev/null`)**:
+Send error output to `/dev/null` to completely suppress it. `/dev/null` is a special 'black hole' device that discards all data written to it.
+
+```bash
+ls --invalid-flag 2> /dev/null # Error message is hidden
+```
+
+**Redirecting Both STDOUT and STDERR**:
+You can manage both streams simultaneously.
+
+To separate regular output and errors into different files:
+
+```bash
+# Assuming 'install_package.sh' generates both output and errors
+./install_package.sh > output.txt 2> error.txt
+```
+
+To send both to the same file (concise syntax):
+
+```bash
+./install_package.sh > combined_output.txt 2>&1
+```
+The `2>&1` redirects file descriptor 2 (STDERR) to the same location as file descriptor 1 (STDOUT), which is `combined_output.txt`. Alternatively, an even shorter Bash 4+ syntax: `&> combined_output.txt`.
+
+---
+
+### Piping (`|`)
+
+Pipes connect the standard output of one command directly to the standard input of another, creating powerful command chains.
+
+**Basic Example:** Find `.txt` files in a directory listing.
+
+```bash
+ls | grep ".txt"
+```
+This sends `ls`'s output to `grep` for filtering.
+
+**Chaining Commands:** Analyze file ownership in a directory.
+
+```bash
+ls -l /projects/bash_scripts | tail -n +2 | sed 's/\s\s*/ /g' | cut -d ' ' -f 3 | sort | uniq -c
+```
+This sequence lists files, removes the header, cleans spacing, extracts owner names, sorts them, and counts unique occurrences.
+
+---
+
+### Here-Documents (`<< DELIMITER`)
+
+Here-documents (`<<`) provide multi-line input directly within your script or terminal, ideal for commands that read from STDIN without needing a separate temporary file.
+
+Specify a custom delimiter (e.g., `EOF`):
+
+```bash
+cat << END_MESSAGE
+This text will be passed as input to cat.
+It spans multiple lines.
+END_MESSAGE
+```
+Output:
+```
+This text will be passed as input to cat.
+It spans multiple lines.
+```
+You can also pipe the output of a here-document to another command:
+
+```bash
+wc -l << MESSAGE_LINES
+Line one.
+Line two.
+Line three.
+MESSAGE_LINES
+```
+Output: `3` (counts the lines). Variables are expanded within here-documents.
+
+---
+
+### Here-Strings (`<<<`)
+
+For piping a single string into a command's STDIN, use a here-string (`<<<`). This is cleaner than a here-document for single lines.
+
+```bash
+wc -w <<<"This is a sample string for word count."
+```
+Output: `7` (counts words).
+Variables are also supported in here-strings.
+
+---
+
+### Summary of Redirection and Piping Operators
+
+| **Operator** | **Description** |
+| :--- | :--- |
+| `>` | Redirect STDOUT to a file, overwriting existing content. |
+| `>>` | Redirect STDOUT to a file, appending to existing content. |
+| `<` | Redirect STDIN from a file. |
+| `2>` | Redirect STDERR to a file, overwriting existing content. |
+| `|` | Pipe STDOUT of one command to STDIN of another. |
+| `<<` | Here-document: Provide multi-line STDIN from the script. |
+| `<<<` | Here-string: Provide single-line STDIN from the command line. |