# Linux I/O redirection and piping.

1. Basic Concepts
2. Input Redirection
3. Output Redirection
4. Error Redirection
5. Piping
6. Advanced Techniques

## 1. Basic Concepts:

In Unix/Linux, there are three standard streams:
- Standard Input (stdin): 0
- Standard Output (stdout): 1
- Standard Error (stderr): 2

By default, stdin is the keyboard, while stdout and stderr are both the terminal.

## 2. Input Redirection:

The < symbol is used for input redirection.

Example:
```bash
sort < file.txt
```
This command sorts the contents of file.txt.

## 3. Output Redirection:

The > symbol is used for output redirection. It creates a new file or overwrites an existing one.
The >> symbol appends to an existing file or creates a new one if it doesn't exist.

Examples:
```bash
echo "Hello, World!" > greeting.txt
echo "How are you?" >> greeting.txt
```

## 4. Error Redirection:

You can redirect stderr using 2> or 2>>.

Example:
```bash
ls /nonexistent 2> error.log
```

To redirect both stdout and stderr to the same file:
```bash
command > output.log 2>&1
```

## 5. Piping:

The | symbol is used for piping. It sends the output of one command as input to another.

Example:
```bash
ls -l | grep "\.txt"
```
This lists all files and then filters for those ending in .txt.

## 6. Advanced Techniques:

- a) Here Documents:
```bash
cat << EOF > file.txt
Line 1
Line 2
EOF
```

- b) Process Substitution:
```bash
diff <(ls dir1) <(ls dir2)
```

- c) Redirecting stdout and stderr to different files:
```bash
command 1> output.log 2> error.log
```

- d) Discarding output:
```bash
command > /dev/null 2>&1
```

- e) tee command (writing to both file and stdout):
```bash
echo "Hello" | tee file.txt
```

- f) Named Pipes (FIFOs):
```bash
mkfifo mypipe
command1 > mypipe & command2 < mypipe
```

These techniques allow for powerful data manipulation and process control in the Linux environment. They're essential for scripting, data processing, and system administration tasks.