2.3 Pipes, Redirections, and Filters

Redirection

Control where output goes.

Standard Streams

stdin (0): Input.
stdout (1): Output.
stderr (2): Errors.

Operators

>: Overwrite file with output.
```
echo "Hello" > file.txt
```
>>: Append output to end of file.
```
echo "World" >> file.txt
```
2>: Redirect errors only.
```
ls /nonexistent 2> errors.log
```

Pipes (`|`)

Take the stdout of the command on the left and pass it as stdin to the command on the right.

command1 | command2 | command3

Filters

Commands designed to process text streams.

Command	Function	Example
`grep`	Search for patterns	`cat log.txt
`ls	grep`	List specific files
`less`	Pager (scrollable view)	`cat hugefile.txt
`head`	First 10 lines	`head -n 5 file.txt`
`tail`	Last 10 lines	`tail -f /var/log/syslog` (Follow live!)
`sort`	Sort lines	`sort names.txt`
`uniq`	Remove adjacent duplicates	`sort names.txt
`wc`	Word Count	`wc -l` (Count lines)

Real World Example

Count how many running processes are owned by “root”:

ps aux | grep "^root" | wc -l

Text Processing Tools

Commands to protect, sort, merge, and transform text.

Command	Function	Example
`cut`	Remove sections from each line of files	`cut -d: -f1 /etc/passwd` (Get usernames)
`tr`	Translate or delete characters	`echo “hello”
`sort`	Sort lines of text files	`sort file.txt`
`uniq`	Report or omit repeated lines	`sort file.txt
`wc`	Print newline, word, and byte counts	`wc -l file.txt`

Stream Editors (`sed`)

sed is a stream editor for filtering and transforming text.

Substitute (Search & Replace):

# Replace the first occurrence of 'foo' with 'bar' in each line
sed 's/foo/bar/' file.txt

# Replace ALL occurrences ('g' for global)
sed 's/foo/bar/g' file.txt

# Edit the file in-place ('-i')
sed -i 's/foo/bar/g' file.txt

Delete Lines:

# Delete the 3rd line
sed '3d' file.txt

# Delete lines containing "error"
sed '/error/d' file.txt