> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Use input output redirection
> Guide to redirecting stdin, stdout, and stderr in Unix shells, covering redirection operators, pipes, here-documents, suppression, and common patterns for command-line workflows.
We'll cover how to redirect input and output in Linux — a foundational skill for shell scripting, automation, and command-line workflows.
Why this matters: most Unix utilities read from stdin (standard input) and write to stdout (standard output). Redirecting these streams — and stderr (standard error) — lets you capture program output, suppress errors, chain commands, and feed files into programs that expect interactive input.
## Basic example (sort)
Many commands accept a filename argument, but they also work with stdin/stdout. For example, sort reads text and prints sorted lines:
```bash theme={null}
$ cat file.txt
6
5
1
3
4
2
$ sort file.txt
1
2
3
4
5
6
# Save sorted output to a new file
$ sort file.txt > sortedfile.txt
$ cat sortedfile.txt
1
2
3
4
5
6
```
## Redirecting stdout: overwrite vs append
* > overwrites (creates the file if it doesn't exist).
* > > appends to the file.
Example — overwrite (only the last run remains):
```bash theme={null}
$ date > file.txt
$ date > file.txt
$ cat file.txt
Mon Nov 8 18:50:30 CST 2021
```
Example — append (each timestamp preserved):
```bash theme={null}
$ date >> file.txt
$ date >> file.txt
$ cat file.txt
Mon Nov 8 18:50:30 CST 2021
Mon Nov 8 18:50:31 CST 2021
```
Quick comparison:
| Operator | Behavior | Creates file if missing |
| -------: | ---------------- | :---------------------: |
| > | Overwrite stdout | Yes |
| >> | Append stdout | Yes |
## File descriptors and common redirections
Programs use three standard streams:
| Descriptor | Name | Purpose |
| ---------: | ------ | -------------------------------------------------- |
| 0 | stdin | Input to the process (keyboard or redirected file) |
| 1 | stdout | Normal program output |
| 2 | stderr | Error messages and diagnostics |
Common redirection operators:
| Syntax | Meaning |
| --------------------------- | ----------------------------------------- |
| \< file.txt | Redirect stdin from file.txt |
| > file.txt or 1> file.txt | Redirect stdout to file.txt (overwrite) |
| >> file.txt or 1>> file.txt | Append stdout to file.txt |
| 2> errors.txt | Redirect stderr to errors.txt (overwrite) |
| 2>> errors.txt | Append stderr to errors.txt |
File descriptors: 0 = stdin, 1 = stdout, 2 = stderr. Use 2> to redirect error messages separately from normal output.
## Discard unwanted output: /dev/null
Send output you don't want to see to /dev/null — a special sink that discards everything.
Example: hide permission-denied messages from a recursive grep:
```bash theme={null}
# Without suppression: stderr clutter
$ grep -r '^The' /etc/
grep: /etc/cups/ssl: Permission denied
...
# Suppress stderr by redirecting it to /dev/null
$ grep -r '^The' /etc/ 2>/dev/null
/etc/brltty/Input/tn/all.txt:The two keys at the left rear (2 columns, 1 row):
...
```
## Redirect stdout and stderr separately
Capture normal output and errors in different files:
```bash theme={null}
# Overwrite files
$ grep -r '^The' /etc/ 1>output.txt 2>errors.txt
# Append to files
$ grep -r '^The' /etc/ 1>>output.txt 2>>errors.txt
```
## Redirect both stdout and stderr to the same file
To collect both streams into one file, redirect stdout first, then redirect stderr to stdout with 2>&1. The order matters:
```bash theme={null}
# Correct: both streams go into all_output.txt
$ grep -r '^The' /etc/ > all_output.txt 2>&1
# Equivalent with explicit descriptor
$ grep -r '^The' /etc/ 1>all_output.txt 2>&1
```
Why order matters:
* 1>all\_output.txt sets stdout to the file.
* 2>&1 then points stderr to wherever stdout is currently going (the file).
If you reverse the order (2>&1 1>file), stderr is redirected to the original stdout (the terminal) before stdout is redirected, so errors still appear on-screen.
## Input redirection (\<) and feeding commands
Some programs read from stdin instead of accepting a filename. Use \< to provide a file as stdin:
```bash theme={null}
# Example: feed an email body from a file to a sendemail utility that reads stdin
$ sendemail someone@example.com < emailcontent.txt
```
## Here-documents and here-strings
Use here-documents (heredocs) for multi-line inline input. Terminate with the chosen delimiter (EOF is common):
```bash theme={null}
$ sort < file.txt | | |
| Append stdout to file | command >> file.txt | | |
| Redirect stderr to file | command 2> errors.txt | | |
| Append stderr to file | command 2>> errors.txt | | |
| Redirect both to same file | command > all.txt 2>&1 | | |
| Suppress stderr | command 2>/dev/null | | |
| Read stdin from file | command \< input.txt | | |
| Pipeline multiple commands | cmd1 | cmd2 | cmd3 |
## Further reading and references
* Bash manual — Redirections: [https://www.gnu.org/software/bash/manual/html\_node/Redirections.html](https://www.gnu.org/software/bash/manual/html_node/Redirections.html)
* Unix I/O and file descriptors overview: [https://en.wikipedia.org/wiki/Standard\_streams](https://en.wikipedia.org/wiki/Standard_streams)
* Advanced shell scripting techniques (pipes, heredocs): [https://tldp.org/LDP/abs/html/](https://tldp.org/LDP/abs/html/)
That’s all for this lesson.