What is Redirect? | DevOps Dictionary

Understanding Linux Redirection

What Is Redirection in Simple Terms

Every process has three standard streams: stdin (input), stdout (output), and stderr (errors). By default, stdin reads from the keyboard and stdout/stderr print to the terminal. Redirection changes these defaults — pointing streams at files, devices, or other commands instead.

How It Works

◈ DIAGRAM

+------------------------------------------+
| File descriptor table for any process    |
|                                          |
| 0 = stdin   <- keyboard (default)        |
| 1 = stdout  -> terminal (default)        |
| 2 = stderr  -> terminal (default)        |
+------------------------------------------+
 
Redirection changes where each FD points:
  command > file.txt
  FD 1 now points to file.txt instead of terminal
 
  command 2>&1
  FD 2 now points wherever FD 1 currently points

Complete redirection reference:

Bash

## Redirect stdout to file (overwrites)
command > output.txt
command 1> output.txt      ## same (1 is stdout FD)
 
## Append stdout to file
command >> output.txt
 
## Redirect stderr to file
command 2> errors.txt
 
## Redirect stderr to stdout (combine both streams)
command 2>&1               ## order matters
command > all.txt 2>&1     ## both go to file
command &> all.txt         ## shorthand for above (bash only)
 
## Discard stdout
command > /dev/null
 
## Discard all output
command > /dev/null 2>&1
command &> /dev/null       ## bash shorthand
 
## Read stdin from file
command < input.txt
 
## Here-document (inline stdin)
cat << 'EOF'
line 1
line 2
EOF
 
## Here-string (single string as stdin)
grep pattern <<< "search in this string"
 
## Tee: write to file AND pass to next command
command | tee output.txt | next-command
 
## Tee append
command | tee -a output.txt

Practical Commands

Bash

## Log script output to file while also showing on terminal
./deploy.sh 2>&1 | tee /var/log/deploy-$(date +%Y%m%d).log
 
## Separate stdout and stderr logs
./build.sh > /var/log/build.log 2> /var/log/build-errors.log
 
## Suppress error output from a command
find / -name 'config.yaml' 2>/dev/null
 
## Read file line by line
while IFS= read -r line; do
  echo "Processing: $line"
done < /etc/hosts
 
## Write to a file requiring root from a non-root script
echo 'vm.swappiness=10' | sudo tee -a /etc/sysctl.conf
## tee runs as sudo, so it can write the file
## Without tee: sudo echo ... >> /etc/sysctl.conf FAILS
## (>> is processed by the shell BEFORE sudo runs)

Troubleshooting

Symptom	Command	What to Check
Error output not captured	Add `2>&1`	Only stdout redirected, not stderr
sudo echo fails to write	Use `sudo tee`	Shell handles >> before sudo
Order of 2>&1 matters	`cmd 2>&1 > file` vs `cmd > file 2>&1`	Wrong order sends stderr to terminal

COMMON MISTAKE / WARNING
**Common Mistake:** Writing `command 2>&1 > file.txt` expecting both stdout and stderr to go to the file. This is wrong. `2>&1` redirects stderr to wherever stdout currently points (the terminal), THEN `> file.txt` redirects stdout to the file. The correct order is `command > file.txt 2>&1`.

PLACEMENT PRO TIP
**Tip:** Use `sudo tee` when you need to write to a root-owned file from a script. `echo 'content' | sudo tee /etc/protected-file` works because `tee` runs as sudo. The alternative `sudo echo 'content' >> /etc/protected-file` fails — the shell processes `>>` before elevating to sudo.