I/O Redirection

Control where commands read from and write to.

Basic Redirections

Output (>)

Redirect stdout to file (overwrites):

echo "Hello" > output.txt
ls -la > filelist.txt

Append (>>)

Append to file:

echo "Line 1" > log.txt
echo "Line 2" >> log.txt

Input (<)

Read from file:

cat < input.txt
wc -l < file.txt
sort < unsorted.txt > sorted.txt

Error Redirection

Stderr (2>)

ls /nonexistent 2> errors.txt
command 2>> errors.log  # Append

Stderr to Stdout (2>&1)

command 2>&1 | grep error
make 2>&1 | tee build.log

Both Streams (&>)

command &> all.log
# Same as: command > all.log 2>&1

Common Patterns

Discard Output

command > /dev/null           # Discard stdout
command 2> /dev/null          # Discard stderr
command &> /dev/null          # Discard both

Logging

./script.sh &> script.log
./daily.sh &>> daily.log      # Append

Separate Streams

command > output.txt 2> errors.txt

Processing Files

tr '[:lower:]' '[:upper:]' < input.txt > output.txt
sort < data.txt > sorted.txt

Order Matters

# Both to file
command > file 2>&1

# Both to pipe
command 2>&1 | grep error

File Descriptors

• 0 = stdin
• 1 = stdout
• 2 = stderr

2> file    # Redirect fd 2
1> file    # Same as > file
0< file    # Same as < file

Troubleshooting

Permission Denied

Check file/directory permissions.

Accidental Overwrite

Use >> to append, or check first:

test -f file.txt && echo "File exists!"

Limitations

• No heredoc (<< EOF)
• No here-string (<<< "string")
• No process substitution (<(command))
• No noclobber protection