Linux Redirections Cheatsheet

Basic Syntax

command < inputfile

Read input from file instead of keyboard

Example: Count lines in a file

$ wc -l < file.txt

Same as: cat file.txt | wc -l

Example: Sort file contents

$ sort < names.txt

Example: Read from file for while loop

$ while read line; do echo "$line"; done < data.txt

Overwrite Output (>)

command > outputfile

Creates or overwrites the file with command output

Example: Save directory listing

$ ls -la > directory-listing.txt

Append Output (>>)

command >> outputfile

Appends output to existing file content

Example: Append to log file

$ echo "New log entry" >> application.log
$ date >> application.log

Example: Create empty file

$ > newfile.txt
# Same as: touch newfile.txt

Basic Syntax

command 2> errorfile

Redirect only error messages to file

Example: Separate errors from output

$ find / -name "*.conf" 2> errors.log

Shows results on screen, saves "Permission denied" errors to file

Example: Append errors to log

$ command 2>> error.log

Example: Suppress error messages

$ command 2> /dev/null

Discards all error messages

Redirect Both STDOUT and STDERR to Same File

command > output.log 2>&1
# Or modern syntax:
command &> output.log

Both output and errors go to the same file

Example: Capture all output

$ ./script.sh > full-log.txt 2>&1
$ ./script.sh &> full-log.txt  # Modern syntax

Redirect STDOUT and STDERR to Different Files

command > output.log 2> error.log

Output and errors go to separate files

Example: Separate logs

$ make all > build-output.txt 2> build-errors.txt

Redirect STDERR to STDOUT (for piping)

command 2>&1 | less

Sends both output and errors through the pipe

Discard Standard Output

command > /dev/null

Shows only errors, discards normal output

Discard Error Messages

command 2> /dev/null

Shows only normal output, discards errors

Silent Execution (Discard Everything)

command > /dev/null 2>&1
# Or modern syntax:
command &> /dev/null

Completely silent - no output or errors

Example: Check if command succeeds without output

$ if ping -c 1 google.com &> /dev/null; then
    echo "Internet connection OK"
else
    echo "No internet connection"
fi

Example: Silent cron jobs

$ 0 2 * * * /path/to/backup.sh > /dev/null 2>&1

Prevents cron from sending email about the job

Basic Syntax

command << DELIMITER
text line 1
text line 2
DELIMITER

Example: Create multi-line file

$ cat << EOF > config.txt
server=localhost
port=8080
timeout=30
EOF

Example: Send email with heredoc

$ mail -s "Server Alert" [email protected] << EOF
Dear Admin,

Server load is high: $LOAD
Memory usage: $MEMORY%

Please check immediately.
EOF

Variables are expanded in heredocs

Example: Prevent variable expansion (quoted delimiter)

$ cat << 'EOF' > script.sh
#!/bin/bash
echo "Current user: $USER"
echo "Home dir: $HOME"
EOF

Variables are NOT expanded when delimiter is quoted

Example: Interactive SQL

$ mysql -u root -p database << SQL
CREATE TABLE users (
    id INT PRIMARY KEY,
    name VARCHAR(100)
);
INSERT INTO users VALUES (1, 'John');
SQL

Basic Syntax

command <<< "string"

Example: Count words in string

$ wc -w <<< "Hello World from Linux"
4

Example: Calculate MD5 hash

$ md5sum <<< "password123"

Example: Read variable into command

$ URL="https://example.com"
$ grep "example" <<< "$URL"

Example: Compare with echo piping

# Using here string (more efficient)
$ bc <<< "2 + 2"

# Using echo and pipe
$ echo "2 + 2" | bc

Basic Syntax

command1 | command2 | command3

Output of command1 becomes input for command2, and so on

Example: Filter and sort

$ ls -l | grep ".txt" | sort -k5 -n

List files, filter .txt files, sort by size

Example: Count unique users

$ cat access.log | cut -d' ' -f1 | sort | uniq | wc -l

Example: Search in command output

$ ps aux | grep nginx | grep -v grep

Learn more about grep patterns and options for powerful text filtering.

Example: Real-time log monitoring

$ tail -f application.log | grep ERROR

Pipe with error redirection

$ command1 2>&1 | command2

Pipes both stdout and stderr to next command

Opening Custom File Descriptors

# Open FD 3 for reading
$ exec 3< inputfile

# Open FD 4 for writing
$ exec 4> outputfile

# Open FD 5 for appending
$ exec 5>> logfile

Reading from Custom FD

$ exec 3< data.txt
$ while read -u 3 line; do
    echo "Line: $line"
done
$ exec 3<&-  # Close FD 3

Writing to Custom FD

$ exec 4> output.log
$ echo "Log entry 1" >&4
$ echo "Log entry 2" >&4
$ exec 4>&-  # Close FD 4

Swapping STDOUT and STDERR

$ command 3>&1 1>&2 2>&3 3>&-

Temporarily swaps stdout and stderr streams

Duplicating File Descriptors

# Save stdout to FD 6
$ exec 6>&1

# Redirect stdout to file
$ exec 1> output.log

# Commands go to output.log...
$ echo "This goes to file"

# Restore original stdout
$ exec 1>&6 6>&-

Closing File Descriptors

$ exec 3>&-    # Close FD 3 (output)
$ exec 4<&-    # Close FD 4 (input)

Logging Script Output

#!/bin/bash
# Log both output and errors with timestamps
exec > >(tee -a script.log)
exec 2>&1

echo "Script started at $(date)"
# ... script commands ...
echo "Script completed at $(date)"

Backup with Progress

$ tar czf - /data | pv -s $(du -sb /data | cut -f1) > backup.tar.gz 2> backup.log

Shows progress, saves errors to log

Multi-destination Output

$ command 2>&1 | tee output.log | grep ERROR > errors.log

Saves full output and extracts errors separately

Conditional Execution Based on Output

$ if grep -q "ERROR" < application.log; then
    echo "Errors found in log" | mail -s "Alert" [email protected]
fi

Batch File Processing

$ while read filename; do
    echo "Processing $filename"
    process "$filename" >> results.log 2>> errors.log
done < filelist.txt

Creating Detailed Reports

$ {
    echo "=== System Report ==="
    echo "Date: $(date)"
    echo ""
    echo "=== Disk Usage ==="
    df -h
    echo ""
    echo "=== Memory Usage ==="
    free -h
} > system-report.txt

Error-Only Logging

$ find / -name "*.conf" > found.txt 2> notfound.txt

Separate successful finds from permission denied errors. For advanced log analysis, check our log parsing cheatsheet.