Bash I/O Redirection

Bash Input/Output Redirection

Bash provides powerful tools for controlling input and output streams. Understanding I/O redirection is essential for scripting and system administration.

Standard Streams

Bash has three standard streams:

stdin (0): Standard input (keyboard by default)
stdout (1): Standard output (terminal by default)
stderr (2): Standard error (terminal by default)

Basic Redirection Operators


# Redirect stdout to a file (overwrite)
command > file.txt

# Redirect stdout to a file (append)
command >> file.txt

# Redirect stderr to a file
command 2> error.log

# Redirect both stdout and stderr to a file
command &> output.log

# Redirect stdout and stderr to separate files
command > output.log 2> error.log

Here Documents

A here document (heredoc) is a way to pass multiline input to a command:


cat << EOF
This is a multiline
text block that will be
passed to the cat command.
EOF

Here Strings

Here strings are a shorter alternative to here documents for single-line input:


grep "search" <<< "This string contains the search term"

Pipes

Pipes connect the stdout of one command to the stdin of another:


# Count the number of files in a directory
ls -1 | wc -l

# Find processes and filter results
ps aux | grep "nginx"

Advanced Redirection


# Redirect stderr to stdout
command 2>&1

# Redirect stdout to stderr
echo "Error message" 1>&2

# Discard output (send to /dev/null)
command > /dev/null

# Redirect stdout to file and also display it
command | tee output.txt

# Redirect stdout to file and append
command | tee -a output.txt

File Descriptors

You can create custom file descriptors for advanced I/O operations:


# Create a custom file descriptor
exec 3<> file.txt

# Read from descriptor 3
read -u 3 line

# Write to descriptor 3
echo "New content" >&3

# Close descriptor 3
exec 3>&-

Process Substitution

Process substitution allows you to use command output as if it were a file:


# Compare output of two commands
diff <(ls dir1) <(ls dir2)

# Feed output of multiple commands to another command
paste <(command1) <(command2) > output.txt

Practical Examples


# Log both output and errors with timestamp
command 2>&1 | while read line; do
    echo "$(date '+%Y-%m-%d %H:%M:%S'): $line"
done >> logfile.txt

# Read file line by line
while read -r line; do
    echo "Processing: $line"
done < input.txt

# Read from multiple files
while read -u 3 line1 && read -u 4 line2; do
    echo "$line1 and $line2"
done 3< file1.txt 4< file2.txt

Common Pitfalls

Forgetting that redirection order matters (command 2>&1 >file is different from command >file 2>&1)
Not quoting variables properly when they contain spaces or special characters
Using pipes in loops can create subshells that limit variable scope
Forgetting to close custom file descriptors

Next: Command Substitution