Input/Output & Redirection

# 0 = stdin (standard input)
# 1 = stdout (standard output)
# 2 = stderr (standard error)
 
# Name references
0  = /dev/stdin
1  = /dev/stdout
2  = /dev/stderr

# Send output to file (overwrite)
echo "Hello" > output.txt
 
# Append to file
echo "World" >> output.txt
 
# Redirect stdout explicitly
command 1> output.txt
 
# Redirect to /dev/null (discard)
command > /dev/null

# Send errors to file
command 2> errors.txt
 
# Append errors
command 2>> errors.txt
 
# Discard errors
command 2> /dev/null

# Both to same file
command > output.txt 2>&1
 
# Or using &>
command &> output.txt
 
# Or using |& (pipe both)
command |& grep "pattern"

# Redirect stdin from file
python script.py < input.txt
 
# Basic example
sort < unsorted.txt

cat << 'EOF'
Line 1
Line 2
Line 3
EOF

# Pass string as input
wc -w <<< "This is a test"
 
# Variable expansion
data="Important info"
grep "info" <<< "$data"

# From multiple files
cat file1.txt file2.txt | sort
 
# From command output
sort < <(ls -la)

# Chain commands
ps aux | grep "firefox" | awk '{print $2}'
 
# Multiple pipes
cat data.txt | sort | uniq | wc -l
 
# Show intermediate results
echo "1 2 3" | tee intermediate.txt | awk '{print $1}'

# Use command output as file
diff <(sort file1.txt) <(sort file2.txt)
 
# In loops
while read line; do
    echo "Processing: $line"
done < <(cat input.txt)

# Create named pipe
mkfifo my_pipe
 
# Write to pipe (background)
cat input.txt > my_pipe &
 
# Read from pipe
cat < my_pipe
 
# Cleanup
rm my_pipe

# Redirect to specific file descriptor
exec 3> output.txt              # Open fd 3 for writing
echo "Output" >&3               # Write to fd 3
exec 3>&-                       # Close fd 3
 
# Multiple file descriptors
exec 3> file1.txt
exec 4> file2.txt
echo "Line 1" >&3
echo "Line 2" >&4
exec 3>&- 4>&-

# Duplicate stdout to fd 3
exec 3>&1
 
# Redirect stdout to file
exec 1> output.txt
 
# Redirect back to terminal
exec 1>&3
exec 3>&-
 
# Usage - capture both, but still output
exec 3>&1 1> output.txt 2>&1
# ... commands run with output to file
exec 1>&3 3>&-

# Trap all output (POSIX portable)
{
    command1
    command2
    command3
} > output.txt 2>&1

# Show and save output
command | tee output.txt
 
# Append
command | tee -a output.txt
 
# Send to multiple files
command | tee file1.txt file2.txt

#!/bin/bash
 
# All output from this point goes to log.txt
exec >> log.txt 2>&1
 
echo "Starting script..."
echo "This goes to log file"

# Swap stdout and stderr
command 3>&1 1>&2 2>&3 3>&-
 
# Capture stderr to stdout, discard stdout
command 2>&1 1>/dev/null
 
# Send stderr to stdout
(command 2>&1) 1>/dev/null

#!/bin/bash
 
LOG_FILE="script.log"
ERROR_FILE="script.error"
 
exec 1> >(tee -a "$LOG_FILE")
exec 2> >(tee -a "$ERROR_FILE" >&2)
 
echo "Starting..."
echo "Some error" >&2

VERBOSE=0
 
if [ $VERBOSE -eq 1 ]; then
    output_redirect=""
else
    output_redirect="> /dev/null 2>&1"
fi
 
eval "my_command $output_redirect"

# Combine multiple inputs
{
    cat file1.txt
    echo "---"
    cat file2.txt
} | sort | uniq

# Check exit code of all commands in pipeline
set -o pipefail
 
if ! cat data.txt | \
    grep "pattern" | \
    sort | \
    uniq > output.txt; then
    echo "Pipeline failed"
    exit 1
fi

# Discard all output
command > /dev/null 2>&1
 
# Log errors separately
command 2> error.log | process_output
 
# Debug mode - show commands
exec 5>&1          # Save stdout
set -x             # Debug on
# ... script runs ...
set +x             # Debug off
exec 1>&5 5>&-     # Restore stdout
 
# Split output
command 1> >(sort > sorted.txt) 2> >(sort > sorted_errors.txt)