Command documentation sourced from the linux-command project This comprehensive command reference is part of the linux-command documentation project.

cu - Call Up Another System

The cu command (Call Up) is a Unix/Linux utility used to establish connections with remote systems over serial lines, modems, or direct connections. It functions as a dial-in terminal that allows users to connect to other computers, network devices, or systems via telephone lines or serial ports. The cu command is part of the UUCP (Unix-to-Unix Copy) system and provides both interactive terminal sessions and simple file transfer capabilities without error checking.

Basic Syntax

cu [options] [system | phone | "dir"]

Command Line Options

Connection Options

Option	Long Option	Description
-a port	--port port	Use named port for connection
-l line	--line line	Use named device (e.g., ttyS0, ttyUSB0)
-s speed	--speed speed --baud speed -#	Set connection speed/baud rate
-c phone	--phone phone	Phone number to call
-z system	--system system	System name to call from UUCP config
-n	--prompt	Prompt for phone number

Terminal Options

Option	Long Option	Description
-E char	--escape char	Set escape character (default: ~)
-h	--halfduplex	Enable local echo (half duplex)
--nostop		Turn off XON/XOFF flow control
-t	--mapcr	Map carriage return to CR/LF

Parity Options

Option	Long Option	Description
-e		Set even parity
-o		Set odd parity
--parity={odd,even,none}		Set specific parity mode

Debug and Configuration

Option	Long Option	Description
-d		Set maximum debugging level
-x debug	--debug debug	Set specific debugging type
-I file	--config file	Specify configuration file
-v	--version	Display version information
--help		Display help information

In-Session Commands

Once connected, cu recognizes several commands that begin with the escape character (default: ~). These commands must be at the beginning of a line.

Connection Control

Command	Description
~.	Terminate the conversation and exit cu
~^Z	Suspend cu (only works with ~escape char)
~%break	Send break signal to remote system
~%debug	Toggle debug mode

Local Shell Commands

Command	Description
~!command	Execute local shell command
~!	Start local shell
~$command	Execute command, send output to remote
~%cd directory	Change local directory
~%old	Toggle between old/new cu protocols

File Transfer

Command	Description
~%put file [remotefile]	Send file to remote system
~%take file [localfile]	Receive file from remote system
~%baud rate	Change transmission rate

Usage Examples

Basic Connection Examples

Direct Serial Connection

# Connect to device on specified serial port
cu -l /dev/ttyS0 -s 9600

# Connect with specific line and speed
cu -l ttyUSB0 -s 115200

# Direct connection to port (requires write access)
cu -l /dev/ttyS0 dir

Modem Connection

# Connect via modem to phone number
cu -s 115200 -l /dev/ttyS0 5551234567

# Using phone option explicitly
cu -c 5557654321 -s 9600 -l /dev/ttyS0

# Prompt for phone number
cu -n -l /dev/ttyS0 -s 9600

System Connection (UUCP)

# Connect to system defined in UUCP config
cu -z remotehost

# Connect to system with numeric name
cu --system 1234host

# Use specific port from config
cu -p modemport remotehost

Advanced Configuration Examples

Custom Escape Character

# Use Ctrl-A as escape character
cu -E ^A -l /dev/ttyS0 remotehost

# Use different escape character
cu -E '#' -l /dev/ttyS0 -s 9600 remotehost

Parity Settings

# Connect with even parity
cu -e -l /dev/ttyS0 -s 9600 remotehost

# Connect with odd parity
cu -o -l /dev/ttyS0 -s 9600 remotehost

# No parity
cu --parity=none -l /dev/ttyS0 -s 9600

File Transfer Examples

Sending Files

# Connect first, then send file
cu -l /dev/ttyS0 -s 9600 remotehost
# Once connected:
~%put localfile.txt remotefile.txt
~%put config.conf

# Send multiple files
~%put data1.txt
~%put data2.txt
~%put data3.txt

Receiving Files

# Connect first, then receive file
cu -l /dev/ttyS0 -s 9600 remotehost
# Once connected:
~%take remotefile.txt localfile.txt
~%take backup.tar.gz

# Receive with same name
~%take document.pdf

In-Session Command Examples

Local Shell Integration

# Connected to remote system
~!ls -la                    # List local files
~!ps aux                    # Show local processes
~!                          # Start local shell
exit                        # Return to cu session

~$date                      # Send local date to remote
~$hostname                  # Send local hostname to remote

Directory Management

~%cd /tmp                   # Change local directory
~%cd ~/documents            # Change to local home dir
~%put report.txt            # Send from current local dir

Connection Management

~%break                     # Send break signal
~%debug                     # Toggle debugging
~.                          # Exit cu

Real-World Applications

Network Device Management

Router/Switch Configuration

# Connect to Cisco router via console
cu -l /dev/ttyUSB0 -s 9600
# Access router CLI for configuration

# Connect to network switch
cu -l /dev/ttyS1 -s 19200 switch01
# Configure switch settings

Embedded System Debugging

# Connect to embedded device
cu -l /dev/ttyACM0 -s 115200
# Debug embedded Linux system

# Connect to Arduino/ESP32
cu -l /dev/ttyUSB0 -s 115200
# Monitor serial output

Industrial Equipment

PLC and SCADA Systems

# Connect to industrial controller
cu -l /dev/ttyS2 -s 38400 -e
# Even parity for industrial equipment

# Monitor SCADA terminal
cu -h -l /dev/ttyS0 -s 9600 plc01
# Half duplex for legacy equipment

Modem-Based Systems

# Dial-up connection to remote site
cu -s 57600 -l /dev/ttyS0 18005551234

# Connect to alarm system
cu -c 5558765432 -s 1200 -o -l /dev/ttyS0
# Odd parity for alarm systems

System Administration

Remote Server Console

# IPMI serial over LAN connection
cu -l /dev/ttyS0 -s 115200 server_console

# KVM over serial
cu -z kvm_host --port kvm_port

Legacy System Support

# Connect to legacy Unix system
cu -s 2400 -e -o legacy_unix

# AS/400 or mainframe connection
cu -l /dev/ttyS8 -s 19200 -h as400_system

Advanced Usage and Techniques

Automation Scripts

Batch File Transfers

#!/bin/bash
# auto_transfer.sh - Automated file transfer script

REMOTE_HOST="sensor01"
DEVICE="/dev/ttyUSB0"
SPEED="9600"

# Connect and send configuration
(
    echo "~%put sensor.conf"
    echo "~%take data.log"
    echo "~."
    sleep 2
) | cu -l $DEVICE -s $SPEED $REMOTE_HOST

echo "Transfer completed"

Monitoring Script

#!/bin/bash
# monitor_device.sh - Monitor remote device output

cu -l /dev/ttyACM0 -s 115200 sensor02 | \
while read line; do
    echo "$(date): $line"
    # Log monitoring data with timestamp
done

Connection Profiles

UUCP Configuration

# /etc/uucp/config - Main UUCP configuration

# System definitions
system router01 {
    phone 5551234567
    port modem1
    speed 115200
    chat "" ATZ OK ATDT\T CONNECT
}

system switch02 {
    phone 5559876543
    port serial2
    speed 19200
    chat "" ATZ OK ATDT\T CONNECT
}

# Port definitions
port modem1 {
    type modem
    device /dev/ttyS0
    speed 115200
    dialer hayes
}

port serial2 {
    type direct
    device /dev/ttyUSB0
    speed 19200
}

Custom cu Wrapper

#!/bin/bash
# mycu - Enhanced cu wrapper with profiles

case "$1" in
    "router")
        cu -l /dev/ttyUSB0 -s 115200 router01
        ;;
    "switch")
        cu -l /dev/ttyS1 -s 19200 -e switch02
        ;;
    "sensor")
        cu -l /dev/ttyACM0 -s 9600 sensor_array
        ;;
    *)
        echo "Usage: $0 {router|switch|sensor}"
        exit 1
        ;;
esac

Troubleshooting and Common Issues

Connection Problems

Permission Denied

# Error: cu: /dev/ttyS0: Permission denied
# Solution: Add user to dialout group
sudo usermod -a -G dialout $USER
# Log out and back in

# Alternative: Use sudo
sudo cu -l /dev/ttyS0 -s 9600 remotehost

Device Busy

# Error: cu: /dev/ttyS0: Device busy
# Solution: Find and kill processes using the device
sudo fuser -k /dev/ttyS0
# Or check what's using it
sudo lsof /dev/ttyS0

No Carrier Detection

# Error: cu: No carrier
# Solution: Check modem configuration
cu -d -l /dev/ttyS0 -s 9600  # Debug mode
# Verify modem responds to AT commands
cu -l /dev/ttyS0
ATZ        # Should respond with OK

Speed and Flow Control

Garbled Output

# Problem: Garbled text or unreadable output
# Solution: Match correct baud rate and parity
cu -l /dev/ttyS0 -s 9600 -e      # Try even parity
cu -l /dev/ttyS0 -s 19200 -o     # Try odd parity
cu -l /dev/ttyS0 -s 115200 --parity=none

Flow Control Issues

# Problem: Data loss or hanging
# Solution: Disable software flow control
cu --nostop -l /dev/ttyS0 -s 115200

# Or enable hardware flow control
cu -l /dev/ttyS0 -s 115200 remotehost
# May need stty configuration

File Transfer Issues

Transfer Failures

# Problem: File transfer incomplete
# Solution: Use smaller blocks or slower speed
cu -l /dev/ttyS0 -s 1200 remotehost
~%put large_file.txt

# Or try different protocol
cu -l /dev/ttyS0 remotehost
~%old    # Switch to old protocol
~%put file.txt

Permission Issues

# Problem: Cannot write file locally
# Solution: Check local directory permissions
~%cd /tmp               # Use writable directory
~%take remote_file.txt

# Or specify full path
~%take remote_file.txt /home/user/downloads/local_file.txt

Security Considerations

Network Security

Sensitive Data Transmission

# Avoid sending sensitive data over unencrypted connections
# Use SSH instead when available:
ssh remotehost "cat sensitive_file"

# If cu must be used, encrypt data first
gpg -c sensitive_file
cu remotehost
~%put sensitive_file.gpg

Authentication Security

# Avoid storing phone numbers in scripts
# Use environment variables instead
export REMOTE_PHONE="5551234567"
cu -c $REMOTE_PHONE -l /dev/ttyS0

# Limit file access permissions
chmod 600 /etc/uucp/config

System Security

Access Control

# Restrict cu access to specific users
# Create dialout group and control membership
sudo groupadd dialout
sudo usermod -a -G dialout operator1
sudo usermod -a -G dialout operator2

# Set device permissions
sudo chown root:dialout /dev/ttyS*
sudo chmod 660 /dev/ttyS*

Audit Logging

# Enable logging for cu connections
# Add to syslog configuration
echo "cu.* /var/log/cu.log" | sudo tee -a /etc/rsyslog.d/cu.conf

# Monitor cu usage
sudo tail -f /var/log/cu.log

Performance Optimization

Connection Speed

Baud Rate Optimization

# Test different speeds for optimal performance
for speed in 9600 19200 38400 57600 115200; do
    echo "Testing $speed baud..."
    cu -l /dev/ttyS0 -s $speed test_device
    # Test file transfer speed
done

Hardware Flow Control

# Enable hardware flow control for better performance
# Configure serial port before cu
sudo stty -F /dev/ttyS0 crtscts
cu -l /dev/ttyS0 -s 115200 remotehost

File Transfer Efficiency

Batch Operations

# Send multiple files efficiently
# Create a tar archive first
tar -czf files.tar.gz file1.txt file2.txt file3.txt

# Send single archive instead of multiple files
cu -l /dev/ttyS0 remotehost
~%put files.tar.gz

Compression for Slow Links

# Compress files before transfer over slow connections
gzip -9 large_document.txt
cu -l /dev/ttyS0 -s 2400 remotehost
~%put large_document.txt.gz

Integration with Other Tools

Shell Integration

Command Pipelines

# Pipe command output to remote system
dmesg | cu -l /dev/ttyS0 remotehost
# Sends dmesg output to remote

# Receive remote data to local command
cu -l /dev/ttyS0 remotehost | tee remote_output.log
# Saves remote output to file

Expect Script Integration

#!/usr/bin/expect -f
# cu_expect.exp - Automated cu interaction

spawn cu -l /dev/ttyS0 -s 9600 remotehost
expect "Connected"

expect "~"
send "~%put config.conf\r"

expect "Transfer complete"
send "~.\r"

expect eof

Monitoring and Logging

Session Recording

# Record cu sessions for documentation
script cu_session.log
cu -l /dev/ttyS0 -s 9600 remotehost
# Commands and output recorded to cu_session.log
exit

Real-time Monitoring

# Monitor remote device with logging
cu -l /dev/ttyACM0 -s 115200 sensor | \
while read line; do
    echo "$(date '+%Y-%m-%d %H:%M:%S'): $line" >> sensor_log.log
done

Best Practices

Connection Management

Connection Scripts

#!/bin/bash
# connect_device.sh - Standardized connection script

DEVICE="$1"
SPEED="$2"
REMOTE="$3"

if [ $# -ne 3 ]; then
    echo "Usage: $0 <device> <speed> <remote>"
    exit 1
fi

# Check device permissions
if [ ! -w "$DEVICE" ]; then
    echo "Error: No write access to $DEVICE"
    echo "Try: sudo usermod -a -G dialout $USER"
    exit 1
fi

# Make connection with error handling
if ! cu -l "$DEVICE" -s "$SPEED" "$REMOTE"; then
    echo "Connection failed"
    exit 1
fi

Documentation

# Document connection parameters
cat > device_connections.txt << EOF
Router Console:
  Device: /dev/ttyUSB0
  Speed: 115200
  Parity: None
  Command: cu -l /dev/ttyUSB0 -s 115200 router01

Sensor Array:
  Device: /dev/ttyACM0
  Speed: 9600
  Parity: Even
  Command: cu -e -l /dev/ttyACM0 -s 9600 sensor01
EOF

Error Handling

Robust Scripts

#!/bin/bash
# robust_connect.sh - Error-handling connection

connect_to_device() {
    local device="$1"
    local speed="$2"
    local remote="$3"
    local max_attempts=3
    local attempt=1

    while [ $attempt -le $max_attempts ]; do
        echo "Attempt $attempt of $max_attempts"

        if cu -l "$device" -s "$speed" "$remote"; then
            echo "Connection successful"
            return 0
        else
            echo "Connection failed, retrying..."
            sleep 2
            ((attempt++))
        fi
    done

    echo "All connection attempts failed"
    return 1
}

connect_to_device /dev/ttyS0 9600 remotehost

Maintenance

Device Health Checks

#!/bin/bash
# check_serial_devices.sh - Verify serial device health

for device in /dev/ttyS* /dev/ttyUSB*; do
    if [ -e "$device" ]; then
        echo "Checking $device..."

        # Test device accessibility
        if stty -F "$device" >/dev/null 2>&1; then
            echo "  Device is accessible"

            # Show current settings
            stty -F "$device"
        else
            echo "  Device is not accessible"
        fi
    fi
done

Related Commands

• minicom - More feature-rich serial communication program
• picocom - Minimal serial communication program
• screen - Terminal multiplexer with serial support
• tip - Another serial communication utility
• kermit - File transfer and serial communication program
• sz/rz - ZMODEM file transfer protocols
• stty - Serial terminal settings configuration
• setserial - Serial port configuration
• dmesg - Check kernel messages for serial devices
• lspci | lsusb - List serial hardware devices
• chat - Automated conversational script for modems

Additional Resources

Manual Pages

• man cu - Complete cu command reference
• man uucp - UUCP system overview
• man stty - Terminal settings configuration
• man tty - Terminal device information

Configuration Files

• /etc/uucp/config - UUCP configuration
• /etc/uucp/sys - System definitions
• /etc/uucp/port - Port configurations
• /etc/inittab - Init configuration (legacy systems)

Online Documentation

• UUCP Documentation - Official UUCP project
• Serial Programming Guide - Serial programming reference
• Linux Serial HOWTO - Serial setup guide