SCP and SFTP on AIX: Complete File Transfer Guide

Secure file transfer is a fundamental task in AIX system administration. This guide covers SCP (Secure Copy) and SFTP (SSH File Transfer Protocol) on AIX, including practical examples, batch operations, and troubleshooting.

SCP vs SFTP: Which to Use?

Feature	SCP	SFTP
Best for	Quick, single file transfers	Interactive sessions, complex operations
Resume interrupted transfer	No	Yes
Directory listing	No	Yes
Delete remote files	No	Yes
Bandwidth limiting	Yes (-l)	No (use external tools)
Scripting	Easy	Requires batch mode

Rule of thumb: Use SCP for quick transfers, SFTP for interactive work or when you need to browse remote directories.

SCP on AIX

Basic SCP Syntax

scp [options] source destination

Copy File to Remote Server

# Copy local file to remote server
scp /tmp/data.tar user@remote-aix:/home/user/

# Copy to specific remote directory
scp /tmp/config.cfg [email protected]:/etc/myapp/

# Copy with different remote filename
scp /tmp/backup.tar user@remote:/backups/backup_20240101.tar

Copy File from Remote Server

# Copy remote file to local directory
scp user@remote-aix:/var/log/app.log /tmp/

# Copy to current directory
scp user@remote-aix:/etc/hosts .

# Copy with different local filename
scp root@remote:/etc/ssh/sshd_config ./sshd_config.backup

Copy Directories Recursively

# Copy entire directory to remote
scp -r /home/user/project user@remote:/backup/

# Copy remote directory to local
scp -r user@remote:/var/www/html /tmp/website_backup/

SCP with Different Port

# Use port 2222 instead of default 22
scp -P 2222 file.txt user@remote:/tmp/

SCP with Identity File (Key)

# Use specific SSH key
scp -i ~/.ssh/my_private_key file.txt user@remote:/tmp/

# Combine with other options
scp -i ~/.ssh/aix_key -r /data user@aix-server:/backup/

Limit Bandwidth

# Limit to 1000 Kbit/s (useful for slow links)
scp -l 1000 large_file.tar user@remote:/tmp/

# Limit to ~500 KB/s
scp -l 4000 huge_backup.tar user@remote:/backups/

Preserve File Attributes

# Preserve modification times, access times, and modes
scp -p /etc/important.conf user@remote:/etc/

Compress During Transfer

# Enable compression (good for text files, slow links)
scp -C large_text_file.log user@remote:/logs/

# Combine with recursive
scp -Cr /var/log/ user@remote:/backup/logs/

Copy Between Two Remote Servers

# Copy from server1 to server2 (via local machine)
scp user@server1:/data/file.txt user@server2:/backup/

# Note: This routes through your local machine
# For direct server-to-server, SSH into server1 and scp from there

SCP with Verbose Output

# Show progress and debug info
scp -v large_file.tar user@remote:/tmp/

# Very verbose (for debugging)
scp -vvv file.txt user@remote:/tmp/

Batch SCP Examples

# Copy multiple files
scp file1.txt file2.txt file3.txt user@remote:/data/

# Copy using wildcards
scp /logs/*.log user@remote:/backup/logs/

# Copy all .conf files from /etc
scp /etc/*.conf user@remote:/backup/etc/

SFTP on AIX

Starting an SFTP Session

# Connect to remote server
sftp user@remote-aix

# Connect on different port
sftp -P 2222 user@remote-aix

# Connect with specific key
sftp -i ~/.ssh/my_key user@remote-aix

SFTP Interactive Commands

Once connected, you have access to these commands:

# Navigation
pwd                 # Print remote working directory
lpwd                # Print local working directory
cd /path            # Change remote directory
lcd /local/path     # Change local directory
ls                  # List remote directory
lls                 # List local directory

# File Transfer
get remote_file           # Download file
get remote_file local     # Download with different name
put local_file            # Upload file
put local_file remote     # Upload with different name
mget *.log                # Download multiple files
mput *.txt                # Upload multiple files

# Directory Transfer
get -r remote_dir         # Download directory recursively
put -r local_dir          # Upload directory recursively

# File Operations
rm file                   # Delete remote file
rmdir directory           # Delete remote directory
mkdir directory           # Create remote directory
rename old new            # Rename remote file
chmod 755 file            # Change remote file permissions
chown uid file            # Change remote file owner
chgrp gid file            # Change remote file group

# Information
df -h                     # Remote disk usage
!command                  # Run local shell command
help                      # Show all commands
exit / quit / bye         # End session

SFTP Practical Examples

Interactive session example:

$ sftp admin@aix-server
Connected to aix-server.
sftp> cd /var/log
sftp> ls -la
-rw-r--r--    1 root     system     123456 Feb 12 09:00 syslog
-rw-r--r--    1 root     system      54321 Feb 12 08:30 messages
sftp> get syslog /tmp/syslog_backup
Fetching /var/log/syslog to /tmp/syslog_backup
sftp> lcd /home/admin/uploads
sftp> put newconfig.conf /etc/myapp/
Uploading newconfig.conf to /etc/myapp/newconfig.conf
sftp> exit

SFTP Batch Mode

For scripting and automation, use batch mode:

Create a batch file (sftp_commands.txt):

cd /var/log
get syslog /backup/syslog
get messages /backup/messages
get secure /backup/secure
quit

Execute batch:

sftp -b sftp_commands.txt user@remote-aix

Inline batch commands:

echo "get /var/log/syslog /tmp/syslog" | sftp user@remote

SFTP in Scripts

Backup script example:

#!/bin/ksh
# backup_logs.sh - Backup remote AIX logs via SFTP

REMOTE_HOST="aix-server"
REMOTE_USER="backup"
BACKUP_DIR="/backup/$(date +%Y%m%d)"
KEY_FILE="/home/backup/.ssh/backup_key"

# Create local backup directory
mkdir -p "$BACKUP_DIR"

# Create SFTP batch file
cat > /tmp/sftp_batch_$$ << EOF
cd /var/log
get syslog $BACKUP_DIR/syslog
get messages $BACKUP_DIR/messages
get -r audit $BACKUP_DIR/audit
quit
EOF

# Execute SFTP with batch
sftp -i "$KEY_FILE" -b /tmp/sftp_batch_$$ ${REMOTE_USER}@${REMOTE_HOST}

# Cleanup
rm -f /tmp/sftp_batch_$$

echo "Backup completed to $BACKUP_DIR"

SFTP Server Configuration

The SFTP subsystem is configured in /etc/ssh/sshd_config:

# Default SFTP subsystem
Subsystem sftp /usr/sbin/sftp-server

# With logging
Subsystem sftp /usr/sbin/sftp-server -l INFO

# Restrict user to SFTP only (chroot)
Match User sftpuser
    ChrootDirectory /sftp/%u
    ForceCommand internal-sftp
    AllowTcpForwarding no
    X11Forwarding no

Create SFTP-Only User on AIX

# Create user
mkuser -a home=/sftp/sftpuser shell=/bin/false sftpuser

# Set password
passwd sftpuser

# Create chroot directory structure
mkdir -p /sftp/sftpuser/upload
chown root:system /sftp/sftpuser
chmod 755 /sftp/sftpuser
chown sftpuser:staff /sftp/sftpuser/upload
chmod 755 /sftp/sftpuser/upload

# Add to sshd_config
cat >> /etc/ssh/sshd_config << EOF

Match User sftpuser
    ChrootDirectory /sftp/%u
    ForceCommand internal-sftp
    AllowTcpForwarding no
EOF

# Restart SSH
stopsrc -s sshd && startsrc -s sshd

Troubleshooting File Transfers

Permission Denied Errors

# Check remote directory permissions
ssh user@remote "ls -la /path/to/directory"

# Check if user can write
ssh user@remote "touch /path/to/directory/test_write && rm /path/to/directory/test_write"

# For SCP, ensure parent directory exists
ssh user@remote "mkdir -p /path/to/directory"

Connection Issues

# Test SSH connection first
ssh -v user@remote

# Check if SFTP subsystem is enabled
ssh user@remote "grep -i sftp /etc/ssh/sshd_config"

# Verify sftp-server exists
ssh user@remote "ls -la /usr/sbin/sftp-server"

Transfer Interrupted

# SFTP can resume (use reget/reput)
sftp user@remote
sftp> reget partially_downloaded_file
sftp> reput partially_uploaded_file

# For SCP, you need rsync instead
rsync -avP --progress user@remote:/path/to/file /local/path/

Slow Transfer Speeds

# Disable compression if already compressed files
scp -o Compression=no file.tar.gz user@remote:/tmp/

# Use faster cipher
scp -c aes128-ctr file.tar user@remote:/tmp/

# Check for network issues
ping -c 5 remote-server
traceroute remote-server

"Received message too long" Error

Usually indicates the remote server is outputting text before SFTP can start. Check:

# Look for echo/print statements in shell profile
ssh user@remote "cat ~/.profile ~/.bashrc ~/.kshrc 2>/dev/null"

# Fix: Wrap output in login shell check
# In remote user's profile:
if [ -t 0 ]; then
    echo "Welcome message"  # Only shows for interactive shells
fi

SCP/SFTP Best Practices on AIX

Use Key-Based Authentication

# Generate key
ssh-keygen -t ed25519 -f ~/.ssh/transfer_key -N ""

# Copy to remote
ssh-copy-id -i ~/.ssh/transfer_key.pub user@remote

# Use in transfers
scp -i ~/.ssh/transfer_key file user@remote:/path/

Create SSH Config for Convenience

Add to ~/.ssh/config:

Host aix-prod
    HostName 192.168.1.100
    User admin
    IdentityFile ~/.ssh/aix_key
    Port 22

Host aix-backup
    HostName 192.168.1.101
    User backup
    IdentityFile ~/.ssh/backup_key

Then simply:

scp file.txt aix-prod:/tmp/
sftp aix-backup

Log All Transfers

For audit purposes, create wrapper scripts:

#!/bin/ksh
# /usr/local/bin/scp-logged
LOG="/var/log/scp_transfers.log"
echo "$(date '+%Y-%m-%d %H:%M:%S') USER=$USER CMD=scp ARGS=$*" >> "$LOG"
/usr/bin/scp "$@"

Related Guides

• AIX SSH Configuration - Complete SSH setup guide
• SSH Tunneling on AIX - Port forwarding guide
• How to Restart SSH in AIX - SSH service management

Summary

SCP and SFTP provide secure file transfer capabilities on AIX:

• SCP: Quick, command-line transfers with scp source dest
• SFTP: Interactive or batch file management
• Use -r for directories, -P for different ports, -i for keys
• SFTP batch mode (-b) enables scripted automation
• Always verify permissions and test SSH connectivity first