The -t flag in SSH forces pseudo-terminal allocation, which enables terminal features when running interactive commands remotely. This is particularly useful for:

# Editing files remotely
ssh -t user@server vim /path/to/file

# Running interactive tools
ssh -t user@server top
ssh -t user@server htop

While ssh -t solves many interactive use cases, there are specific scenarios where it can cause issues:

1. Automated Script Execution

When running scripts remotely, TTY allocation can interfere with output processing:

# Bad practice for automation
ssh -t user@server "/path/to/script.sh"

# Better approach
ssh user@server "/path/to/script.sh"

2. Command Chaining and Piping

TTY allocation adds control characters that can break pipe operations:

# Problematic with -t
ssh -t user@server "ls -l /tmp" | grep ".log"

# Correct approach
ssh user@server "ls -l /tmp" | grep ".log"

3. Background Processes

Processes started with TTY allocation may terminate when SSH disconnects:

# Process may die on disconnect
ssh -t user@server "nohup long_running_process &"

# More reliable without -t
ssh user@server "nohup long_running_process > /dev/null 2>&1 &"

The pseudo-terminal allocation performs several behind-the-scenes operations that can cause unexpected behavior:

• Line buffering instead of block buffering
• Special character interpretation (like Ctrl+C handling)
• Terminal control sequence injection
• Different handling of stdin/stdout/stderr

Follow these guidelines for optimal SSH usage:

# Use -t for:
1. Interactive applications (vim, less, top)
2. Commands requiring terminal features
3. When you need proper signal handling

# Avoid -t for:
1. Script automation
2. Output processing pipelines
3. Background processes
4. Non-interactive bulk operations

Some commands behave differently with/without TTY. You can force non-TTY behavior even when connected interactively:

# Force non-TTY mode for specific command
ssh user@server "script -q -c 'your_command' /dev/null"

The ssh -t flag forces pseudo-terminal allocation, which enables:

# Working interactive commands
ssh -t user@server vim /path/to/file

# Proper terminal emulation
ssh -t user@server top

Automation and Scripting:

#!/bin/bash
# Bad practice:
result=$(ssh -t user@server "command")
# Better:
result=$(ssh user@server "command")

Command Chaining and Pipes:

# Problematic:
ssh -t user@server "command1 | command2" > output.log
# Preferred:
ssh user@server "command1 | command2" > output.log

Line Buffering Problems:

# With -t: Immediate output
ssh -t user@server "while true; do echo 'test'; sleep 1; done"
# Without -t: Buffered output
ssh user@server "while true; do echo 'test'; sleep 1; done"

Exit Code Corruption:

# With -t (may report incorrect exit status)
ssh -t user@server "false; echo $?"
# Without -t (accurate exit code)
ssh user@server "false; echo $?"

• Use -t only when you need terminal features (vim, top, etc.)
• Avoid -t in scripts and automated processes
• Combine carefully with -T when needed: ssh -t host1 ssh -T host2
• For complex commands, test both with and without -t to observe behavior differences

# Problematic chain:
ssh -t host1 ssh -t host2
# Better approach:
ssh -t host1 ssh -T host2