切换SSH登录设备后提取数据出现BASH不可见无效字符求助

Got it, let's tackle this invisible character issue you're seeing after switching your script from Telnet to SSH. I’ve run into this exact kind of headache before—here are some practical, step-by-step fixes to clean up your extracted data:

1. First, Identify What Those Invisible Characters Are #

Before blindly filtering, it helps to know exactly what you’re dealing with. Save a problematic line to a file (e.g., bad_line.txt) and run one of these commands to see the raw ASCII values:

# Show hex values with readable ASCII
hexdump -C bad_line.txt

# Or use od for a different view
od -t x1c bad_line.txt

This will reveal if you’re dealing with control characters (like \r, \x00, or terminal escape sequences), which are common culprits when switching between Telnet and SSH.

2. Disable SSH Pseudo-Terminal Mode #

By default, SSH might allocate a pseudo-terminal (pty) for your command, which makes the remote device think it’s talking to an interactive terminal. This can trigger extra output like color codes, cursor moves, or other control characters. Fix this by adding the -T flag to your SSH command:

ssh -T your_user@your_device "your_command_to_extract_data"

This tells the remote device not to send terminal-specific control sequences, which often eliminates the invisible characters right away.

3. Filter Out Non-Printable Characters #

If disabling the pty doesn’t fix everything, use tr to strip out all non-printable characters (while keeping newlines intact):

ssh -T your_user@your_device "your_command" | tr -dc '[:print:]\n'

The [:print:] POSIX character class includes all readable characters (letters, numbers, symbols, spaces), and \n preserves line breaks so your report structure stays intact.

4. Fix Line Ending Differences #

Telnet typically uses CRLF (\r\n) for line endings, while SSH often uses just LF (\n). Leftover \r characters can show up as invisible junk (or cause lines to overwrite themselves). Strip them with sed:

ssh -T your_user@your_device "your_command" | tr -dc '[:print:]\n' | sed 's/\r//g'

5. Targeted Filtering with Awk #

If you only need to clean specific parts of the line (like the area after PON7), use awk to remove unwanted characters with a regex. For example, to keep only letters, numbers, colons, and spaces:

ssh -T your_user@your_device "your_command" | awk '{gsub(/[^a-zA-Z0-9: ]/, "", $0); print}'

Adjust the regex [^a-zA-Z0-9: ] to include any other characters you need to preserve.

Why This Happens #

Telnet and SSH handle terminal emulation differently: Telnet often uses simpler, minimal terminal settings, while SSH defaults to emulating a full interactive terminal (which adds control characters). Switching without adjusting your script means those extra characters end up in your extracted data instead of being handled by a human terminal.

内容的提问来源于stack exchange，提问作者Ibraheem