bash – How Does Bash Know When to Do Filename Expansion?

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Typing ssh-copy-id -i ~/.ssh/, then Tab causes filename expansion, which lists all files in ~/.ssh.

Typing ssh-copy-id without the -i flag, then Spacebar, Tab doesn't cause file expansion.

Typing ssh-copy-id -x, Spacebard, Tab(note that -x is an invalid flag) also does not lead to file expansion.

How does Bash "know" to do file expansion after typing -i? Does the program ssh-copy-id have to be programmed in a way to let Bash know to do it? I read this page and others on filename expansion but could not find an answer for my question:
https://www.gnu.org/software/bash/manual/html_node/Filename-Expansion.html

Bash version:
GNU bash, version 4.4.20(1)-release (x86_64-pc-linux-gnu)

Best Answer

Tab-completion is different from filename expansion.

Tab-completion is a native feature of Bash GNU Readline for interactive Bash sessions. For example, it completes variables (try echo $SH<TAB>) and also commands arguments with file names.

Additionally, if bash-completion package is installed, the completion becomes more intelligent for the commands comprised in in /usr/share/bash-completion/completions, one of which is ssh-copy-id. Since the -i flag for ssh-copy-id requires a file, it tab-completes files. Without -i, there does not make sense to supply a file, so no completion is performed.

Again, that is only true for commands in the completions directory. If you have a foobar command that does not accept a file as an argument, Bash will still complete with files because bash-completion has no idea of foobar.

On the other hand, filename expansion is a POSIX shell behavior. It occurs regardless of whether the shell is interactive or not. Example: ssh-copy-id -x * causes * to expand to all files in the current directory, the command wanting them or not.

Related Solutions

PuTTY – Pasting Text with Tabs into a Here-Document

When you type into a shell, the shell recognizes some characters as commands. For example, the carriage return character (the character sent by the Enter key) causes the shell to execute the command. The tab character causes the shell to perform completion. When you paste something into the PuTTY terminal window, from the shell's point of view, that's the same thing as if you'd typed these characters. So at the point the tab character is pasted, the shell performs completion, it doesn't insert a tab.

The easiest way to copy a file without it being transformed would be to use PuTTY's companion program PSCP or PSFTP to copy the file. This is the simplest way conceptually, but it does have the overhead of running another program, authenticating, choosing a directory, etc.

If you want something inline, you can paste directly into cat, rather than in a here document. Then you'd be pasting into the terminal's line editor, not into the shell's line editor. As the terminal's line editor is very primitive, only a few control characters have a special meaning there, not including tab. Press Ctrl+D at the beginning of a line to terminate the input.

[darkstar /]$ cat >text.txt
Paste
Ctrl+D
[darkstar /]$

I you want to transfer arbitrary data over a medium that interprets control characters, you can encode it into a form that uses only “tame” characters. Base64 is one; it doesn't use any control character and ignores whitespace and newlines. GNU coreutils, which is part of the basic installation on Linux and Cygwin, includes a base64 command. On the sender side, run base64 <file-to-decode, e.g.

On Windows: run base64 c:/path/to/test.txt from a Cygwin terminal
Copy the output.
In the shell in the PuTTY window, type `base64 -d >/tmp/test.txt and press Enter.
Paste the output from base64.
Press Ctrl+D.

Bash – Autocompletion with Wildcards for CD Command

I'm fighting with the same problem. As @Gilles mentioned, there is no real bash-based solution right now.

Since bash-completion doesn't really add much functionality to cd (AFAIK only the use of $CDPATH directories), so if you're not using that then you can add this to your ~/.bashrc:

compopt -o bashdefault cd

This will reset the behavior of the cd completion to default (where wildcard completion works), and will not change anything else.

Best Answer

Related Solutions

PuTTY – Pasting Text with Tabs into a Here-Document

Bash – Autocompletion with Wildcards for CD Command

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