Bash: pressing escape 4 times

$ bind -p | grep 'complete$'
"\C-i": complete
"\M-\e": complete

This shows that the default key binding of Meta+Esc (and Ctrl+i) in Emacs command line editing mode is the Readline function complete. The Meta key is usually Esc on keyboards without an explicit Meta key. The Readline documentation for this function says

Attempt to perform completion on the text before point. The actual completion performed is application-specific. Bash, for instance, attempts completion treating the text as a variable (if the text begins with $), username (if the text begins with ~), hostname (if the text begins with @), or command (including aliases and functions) in turn. If none of these produces a match, filename completion is attempted. Gdb, on the other hand, allows completion of program functions and variables, and only attempts filename completion under certain circumstances.

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Regarding your comment to Anthon's answer: No, pressing Esc twice is not the same as pressing Tab generally (unless it's in a program that maps them both to the same action, as Readline does by default). However Ctrl+i is the same as Tab, just like Ctrl+[ is the same as Esc. This means that you can do completion with Ctrl+[ Ctrl+[ in bash if you wish, as long as double Esc is bound to the Readline complete function. This is handy if you're working at a VT220 terminal, for example, which lacks the Escape key:

That is called file completion, and if you don't have any preceding command on the commandline, bash will offer you to complete all 2837 commands it knows about, both built-in and the ones found in your PATH

The are presented in sorted order