Bash – How does bash retrieves what was written on the terminal (without Enter being pressed)

bashinputterminaltty

Say that the ping command is running, and I type something on the terminal while ping is still running.

Now when ping terminates and bash gain back control, bash will print on the terminal what I typed while ping was running. This is a screenshot that shows what I mean:

How did bash get this information? I am sure it did not get it from stdin, because when I typed "I typed this while ping was running", I did not press Enter (and so stdin is empty).

Best Answer

Bash did get your input from its standard input (stdin), which is the terminal.

A terminal can be in one of two modes: raw or cooked (also known as “character mode” and “line mode”, the terms “raw” and “cooked” are mostly used in the Unix world) (the word “cooked” was introduced as a pun to make an opposite to “raw”). In raw mode, the terminal transmits input immediately to the application. In cooked mode, the terminal reads a full line and provides a (very primitive) line edition mechanism, and only transmits the data to the application when the user presses Enter.

Terminals start in cooked mode. Applications that want more control over input, such as bash, set the terminal in raw mode while they're waiting for the user to type a command.

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Bash – How to a Bash script tell how it was run

From man bash under CONDITIONAL EXPRESSIONS:

-t fd  
    True if file descriptor fd is open and refers to a terminal.

Assuming fd 1 is standard out, if [ -t 1 ]; then should work for you. The Advanced Shell Scripting Guide claims that -t used this way will fail over ssh, and that the test (using stdin, not stdout) should therefore be:

if [[ -t 0 || -p /dev/stdin ]]

-p tests if a file exists and is a named pipe. However, I'd note experientially this is not true for me: -p /dev/stdin fails for both normal terminals and ssh sessions whereas if [ -t 0 ] (or -t 1) works in both cases (see also Gilles comments below about issues in that section of the Advanced Shell Scripting Guide).

If the primary issue is a specialized context from which you wish to call the script to behave in a way appropriate to that context, you can sidestep all these technicalities and save your self some fuss by using a wrapper and a custom variable:

!#/bin/bash

export SPECIAL_CONTEXT=1
/path/to/real/script.sh

Call this live_script.sh or whatever and double click that instead. You could of course accomplish the same thing with command line arguments, but a wrapper would still be needed to make point and click in a GUI file browser work.

Bash – The effect of `stty –file= ` is only temporary for consoles in bash. Why

Your shell does this, to help you edit command-lines.

bash's readline library saves/restores terminal modes. You can see this in the rl_prep_terminal and rl_deprep_terminal functions, called indirectly from edit_and_execute_command.

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Bash – How to a Bash script tell how it was run

Bash – The effect of `stty –file= ` is only temporary for consoles in bash. Why

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