Command line input while a running script displays strange symbols

command linespecial charactersterminal

If I use the bash shell cli and run some program which doesn't return immediately, then I input some letters (basically typing the next command before noticing the previous didn't finish) and the input always looks something like this (especially when using arrow up and down keys:)

]]A^ or ]]B^

And I wonder, is this kind of premature input 'harmful' in anyway, or what does the returned string mean?

Best Answer

These strings are the raw representation of the special characters you typed (like home, end, arrows, tab etc...).
Their harmful effect is controlled by the command you are running, if it is receiving input from the terminal, these characters will be part of it's input.
If it does not take input from the terminal, it shouldn't have any adverse effects on any of the Unix flavors I know of.

Related Solutions

Linux – Switching among cli and vim and other files workflow

There are several options to do so:

You can use a terminal multiplexer like screen or tmux.

In screen, for example, the shortcut Ctrl+a - a, has the same functiononality as Alt+Tab in graphical environments: switch to the last screen.
Or you use vim's internal function.

Type :!command in vim's command mode. For example: :!ls -l. After the command finishes press Enter to switch back to vim.
There is one more option: Job conrol.

Press Ctrl+z to stop the current process (vim). You will find yourself in a terminal. To bring the stopped process back to the foreground type fg.

For me, I prefer screen. I have an unwritten rule for myself: "Always open a screen."

Bash – Command line method to find repeat-word typos, with line numbers

Edited: added install and demo

You need to take care of at least some edge cases, like

repeated words at the end (and beginning) of the line.
search should be case insensitive, because of frequent errors like The the apple.
probably you want to restrict search only to word constituent to not match something like ( ( a + b) + c ) (repeated opening parentheses.
only full words should match to eliminate the thesis
When it comes to human language Unicode characters inside words should properly interpreted

All in all I recommend pcregrep solution:

pcregrep -Min --color=auto '\b([^[:space:]]+)[[:space:]]+\1\b' file

Obviously color and line number (n option) is optional, but usually nice to have.

Install

On Debian-based distributions you can install via:

$ sudo apt-get install pcregrep

Example

Run the command on jefferson_typo.txt to see:

$ pcregrep -Min --color=auto '\b([^[:space:]]+)[[:space:]]+\1\b' jefferson_typo.txt
1:He has has refused his Assent to Laws, the most wholesome and necessary
3:He has forbidden his Governors to pass Laws of immediate and
and pressing importance, unless suspended in their operation till his
5:Assent should be be obtained; and when so suspended, he has utterly

The above is just a text capture, but on a color-supported terminal, matches are colorized:

has has
and
and
be be