Linux terminal input: reading user input from terminal truncating lines at 4095 character limit

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Please refer to termios(3) manual page, under section “Canonical and noncanonical mode”.

Typically, the terminal (standard input) is in canonical mode; in this mode the kernel will buffer the input line before returning the input to the application. The hard-coded limit for Linux (N_TTY_BUF_SIZE defined in ${linux_source_path}/include/linux/tty.h) is set to 4096 allowing input of 4095 characters not counting the ending new line. You can also have a look at file ${linux_source_path}/drivers/tty/n_tty.c, function n_tty_receive_buf_common() and the comment above that.

In noncanonical mode there will by default be no buffering by kernel and the read(2) system call returns immediately once a single character of input is returned (key is pressed). You can manipulate the terminal settings to read a specified amount of characters or set a time-out for non-canonical mode, but then too the hard-coded limit is 4095 per the termios(3) manual page (and the comment above the above mentioned n_tty_receive_buf_common()).

Bash read builtin command still works in non-canonical mode as can be demonstrated by the following:

IFS=$'\n'      # Allow spaces and other white spaces.
stty -icanon   # Disable canonical mode.
read line      # Now we can read without inhibitions set by terminal.
stty icanon    # Re-enable canonical mode (assuming it was enabled to begin with).

After this modification of adding stty -icanon you can paste longer than 4096 character string and read it successfully using bash built-in read command (I successfully tried longer than 10000 characters).

If you put this in a file, i.e. make it a script, you can use strace to see the system calls called, and you will see read(2) called multiple times, each time returning a single character when you type input to it.

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