Small Things for Linux

1. alternatives manage multi-version softwares

I have a gcc-13 and a gcc-14 in /usr/bin. Here I use alternatives to manage them.

sudo update-alternatives --install /bin/gcc gcc /usr/bin/gcc-13 100
sudo update-alternatives --install /bin/gcc gcc /usr/bin/gcc-14 200

The command will create a link /bin/gcc which is linked to /etc/alternatives/gcc. /etc/alternatives/gcc is linked to /usr/bin/gcc-13 or /usr/bin/gcc-14. Use sudo update-alternatives --config gcc can change /etc/alternatives/gcc is linked to /usr/bin/gcc-13 or /usr/bin/gcc-14.

Let g++ change with gcc,

sudo update-alternatives --install /bin/gcc gcc /usr/bin/gcc-13 100 --slave /bin/g++ g++ /usr/bin/g++-13
sudo update-alternatives --install /bin/gcc gcc /usr/bin/gcc-14 200 --slave /bin/g++ g++ /usr/bin/g++-14

2. sudo apt update error: “Release file is not yet valid” in WSL.

Run sudo hwclock --hctosys. This command gets the latest time from your Windows machine’s RTC and sets the system time to that.

3. Proxy for apt

/etc/apt is the host directory for apt.

/etc/apt/apt.conf.d/proxy.conf defines the proxy only for apt.

If one wants to define proxy globally, export http(s)_proxy should be used.

4. Shortcuts

I want to find out where the “kill” shortcut, ctrl C, defined. I use bind -P to check the shortcuts. However, do not find ctrl C. Thus, I learned something about the shortcuts on linux. ctrl C is a shortcut of stty. However, bind -P lists the shortcuts of readline.

There may be two kinds of shortcuts, in practice,

• stty shortcuts
• readline shortcuts

4.1. stty (set tty, set teletypewriter)

The difference between shell, terminal and tty

tty: Print the file name of the terminal connected to standard input.

stty: change or print terminal characteristics, change and print terminal line settings.

They are related to terminal.

• terminal = tty = text input/output environment
• console = physical terminal
• shell = command line interpreter

The input of terminal or tty will used by readline, but not always readline.

4.2. gun readline - get a line from a user with editing

4.2.1. Synopsis

#include <stdio.h>
#include <readline/readline.h>
#include <readline/history.h>

char *
readline (const char *prompt)

4.2.2. Description

readline will read a line from the terminal and return it, using prompt as a prompt.

If prompt is NULL or the empty string, no prompt is issued. The line returned is allocated with malloc; the caller must free it when finished. The line returned has the final newline removed, so only the text of the line remains.

4.2.3. Return Value

readline returns the text of the line read. A black line returns the empty string. If EOF is encountered while reading a line, and the line is empty, NULL is returned. If an EOF is read with a non-empty line, it is treated as a newline.

4.2.4. Initialization file

Readline is customized by putting commands in an initialization file (the inputrc file). The name of this file is taken from the value of the INPUTRC environment variable. If that variable is unset, the default is ~/.inputrc. If that file does not exist or cannot be read, the ultimate default is /etc/inputrc.

When a program which uses the readline library starts up, the init file is read, and the key bindings and variables are set.

There are only a few basic constructs allowed in the readline init file.

1. Black lines are ignored.
2. Lines beginning with a # are comments.
3. Lines beginning with a $ indicate conditional constructs.
4. Other lines denote key bindings and variable settings.

Each program using this library may add its own commands and bindings.

For example, placing

M-Control-u: universal-argment

into the inputrc would make M-C-u execute the readline command universal-argument.

5. The difference of help and man on linux

help displays information about builtin shell commands.

man is s system-wide documentation system that provides short reference manuals for individual commands, API functions, concepts, configuration file syntax, file formats. It is the traditional Unix documentation system.

We can get the information of cd by help cd, but not by man cd. We can get the information of git/nvim/stty by man git/nvim/stty, but not by help git/nvim/stty. cd is a shell command and git/nvim/stty is linux projects.