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Команда source в Linux

Командная оболочка играет очень важную роль в работе семейства операционных систем Linux. Она используется не только пользователями для работы в терминале, но и программами, а также компонентами операционной системы для обмена данными между собой. Для этого применяются переменные окружения. Для перезагрузки переменных окружения из файла часто используется команда source.

Эта команда позволяет выполнить скрипт в текущем процессе оболочки bash. По умолчанию для выполнения каждого скрипта запускается отдельная оболочка bash, хранящая все его переменные и функции. После завершения скрипта всё это удаляется вместе с оболочкой. Команда source позволяет выполнить скрипт в текущем командном интерпретаторе, а это значит, что всё переменные и функции, добавленные в этом скрипте, будут доступны также и в оболочке после его завершения. Как вы уже поняли, в этой статье будет рассмотрена команда source linux.

Команда source linux

Синтаксис команды очень прост. Надо вызвать саму команду и передать ей путь к исполняемому файлу:

$ source путь_к_файлу аргументы

Никаких опций более не нужно. Если указан не абсолютный путь к файлу, а просто имя файла, то утилита будет искать исполняемый файл в текущей папке и директориях, указанных в переменной PATH. Давайте разберём несколько примеров работы с утилитой. Создаём скрипт, объявляющий переменную:

Затем загрузим переменную из этого файла:

Теперь можно попытаться вывести содержимое переменной и убедиться, что всё работает:

Однако, переменная есть только в текущем командном интерпретаторе, в других командных интерпретаторах её нет. Это отличие команды source от команды export, позволяющей экспортировать переменные окружения глобально.

Если выполняемому скрипту нужно передать параметры, можно это сделать, просто перечислив их после пути к файлу скрипта. Модифицируем наш скрипт, чтобы переменная бралась из первого параметра:

И снова выполняем:

source losstsource losst.ru

Аналогично работают и функции. Если объявить функцию в скрипте bash, а затем выполнить его с помощью команды source linux, то функция станет доступна в интерпретаторе:

#!/bin/bash
print_site() <
echo «losst.ru»
>

Теперь можно выполнить функцию print_site в терминале или любом другом скрипте:

Для тех, кто знаком с программированием на языке Си, можно провести аналогию с директивой #include, делающей доступными в текущем файле функции из других файлов. Если файл, имя которого передано как параметр команде, не существует, она вернёт код возврата 1 и завершится:

Вместо команды source можно использовать точку (.), однако здесь следует быть осторожными — между точкой и именем файла должен быть пробел для того, чтобы bash интерпретировал эту точку как отдельную команду, а не как часть имени файла:

Однако, нельзя писать .losstsource или ./losstsource, потому что обозначение ./ — это уже отсылка на текущую директорию, скрипт будет выполнен как обычно.

Выводы

В этой небольшой статье мы рассмотрели работу с командой source linux. Как видите, это очень простая и в то же время полезная команда, очень сильно облегчающая работу в терминале. Именно с её помощью работают виртуальные окружения Python и многие другие подсистемы.

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How to Install Software from Source Code… and Remove it Afterwards

Last updated August 17, 2020 By Sylvain Leroux 58 Comments

Brief: This detailed guide explains how to install a program from source code in Linux and how to remove the software installed from the source code.

One of the greatest strength of your Linux distribution is its package manager and the associated software repository. With them, you have all the necessary tools and resources to download and install new software on your computer in a completely automated manner.

But despite all their efforts, the package maintainers cannot handle each and every use cases. Nor can they package all the software available out there. So there are still situations where you will have to compile and install new software by yourself. As for me, the most common reason, by far, I have to compile some software is when I need to run a very specific version, or modify the source code by the use of some fancy compilation options.

If your needs belong to the latter category, chances are you already know what to do. But, for the vast majority of Linux users, compiling and installing software from the source code for the first time might look like an initiation ceremony: somewhat frightening; but with the promise of entering a new world of possibilities and a place of prestige in a privileged community.

A. Installing software from source code in Linux

And that’s exactly what we will do here. For the purpose of this article, let’s say I need to install NodeJS 8.1.1 on my system. That version exactly. A version which is not available from the Debian repository:

Now, installing NodeJs on Ubuntu or Debian is pretty simple if you do it with the package manager. But let’s do it via the source code.

Step 1: Getting the source code from GitHub

Like many open-source projects, the sources of NodeJS can be found on GitHub: https://github.com/nodejs/node

So, let’s go directly there.

If you’re not familiar with GitHub, git or any other version control system worth mentioning the repository contains the current source for the software, as well as a history of all the modifications made through the years to that software. Eventually up to the very first line written for that project. For the developers, keeping that history has many advantages. For us today, the main one is we will be able to get the sources from for the project as they were at any given point in time. More precisely, I will be able to get the sources as they were when the 8.1.1 version I want was released. Even if there were many modifications since then.

On GitHub, you can use the “branch” button to navigate between different versions of the software. “Branch” and “tags” are somewhat related concepts in Git. Basically, the developers create “branch” and “tags” to keep track of important events in the project history, like when they start working on a new feature or when they publish a release. I will not go into the details here, all you need to know is I’m looking for the version tagged “v8.1.1”

After having chosen on the “v8.1.1” tag, the page is refreshed, the most obvious change being the tag now appears as part of the URL. In addition, you will notice the file change date are different too. The source tree you are now seeing is the one that existed at the time the v8.1.1 tag was created. In some sense, you can think of a version control tool like git as a time travel machine, allowing you to go back and forth into a project history.

At this point, we can download the sources of NodeJS 8.1.1. You can’t miss the big blue button suggesting to download the ZIP archive of the project. As for me, I will download and extract the ZIP from the command line for the sake of the explanation. But if you prefer using a GUI tool, don’t hesitate to do that instead:

Downloading the ZIP archive works great. But if you want to do it “like a pro”, I would suggest using directly the git tool to download the sources. It is not complicated at all— and it will be a nice first contact with a tool you will often encounter:

By the way, if you have an issue, just consider the first part of this article as a general introduction. Later I have more detailed explanations for Debian- and RedHat-based distributions in order to help you troubleshoot common issues.

Anyway, whenever you downloaded the source using git or as a ZIP archive, you should now have exactly the same source files in the current directory:

Step 2: Understanding the Build System of the program

We usually talk about “compiling the sources”, but the compilation is only one of the phases required to produce a working software from its source. A build system is a set of tool and practices used to automate and articulate those different tasks in order to build entirely the software just by issuing few commands.

If the concept is simple, the reality is somewhat more complicated. Because different projects or programming language may have different requirements. Or because of the programmer’s tastes. Or the supported platforms. Or for historical reason. Or… or.. there is an almost endless list of reasons to choose or create another build system. All that to say there are many different solutions used out there.

NodeJS uses a GNU-style build system, it is a popular choice in the open source community and once again, a good way to start your journey.

Writing and tuning a build system is a pretty complex task, but for the “end user”, GNU-style build systems ease the task by using two tools: configure and make .

The configure file is a project-specific script that will check the destination system configuration and available feature in order to ensure the project can be built, eventually dealing with the specificities of the current platform.

An important part of a typical configure job is to build the Makefile . That is the file containing the instructions required to effectively build the project.

The make tool, on the other hand, is a POSIX tool available on any Unix-like system. It will read the project-specific Makefile and perform the required operations to build and install your program.

But, as always in the Linux world, you still have some leniency in customising the build to your specific needs .

The configure -help command will show you all the available configuration options. Once again, this is very project-specific. And to be honest, it is sometimes necessary to dig into the project before fully understanding the meaning of each and every configure option.

But there is at least one standard GNU Autotools option that you must know: the —prefix option. This has to do with the file system hierarchy and the place your software will be installed.

Step 3: The FHS

The Linux file system hierarchy on a typical distribution mostly comply with the Filesystem Hierarchy Standard (FHS)

That standard explains the purpose of the various directories of your system: /usr , /tmp , /var and so on.

When using the GNU Autotools— and most other build systems— the default installation location for your new software will be /usr/local . Which is a good choice as according to the FSH “The /usr/local hierarchy is for use by the system administrator when installing software locally? It needs to be safe from being overwritten when the system software is updated. It may be used for programs and data that are shareable amongst a group of hosts, but not found in /usr.”

The /usr/local hierarchy somehow replicates the root directory, and you will find there /usr/local/bin for the executable programs, /usr/local/lib for the libraries, /usr/local/share for architecture independent files and so on.

The only issue when using the /usr/local tree for custom software installation is the files for all your software will be mixed there. Especially, after having installed a couple of software, it will be hard to track to which file exactly of /usr/local/bin and /usr/local/lib belongs to which software. That will not cause any issue to the system though. After all, /usr/bin is just about the same mess. But that will become an issue the day you will want to remove a manually installed software.

To solve that issue, I usually prefer installing custom software in the /opt sub-tree instead. Once again, to quote the FHS:

_”/opt is reserved for the installation of add-on application software packages.

A package to be installed in /opt must locate its static files in a separate /opt/

directory tree, where

is a name that describes the software package and

is the provider’s LANANA registered name.”_

So we will create a sub-directory of /opt specifically for our custom NodeJS installation. And if someday I want to remove that software, I will simply have to remove that directory:

Anything but “ok” after the make command has completed would mean there was an error during the build process. As we ran a parallel build because of the -j option, it is not always easy to retrieve the error message given the large volume of output produced by the build system.

In the case of an issue, just restart make , but without the -j option this time. And the error should appear near the end of the output:

Finally, once the compilation has gone to the end, you can install your software to its location by running the command:

B. What if things go wrong while installing from source code?

What I’ve explained above is mostly what you can see on the “build instruction” page of a well-documented project. But given this article goal is to let you compile your first software from sources, it might worth taking the time to investigate some common issues. So, I will do the whole procedure again, but this time from a fresh and minimal Debian 9.0 and CentOS 7.0 systems so you can see the errors I encountered and how I solved them.

From Debian 9.0 “Stretch”

This problem is quite easy to diagnosis and solve. Just install the git package:

No problem here.

Obviously, to compile a project, you need a compiler. NodeJS being written using the C++ language, we need a C++ compiler. Here I will install `g++`, the GNU C++ compiler for that purpose:

One other missing tool. Same symptoms. Same solution:

Please notice: I’ve installed the various tools one by one to show how to diagnosis the compilation issues and to show you the typical solution to solve those issues. But if you search for more information about the topic or read other tutorials, you will discover that most distributions have “meta-packages” acting as an umbrella to install some or all the typical tools used for compiling a software. On Debian-based systems, you will probably encounter the build-essentials package for that purpose. And on Red-Hat-based distributions, that will be the “Development Tools” group.

From CentOS 7.0

Command not found? Just install it using the yum package manager:

You guess it: NodeJS is written using the C++ language, but my system lacks the corresponding compiler. Yum to the rescue. As I’m not a regular CentOS user, I actually had to search on the Internet the exact name of the package containing the g++ compiler. Leading me to that page: https://superuser.com/questions/590808/yum-install-gcc-g-doesnt-work-anymore-in-centos-6-4

C. Making changes to the software installed from source code

You may install software from the source because you need a very specific version not available in your distribution repository, or because you want to modify the program to fix a bug or add a feature. After all, open-source is all about making modifications. So, I will take this opportunity to give you a taste of the power you have at hand now that you are able to compile your own software.

Here, we will make a minor change to the sources of NodeJS. And we will see if our change will be incorporated into the compiled version of the software:

Open the file node/src/node.cc in your favorite text editor (vim, nano, gedit, … ). And try to locate that fragment of code:

It is around line 3830 of the file. Then modify the line containing printf to match that one instead:

Then head back to your terminal. Before going further— and to give you some more insight of the power behind git— you can check if you’ve modified the right file:

You should see a “-” (minus sign) before the line as it was before you changed it. And a “+” (plus sign) before the line after your changes.

It is now time to recompile and re-install your software:

This times, the only reason it might fail is that you’ve made a typo while changing the code. If this is the case, re-open the node/src/node.cc file in your text editor and fix the mistake.

Once you’ve managed to compile and install that new modified NodeJS version, you will be able to check if your modifications were actually incorporated into the software:

Congratulations! You’ve made your first change to an open-source program!

D. Let the shell locate our custom build software

You may have noticed that I’ve always launched my newly compiled NodeJS software by specifying the absolute path to the binary file.

It works. But this is annoying, to say the least. There are actually two common ways of fixing that.

There are actually two common ways of fixing the annoying problem of specifying the absolute path to the binary files,
but to understand them you must first know that your shell locates the executable files by looking for them only in the directories specified by the PATH environment variable.

Here, on that Debian system, if you do not specify explicitly any directory as part of a command name, the shell will first look for the executable programs in /usr/local/bin , then if not found into /usr/bin , then if not found into /bin then if not found into /usr/local/games then if not found into /usr/games , then if not found … the shell will report an error “command not found”.

Given that, we have two way to make a command accessible to the shell: by adding it to one of the already configured PATH directories. Or by adding the directory containing our executable file to the PATH .

Adding a link from /usr/local/bin

Just copying the node binary executable from /opt/node/bin to /usr/local/bin would be a bad idea since by doing so, the executable program would no longer be able to locate the other required components belonging to /opt/node/ (it’s a common practice for software to locate its resource files relative to its own location).

So, the traditional way of doing that is by using a symbolic link:

This is a simple and effective solution, especially if a software package is made of just few well known executable programs— since you have to create a symbolic link for each and every user-invokable command. For example, if you’re familiar with NodeJS, you know the npm companion application I should symlink from /usr/local/bin too. But I let that to you as an exercise.

Modifying the PATH

First, if you tried the preceding solution, remove the node symbolic link created previously to start from a clear state:

And now, here is the magic command to change your PATH :

Simply said, I replaced the content of the PATH environment variable by its previous content, but prefixed by /opt/node/bin . So, as you can imagine it now, the shell will look first into the /opt/node/bin directory for executable programs. We can confirm that using the which command:

Whereas the “link” solution is permanent as soon as you’ve created the symbolic link into /usr/local/bin , the PATH change is effective only into the current shell. I will leave you to do some research on how to make changes in the PATH permanents. As a hint, it has to do with your “profile”. If you find the solution, don’t hesitate to share that with the other readers by using the comment section below!

E. How to remove that newly installed software from source code

Since our custom compiled NodeJS software sits completely in the /opt/node-v8.1.1 directory, removing that software requires no more effort than using the rm command to remove that directory:

BEWARE: sudo and rm -rf are a dangerous cocktail! Always check your command twice before pressing the “enter” key. You won’t have any confirmation message and no undelete if you remove the wrong directory…

Then, if you’ve modified your PATH , you will have to revert those changes, which is not complicated at all.

And if you’ve created links from /usr/local/bin you will have to remove them all:

Wait? Where was the Dependency Hell?

As a final comment, if you read about compiling your own custom software, you might have heard about the dependency hell. This is a nickname for that annoying situation where before being able to successfully compile a software, you must first compile a pre-requisite library, which in its turn requires another library that might in its turn be incompatible with some other software you’ve already installed.

Part of the job of the package maintainers of your distribution is to actually resolve that dependency hell and to ensure the various software of your system are using compatible libraries and are installed in the right order.

For this article, I chose, on purpose, to install NodeJS as it virtually doesn’t have dependencies. I said “virtually” because, in fact, it has dependencies. But the source code of those dependencies are present in the source repository of the project (in the node/deps subdirectory), so you don’t have to download and install them manually before hand.

But if you’re interested in understanding more about that problem and learn how to deal with it, let me know that using the comment section below: that would be a great topic for a more advanced article!

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