My book states:
Every program that runs on your computer has a current working directory, or cwd. Any filenames or paths that do not begin with the root folder are assumed to be under the current working directory
As I am on OSX, my root folder is /. When I type in os.getcwd() in my Python shell, I get /Users/apple/Documents. Why am I getting the Documents folder in my cwd? Is it saying that Python is using Documents folder? Isn't there any path heading to Python that begins with / (the root folder)? Also, does every program have a different cwd?
Every process has a current directory. When a process starts, it simply inherits the current directory from its parent process; and it's not, for example, set to the directory which contains the program you are running.
For a more detailed explanation, read on.
When disks became large enough that you did not want all your files in the same place, operating system vendors came up with a way to structure files in directories. So instead of saving everything in the same directory (or "folder" as beginners are now taught to call it) you could create new collections and other new collections inside of those (except in some early implementations directories could not contain other directories!)
Fundamentally, a directory is just a peculiar type of file, whose contents is a collection of other files, which can also include other directories.
On a primitive operating system, that was where the story ended. If you wanted to print a file called term_paper.txt which was in the directory spring_semester which in turn was in the directory 2021 which was in the directory studies in the directory mine, you would have to say
print mine/studies/2021/spring_semester/term_paper.txt
(except the command was probably something more arcane than print, and the directory separator might have been something crazy like square brackets and colons, or something;
lpr [mine:studies:2021:spring_semester]term_paper.txt
but this is unimportant for this exposition) and if you wanted to copy the file, you would have to spell out the whole enchilada twice:
copy mine/studies/2021/spring_semester/term_paper.txt mine/studies/2021/spring_semester/term_paper.backup
Then came the concept of a current working directory. What if you could say "from now on, until I say otherwise, all the files I am talking about will be in this particular directory". Thus was the cd command born (except on old systems like VMS it was called something clunkier, like SET DEFAULT).
cd mine/studies/2021/spring_semester
print term_paper.txt
copy term_paper.txt term_paper.backup
That's really all there is to it. When you cd (or, in Python, os.chdir()), you change your current working directory. It stays until you log out (or otherwise exit this process), or until you cd to a different working directory, or switch to a different process or window where you are running a separate command which has its own current working directory. Just like you can have your file browser (Explorer or Finder or Nautilus or whatever it's called) open with multiple windows in different directories, you can have multiple terminals open, and each one runs a shell which has its own independent current working directory.
So when you type pwd into a terminal (or cwd or whatever the command is called in your command language) the result will pretty much depend on what you happened to do in that window or process before, and probably depends on how you created that window or process. On many Unix-like systems, when you create a new terminal window with an associated shell process, it is originally opened in your home directory (/home/you on many Unix systems, /Users/you on a Mac, something more or less like C:\Users\you on recent Windows) though probably your terminal can be configured to open somewhere else (commonly Desktop or Documents inside your home directory on some ostensibly "modern" and "friendly" systems).
Many beginners have a vague and incomplete mental model of what happens when you run a program. Many will incessantly cd into whichever directory contains their script or program, and be genuinely scared and confused when you tell them that you don't have to. If frobozz is in /home/you/bin then you don't have to
cd /home/you/bin
./frobozz
because you can simply run it directly with
/home/you/bin/frobozz
and similarly if ls is in /bin you most definitely don't
cd /bin
./ls
just to get a directory listing.
Furthermore, like the ls (or on Windows, dir) example should readily convince you, any program you run will look in your current directory for files. Not the directory the program or script was saved in. Because if that were the case, ls could only produce a listing of the directory it's in (/bin) -- there is nothing special about the directory listing program, or the copy program, or the word processor program; they all, by design, look in the current working directory (though again, some GUI programs will start with e.g. your Documents directory as their current working directory, by design, at least if you don't tell them otherwise).
Many beginners write scripts which demand that the input and output files are in a particular directory inside a particular user's home directory, but this is just poor design; a well-written program will simply look in the current working directory for its input files unless instructed otherwise, and write output to the current directory (or perhaps create a new directory in the current directory for its output if it consists of multiple files).
Python, then, is no different from any other programs. If your current working directory is /Users/you/Documents when you run python then that directory is what os.getcwd() inside your Python script or interpreter will produce (unless you separately os.chdir() to a different directory during runtime; but again, this is probably unnecessary, and often a sign that a script was written by a beginner). And if your Python script accepts a file name parameter, it probably should simply get the operating system to open whatever the user passed in, which means relative file names are relative to the invoking user's current working directory.
python /home/you/bin/script.py file.txt
should simply open(sys.argv[1]) and fail with an error if file.txt does not exist in the current directory. Let's say that again; it doesn't look in /home/you/bin for file.txt -- unless of course that is also the current working directory of you, the invoking user, in which case of course you could simply write
python script.py file.txt
On a related note, many beginners needlessly try something like
with open(os.path.join(os.getcwd(), "input.txt")) as data:
...
which needlessly calls os.getcwd(). Why is it needless? If you have been following along, you know the answer already: the operating system will look for relative file names (like here, input.txt) in the current working directory anyway. So all you need is
with open("input.txt") as data:
...
One final remark. On Unix-like systems, all files are ultimately inside the root directory / which contains a number of other directories (and usually regular users are not allowed to write anything there, and system administrators with the privilege to do it typically don't want to). Every relative file name can be turned into an absolute file name by tracing the path from the root directory to the current directory. So if the file we want to access is in /home/you/Documents/file.txt it means that home is in the root directory, and contains you, which contains Documents, which contains file.txt. If your current working directory were /home you could refer to the same file by the relative path you/Documents/file.txt; and if your current directory was /home/you, the relative path to it would be Documents/file.txt (and if your current directory was /home/you/Music you could say ../Documents/file.txt but let's not take this example any further now).
Windows has a slightly different arrangement, with a number of drives with single-letter identifiers, each with its own root directory; so the root of the C: drive is C:\ and the root of the D: drive is D:\ etc. (and the directory separator is a backslash instead of a slash, although you can use a slash instead pretty much everywhere, which is often a good idea for preserving your sanity).
Your python interpreter location is based off of how you launched it, as well as subsequent actions taken after launching it like use of the os module to navigate your file system. Merely starting the interpreter will place you in the directory of your python installation (not the same on different operating systems). On the other hand, if you start by editing or running a file within a specific directory, your location will be the folder of the file you were editing. If you need to run the interpreter in a certain directory and you are using idle for example, it is easiest to start by creating a python file there one way or another and when you edit it you can start a shell with Run > Python Shell which will already be in that directory. If you are using the command line interpreter, navigate to the folder where you want to run your interpreter before running the python/python3/py command. If you need to navigate manually, you can of course use the following which has already been mentioned:
import os
os.chdir('full_path_to_your_directory')
This has nothing to do with osx in particular, it's more of a concept shared by all unix-based systems, and I believe Windows as well. os.getcwd() is the equivalent of the bash pwd command - it simply returns the full path of the current location in which you are in. In other words:
alex#suse:~> cd /
alex#suse:/> python
Python 2.7.12 (default, Jul 01 2016, 15:34:22) [GCC] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.getcwd()
'/'
It depends from where you started the python shell/script.
Python is usually (except if you are working with virtual environments) accessible from any of your directory. You can check the variables in your path and Python should be available. So the directory you get when you ask Python is the one in which you started Python. Change directory in your shell before starting Python and you will see you will it.
os.getcwd() has nothing to do with OSX in particular. It simply returns the directory/location of the source-file. If my source-file is on my desktop it would return C:\Users\Dave\Desktop\ or let say the source-file is saved on an external storage device it could return something like G:\Programs\. It is the same for both unix-based and Windows systems.
I'm currently practicing basic Shell Commands in WSL, Windows Subsystem for Linux (I do not have a linux system but I want to get familiar with commands).
I start a bash session on the command prompt window and navigate to my desktop using cd . In desktop I noticed that after using ls -lF some files with the prefix ~$ appear, such as: '~$executable.x'* or '~$file.txt'
These files are not currently present under the desktop directory, but I was able to remember that they were at one point (varying from a week to months ago).
When I do the same process in powershell windows (not using linux commands) I noticed that files displayed match the desktop and no extra files are listed.
I was wondering if anyone could explain what ~$ means in this context? my intuition is telling me they are backed up files that are somehow hidden in the desktop. After googling, all I could find is that ~ reefers to the home. I also understand that $ is the default prompt symbol for the bash shell when it is waiting for me to type something, but I'm still confused on why it would show up as a prefix for the name of a file.
Hope I made my question clear.
I'm currently reading "Linux® Command Line and Shell Scripting BIBLE" by Blum and Bresnahan but I could not find an answer there, this is my last resource after many googling attempts. Any other source for more information on the topic would be helpful.
On Windows, files that start with ~ are used for hidden files. More specifically,, the prefix ~$ are often used as backups for programs, should they crash before writing updates to a file (e.g. Microsoft Word, etc.)
From Wikipedia:
The tilde symbol is used to prefix hidden temporary files that are created when a document is opened in Windows. For example, when you open a Word document called “Document1.doc,” a file called “~$cument1.doc” is created in the same directory. This file contains information about which user has the file open, to prevent multiple users from attempting to change a document at the same time.
See: Why does Word make temporary files?
Relevant superuser question: https://superuser.com/questions/405257/what-type-of-file-is-file
I have a shell script, and a tarball. The shell script unpacks the tarball, and makes use of the files inside of for performing a task. I need to make this accessible on mac laptops, but in such a way that there is either a .app or .dmg file that when clicked, ultimately calls that shell script. I found several utilities, that can do this (create such an .app file), such as Platypus, or Appify. However, these require Mac to build the file. The thing is, I must package the .app/.dmg file, in an Ubuntu environment.
Is there any good software for creating a dmg or app file which call a shell script when clicked, but such that the software which can be run in Ubuntu (just for the purpose of creating the file)?
This is not an exact answer to your question but an workaround that might be acceptable if you can't find a better solution.
First, a zip file will automatically extract its content if you double click it in OS X so
tar -cvzf your_filename.zip ...
would create a file that can be easily extracted.
Secondly, if you create a shell script that has the extension .command, but otherwise is like any shell script, it can be run from OS X by double clicking on it (by opening a terminal and executing it there), it would mean an extra manual step for the user but like I said, this is a workaround :)
If you create a .command file, remember to make it executable.
I am currently following the guide to setting up and compiling AOSP in Ubuntu. The problem is I do not have the best knowledge of Linux/Ubuntu.
The part that currently has me confused is setting up the ccache found at this link https://source.android.com/source/initializing.html.
What I don't really understand is the following, which .bashrc file do I need to edit/add the information to? And can I have more than 1 .bashrc file in Ubuntu? Not really sure what this file really does.
Thanks
.bashrc is a file which is called by bash before on each start of a new shell.
This file can be used to setup the environment, export variables, create aliases and functions. There are usually multiple copy of this file, One per system and one per user to allow system wide configuration but also customization by users ( users bashrc will be called at last and can overwrite things).
here you edid or add line to your won user's .bashrc file
type cd $HOME
vi .bashrc
after that change your file and then you have to compile the file using one of the commands.
..bashrc
or
source .bashrc
Is it dangerous, insecure or not-so-smart for any other reason to put a custom shell script to /usr/bin and add /usr/bin to $PATH in order to make a custom script executable from everywhere without ./ and the file extension?
Bonus question: can I assign custom icons to custom executable scripts?
Traditionally, /usr/bin is one of the places where operating system binaries are stored. For custom scripts, you'd use /usr/local/bin. This you have to create yourself if it does not exist and add to $PATH, as you mentioned.
Icons are a GUI thing, shell scripts are a CLI thing. They live in separate universes. Nothing prevents you from creating a bridge though. For instance, you can make a shell script and call it foo.command. Opening this from the GUI starts Terminal and runs the script. Since you see the file in the Finder, you can assign it a new icon through the Info pane.
Also, you may want to take a look at the free Platypus application. It allows you to create a full-blown application bundle around a script. The bundle will contain the script, so you won't have to put it in some obscure directory and modify $PATH. If you also need CLI access, this option is less desirable.
I wrote a command line tool for setting the custom icon of a file. You can grab it here.