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 have a the following command in a file called $stat_val_result_command.
I want to add -Xms1g parameter at the end of the file so that is should look like this:
<my command in the file> -Xms1g
However, I want to run this command after append. I am running this in a workflow system called "nextflow". I tied many things, including following, but it does not working. check the script section which runs in Bash by default:
process statisticalValidation {
input:
file stat_val_result_command from validation_results_command.flatten()
output:
file "*_${params.ticket}_statistical_validation.txt" into validation_results
script:
"""
echo " -Xms1g" >> $stat_val_result_command && ```cat $stat_val_result_command```
"""
}
Best to avoid appending to or manipulating input files localized in the workdir as these can be, and are by default, symbolic links to the original files.
In your case, consider instead exporting the JAVA_TOOL_OPTIONS environment variable. This might or might not work for you, but might give you some ideas if you have control over how the scripts are being generated:
export JAVA_TOOL_OPTIONS="-Xms1g"
bash "${stat_val_result_command}"
Also, it's generally better to avoid localizing and running scripts like this. It might be unavoidable, but usually there are better options. For example, third-party scripts, like your Bash script could be handled more simply:
Grant the execute permission to these files and copy them into a
folder named bin/ in the root directory of your project repository.
Nextflow will automatically add this folder to the PATH environment
variable, and the scripts will automatically be accessible in your
pipeline without the need to specify an absolute path to invoke them.
This of course assumes you can control and parameterize the process that creates your Bash scripts.
I am writing a posix compliment shell script that will, amongst other things, clone a git repository and then execute a script (that was cloned along with the repository) inside the repository.
For example:
git clone git#github.com:torvalds/linux.git
cd linux
./Kconfig
The idea would be that people would use it for good, not evil, but you know.... So really I would like stop people from doing putting a line like:
rm -rf /
Inside the script.
Or perhaps something slightly less evil like:
rm -rf ../../
Is it possible for me to somehow change the permissions of the script (after the clone) so that it is only able to modify things inside the cloned repository?
Basically the answer for your question is the chroot command, which allows you to lock in processes in a directory as if it was the root directory. chroot requires root privileges to setup, but there are alternative implementations such has schroot, fakechroot, or proot that don't. Because all file system access (also read) is restricted, you will need to hand in anything that the scripts need to function into the chrooted environment. How to do that conveniently depends on your distribution.
That doesn't necessarily mean it is perfectly secure, because it provides only file system isolation.
I feel like I'm missing something very basic so apologies if this question is obtuse. I've been struggling with this problem for as long as I've been using the bash shell.
Say I have a structure like this:
├──bin
├──command (executable)
This will execute:
$ bin/command
then I symlink bin/command to the project root
$ ln -s bin/command c
like so
├──c (symlink to bin/command)
├──bin
├──command (executable)
I can't do the following (errors with -bash: c: command not found)
$ c
I must do?
$ ./c
What's going on here? — is it possible to execute a command from the current directory without preceding it with ./ and also without using a system wide alias? It would be very convenient for distributed executables and utility scripts to give them one letter folder specific shortcuts on a per project basis.
It's not a matter of bash not allowing execution from the current directory, but rather, you haven't added the current directory to your list of directories to execute from.
export PATH=".:$PATH"
$ c
$
This can be a security risk, however, because if the directory contains files which you don't trust or know where they came from, a file existing in the currently directory could be confused with a system command.
For example, say the current directory is called "foo" and your colleague asks you to go into "foo" and set the permissions of "bar" to 755. As root, you run "chmod foo 755"
You assume chmod really is chmod, but if there is a file named chmod in the current directory and your colleague put it there, chmod is really a program he wrote and you are running it as root. Perhaps "chmod" resets the root password on the box or something else dangerous.
Therefore, the standard is to limit command executions which don't specify a directory to a set of explicitly trusted directories.
Beware that the accepted answer introduces a serious vulnerability!
You might add the current directory to your PATH but not at the beginning of it. That would be a very risky setting.
There are still possible vulnerabilities when the current directory is at the end but far less so this is what I would suggest:
PATH="$PATH":.
Here, the current directory is only searched after every directory already present in the PATH is explored so the risk to have an existing command overloaded by an hostile one is no more present. There is still a risk for an uninstalled command or a typo to be exploited, but it is much lower. Just make sure the dot is always at the end of the PATH when you add new directories in it.
You could add . to your PATH. (See kamituel's answer for details)
Also there is ~/.local/bin for user specific binaries on many distros.
What you can do is add the current dir (.) to the $PATH:
export PATH=.:$PATH
But this can pose a security issue, so be aware of that. See this ServerFault answer on why it's not so good idea, especially for the root account.
Should be an easy question for the gurus here, though it's hard to explain it in text so hopefully this is clear. I've got two directories on a box with some flavor of unix on it. I've got a script that I want to use to move all the files and directories from one location to another.
First, an example of how the directories look:
Directory A: final/results/2012/2012-02/2012-02-25/name/files
Directory B: test/results/2012/2012-02/2012-02-24/name/files
So you see they're very similar. What I want to do is move everything from the Directory B 2012 directory, recursively, to the same level of Directory A. So you'd end up with:
someproject/results/2012/2012-02/2012-02-25/name/files
someproject/results/2012/2012-02/2012-02-24/name/files
etc.
I want this script to be future proof though, meaning I don't want the 2012 hardcoded. Also, towards the end of a month you will potentially have data from two different months and both need to be copied into the 2012 directory. So here is the command I used in the shell script file:
CONS="/someproject";
ROOT="/test";
/bin/cp -r ${ROOT}/results/* ${CONS}/results/*
but this resulted in:
/final/results/2012/2012-02/2012-02-25/name/files
and
/final/results/2012/2012/2012-02/2012-02-24/name/files
So as I hope is clear, it started a level below where I wanted it too. Can anyone fill me in on what I'm doing wrong, if they can understand what I'm even trying to explain. My apologies if it's not clear. I'm sure this is a fairly simple fix but I'm not sure what to do. Shell scripting is not a strong point of mine.
One poster suggests rsync, which is overkill.
cp -rp will work fine. if you want to move the files, just mv the directory -- it and everything under it will move too.
The only real problem here is the use of terminating *'s in the command line in the original script. You don't need the *, you're just trying to pass directories to the cp command, you aren't trying to pass it the names of all the files already in the source (and more importantly, the destination).
You could also use a tool like rsync to make sure your source and target are synchronized.
rsync -av ${ROOT}/results/ ${CONS}/results/
You specified that you want to "move" the files, though. Which means deleting the originals after they're copied:
rsync -av --remove-source-files ${ROOT}/results/ ${CONS}/results/
If you start playing around with rsync, be sure to read the man page about how it treats trailing slashes.