When running scripts in bash, I have to write ./ in the beginning:
$ ./manage.py syncdb
If I don't, I get an error message:
$ manage.py syncdb
-bash: manage.py: command not found
What is the reason for this? I thought . is an alias for current folder, and therefore these two calls should be equivalent.
I also don't understand why I don't need ./ when running applications, such as:
user:/home/user$ cd /usr/bin
user:/usr/bin$ git
(which runs without ./)
Because on Unix, usually, the current directory is not in $PATH.
When you type a command the shell looks up a list of directories, as specified by the PATH variable. The current directory is not in that list.
The reason for not having the current directory on that list is security.
Let's say you're root and go into another user's directory and type sl instead of ls. If the current directory is in PATH, the shell will try to execute the sl program in that directory (since there is no other sl program). That sl program might be malicious.
It works with ./ because POSIX specifies that a command name that contain a / will be used as a filename directly, suppressing a search in $PATH. You could have used full path for the exact same effect, but ./ is shorter and easier to write.
EDIT
That sl part was just an example. The directories in PATH are searched sequentially and when a match is made that program is executed. So, depending on how PATH looks, typing a normal command may or may not be enough to run the program in the current directory.
When bash interprets the command line, it looks for commands in locations described in the environment variable $PATH. To see it type:
echo $PATH
You will have some paths separated by colons. As you will see the current path . is usually not in $PATH. So Bash cannot find your command if it is in the current directory. You can change it by having:
PATH=$PATH:.
This line adds the current directory in $PATH so you can do:
manage.py syncdb
It is not recommended as it has security issue, plus you can have weird behaviours, as . varies upon the directory you are in :)
Avoid:
PATH=.:$PATH
As you can “mask” some standard command and open the door to security breach :)
Just my two cents.
Your script, when in your home directory will not be found when the shell looks at the $PATH environment variable to find your script.
The ./ says 'look in the current directory for my script rather than looking at all the directories specified in $PATH'.
When you include the '.' you are essentially giving the "full path" to the executable bash script, so your shell does not need to check your PATH variable. Without the '.' your shell will look in your PATH variable (which you can see by running echo $PATH to see if the command you typed lives in any of the folders on your PATH. If it doesn't (as is the case with manage.py) it says it can't find the file. It is considered bad practice to include the current directory on your PATH, which is explained reasonably well here: http://www.faqs.org/faqs/unix-faq/faq/part2/section-13.html
On *nix, unlike Windows, the current directory is usually not in your $PATH variable. So the current directory is not searched when executing commands. You don't need ./ for running applications because these applications are in your $PATH; most likely they are in /bin or /usr/bin.
This question already has some awesome answers, but I wanted to add that, if your executable is on the PATH, and you get very different outputs when you run
./executable
to the ones you get if you run
executable
(let's say you run into error messages with the one and not the other), then the problem could be that you have two different versions of the executable on your machine: one on the path, and the other not.
Check this by running
which executable
and
whereis executable
It fixed my issues...I had three versions of the executable, only one of which was compiled correctly for the environment.
Rationale for the / POSIX PATH rule
The rule was mentioned at: Why do you need ./ (dot-slash) before executable or script name to run it in bash? but I would like to explain why I think that is a good design in more detail.
First, an explicit full version of the rule is:
if the path contains / (e.g. ./someprog, /bin/someprog, ./bin/someprog): CWD is used and PATH isn't
if the path does not contain / (e.g. someprog): PATH is used and CWD isn't
Now, suppose that running:
someprog
would search:
relative to CWD first
relative to PATH after
Then, if you wanted to run /bin/someprog from your distro, and you did:
someprog
it would sometimes work, but others it would fail, because you might be in a directory that contains another unrelated someprog program.
Therefore, you would soon learn that this is not reliable, and you would end up always using absolute paths when you want to use PATH, therefore defeating the purpose of PATH.
This is also why having relative paths in your PATH is a really bad idea. I'm looking at you, node_modules/bin.
Conversely, suppose that running:
./someprog
Would search:
relative to PATH first
relative to CWD after
Then, if you just downloaded a script someprog from a git repository and wanted to run it from CWD, you would never be sure that this is the actual program that would run, because maybe your distro has a:
/bin/someprog
which is in you PATH from some package you installed after drinking too much after Christmas last year.
Therefore, once again, you would be forced to always run local scripts relative to CWD with full paths to know what you are running:
"$(pwd)/someprog"
which would be extremely annoying as well.
Another rule that you might be tempted to come up with would be:
relative paths use only PATH, absolute paths only CWD
but once again this forces users to always use absolute paths for non-PATH scripts with "$(pwd)/someprog".
The / path search rule offers a simple to remember solution to the about problem:
slash: don't use PATH
no slash: only use PATH
which makes it super easy to always know what you are running, by relying on the fact that files in the current directory can be expressed either as ./somefile or somefile, and so it gives special meaning to one of them.
Sometimes, is slightly annoying that you cannot search for some/prog relative to PATH, but I don't see a saner solution to this.
When the script is not in the Path its required to do so. For more info read http://www.tldp.org/LDP/Bash-Beginners-Guide/html/sect_02_01.html
All has great answer on the question, and yes this is only applicable when running it on the current directory not unless you include the absolute path. See my samples below.
Also, the (dot-slash) made sense to me when I've the command on the child folder tmp2 (/tmp/tmp2) and it uses (double dot-slash).
SAMPLE:
[fifiip-172-31-17-12 tmp]$ ./StackO.sh
Hello Stack Overflow
[fifi#ip-172-31-17-12 tmp]$ /tmp/StackO.sh
Hello Stack Overflow
[fifi#ip-172-31-17-12 tmp]$ mkdir tmp2
[fifi#ip-172-31-17-12 tmp]$ cd tmp2/
[fifi#ip-172-31-17-12 tmp2]$ ../StackO.sh
Hello Stack Overflow
This question already has answers here:
Why do you need ./ (dot-slash) before executable or script name to run it in bash?
(9 answers)
Closed 2 years ago.
In ubuntu scripts can be executed with following commands:
$ chmod +x manage.py
$ manage.py
However in mac you need to use ./ in order to actually run the script, as follow:
$ chmod +x manage.py
$ ./manage.py
I would like to know what is exactly ./ (especially that both system use bash by default) and if there is a way to run scripts directly in mac?
It's because you (very sensibly) don't have . in your PATH environment variable. If you do, it becomes an attack vector for people to get you to execute their own code instead of real stuff.
For example, let's say your path is:
.:/usr/bin
so that commands will first be searched for in your current directory, then in /usr/bin.
Then another user creates an executable script file ls in their home directory which changes to your home directory and deletes all your files. Then they tell you they've got something interesting in their home directory. You run ls to see what they have, and your files are deleted. All because it ran ls from your current directory first.
This is a particular favorite attack vector against naive system admins.
To be honest, on my home machines, I don't worry too much, since I'm the only user and I'm not prone to downloading stuff I don't trust. So I usually add . to my path for convenience, but usually at the end so it doesn't get in the way of my more regular commands.
When you are executing a command that file (script/binary) needs to be found by the system. That is done by putting directories where to look for scripts into the PATH environment variable. So if it works in ubuntu it means PATH includes '.' (the current directory). If you want the same behavior on mac then put something like export PATH="$PATH:." in your .bashrc (asuming you are using bash..)
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.
In order for me to run a .exe or a .sh in Cyqwin, I have to put a ./ at the beginning of my line. Is there a way for me to change this so that isn't necessary? This is causing problems when I try and run a test script and it can't find files that are right in the directory I'm working in.
The reason you must use a './' is to specifically tell the shell what file you are trying to execute. Without the leading './', your $PATH environment variable is searched. You can try adding the directories with scripts and executables you commonly use to your $PATH if you'd like. Alternatively, you can add the current directory (.) to your $PATH, but this is a Very Bad Idea as it can lead to unintentional executions.
From the cygwin environment variables doc:
The PATH environment variable is used by Cygwin applications as a list
of directories to search for executable files to run. This environment
variable is converted from Windows format (e.g.
C:\Windows\system32;C:\Windows) to UNIX format (e.g.,
/cygdrive/c/Windows/system32:/cygdrive/c/Windows) when a Cygwin
process first starts. Set it so that it contains at least the
x:\cygwin\bin directory where "x:\cygwin is the "root" of your cygwin
installation if you wish to use cygwin tools outside of bash.
I did try to find the answer online but unfortunately came up empty handed. Searching for './' in combination with other keywords brings up many hits, but none that helped...
Anyhow, as you can probably tell, I am rather new to MacOs. I am running mongo from the shell.
I cd into the /bin folder and start the mongo daemon with:
'./mongod'
.
If I just enter 'mongod', I get the following error:
'-bash: mongod: command not found'
What does the
'./'
in './mongod' stand for? Why is it needed? Why can't I just execute mongo by typing mongod. After all, I am in the correct directory.
After all, I am in the correct directory
But that directory isn't in the PATH. The gist of the matter is that when the shell wants to execute something it looks into a list of directories (specified in a variable called PATH). And the current directory isn't in that list.
As explained in the answer by cnicutar the shell tries to match the command you enter with executables in the directories that are listed in the PATH environment variable.
Although you could add the current directory to the PATH, it's probably not a good idea to do it. This article gives some good reasons why Linux doesn't add it by default:
Its a measure to plug possible security holes. You can't run commands
in the current directory without specifically calling them.
ie ./myscript.sh
To stop nasties writing a vicious shell script, placing the script in
an innocent location such as /tmp and getting root to run the script.
Would root run a nefarious script? If the script was named 'ls' and
root did a listing, the script would automatically run.