Basically I am trying to get list of programs in Linux which are installed and can open particular file extension .jpg for example. If not all, At-least default program should get listed.
Linux (the kernel) has no knowledge on file types to application mapping. If you want to use Gnome programs you can look at https://people.gnome.org/~shaunm/admin-guide/mimetypes-7.html. For KDE there is another mechanism. Each toolkit can define it as it likes. And the programmer can use the defaults or not. So it is simply application specific!
What do you want to achieve?
If you (double) click with a explorer/browser application on an icon or file name, exactly the explorer/browser looks for the file type. Typically this is realized via mime type dictionary. But how a program looks for the file type and maybe execute another program is only related to the programmer who writes that program. The GUI tool-chains like Gnome and KDE have a lot of support for that topic and so you have basic conformity for each family of applications.
If you want to know how a application do the job, start it with strace. But it is quite hard to dig into the huge amount of data.
Also you can take a look for xdg-open. Many programs use this helper to start applications. As an example: If you start Dolphin with strace you will find a line like lstat64("/etc/xdg", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0 after clicking on a file.
you can run from command line with:
xdg-open <file-name>
You maybe also want to have a look for applications which registers for file types: /usr/share/applications/*.desktop
Here you can find in each desktop file some mime-types which are registered for the applications. E.g. for audiacity is:
MimeType=application/x-audacity-project;audio/flac;audio/x-flac;audio/basic;audio/x-aiff;audio/x-wav;application/ogg;audio/x-vorbis+ogg;
For your example with jpg:
$ xdg-mime query filetype <any-jpg-file>
image/jpeg
$ grep 'image/jpeg' -R /usr/share/applications/*
...
/usr/share/applications/mimeinfo.cache:image/jpeg2000=kde4-kolourpaint.desktop;gimp.desktop;
So you can see that gimp is one of the default applications for jpg
The place to start looking is at the mailcap (/etc/mailcap) and MIME-types, e.g., in /etc/mime.types in Debian (the filename and path will vary according to who provides it).
The mailcap file gives some rules for opening a file, while MIME-types lists the known filetypes with a tag that allows multiple applications to know about the file types.
Except for embedded or reduced-functionality systems (such as those based on busybox), you would find these files on almost every UNIX-like system.
Related
With more and more programs installed on my computer, I am tired of seeing lots of dotfiles while I have to access them often. For some reason I won't hide dotfiles when browsing files. Is there a way to move them to a better place I want them to stay (e.g. ~/.config/$PROGCONF) without affecting programs while running?
Symlinks still leave file symbols, which is far from my expectation. I expect that operations like listdirs() won't show the files while opening them uses a redirection.
"For some reason it won't hide dotfiles when browsing files.":
That depends on the file manager you use. nautilus hides it by default and most file managers have an option to "show/hide hidden files". The ls command by default omits out hidden files (files starting with a dot). It lists all files with the option -a.
"Is there a way to move them to a better place":
Programs which have support for "XDG user directories" can store their config files in `~/.config/$PROGRAM_NAME/. If the program doesn't support that and expects the config file to be present in the home directory, there is little you can do (Maybe you can give us a list of what programs' config files you want to move). The process differs for each program.
Let me give an example with vim. Its config file is ~/.vimrc. Lets say you move the file to ~/.config/vim/.vimrc. You can make vim read the file by launching vim using the following command.
vim -u ~/.config/vim/.vimrc
You can modify the .desktop entry or create a new shell script to launch vim using the above command and put it inside /usr/local/bin/ or create shell functions / aliases. You can read more about changing vim's config file location in this SO question.
This arch wiki article has application specific information.
"without affecting programs while running":
It depends on a few factors namely the file system used, the program we are dealing with and so on.
Generally, deleting / moving files only unlinks the file name from an inode and programs read / write files using inodes. Read more here. And most programs read the config file at the start, load the values into memory. They rarely read the config files again. So, if you move your config file while the program is running (assuming the program supports config in both places), you won't see a difference until the program is restarted.
"I expect that operations like listdirs() won't show the files"
I am assuming you are talking about os.listdir() in python. If files are present, os.listdir() will list them, there is little you can change about that. But you can write custom functions to omit out the hidden files from being listed.
This SO question can help with that.
Why is bash(.sh) script not executable by default.
I agree that while we touch any file in linux it is created for reading purpose.
But since file name extensions such as sh and csh are for execution purpose.
Won't it be ideal to touch them in an executable mode.
Question might sound redundant to, but i still thought of asking it :)
Ultimately the answer to this question is that that's not what touch was designed to do.
In fact, touch was not even designed to create files; its primary purpose in life is to change file timestamps. It is merely a side effect of that purpose, and the fact that the designers decided to be a little more generous and create the target file if it doesn't already exist (and if you didn't provide the -c option), that it allows you to create files at all.
It should also be mentioned that there are other techniques that create files, such as redirection (echo 'echo '\''I am a script.'\'';' >|script.sh;). The act of file creation is a generic one, and the whole concept of a file is a generic one. A file is just a byte stream; what goes in the byte stream is unspecified at the file abstraction layer. As #AdamGent mentioned, Windows requires certain types of executable files to have certain extensions in order to be executed properly, but even in Windows, you can put executable code in non-executable-extensioned files, and you can put non-executable content in executable-extensioned files. There's no enforcement of file name / file content correspondence at the file layer.
All of that being said, it would often be a convenience if you could easily create a script in Unix that automatically has executable permission set. I've actually written a script to allow me to edit a new file in vim, and then have its permissions set to executable after write-quitting. The reason this potential convenience has not been standardized into a utility is likely related to the concern about security; you don't want people to accidentally make files executable, because that raises the risk of security holes.
You can always write your own program to create a file and make it executable, perhaps based on the extension of the file name.
Another thing that can be added here is that even shell scripts don't always need to be executable. For example, if you write a script that is only intended to be sourced from existing shell processes (via the source or classic . builtins), then the script does not need to be executable at all. Thus, there are cases where the file extension itself does not provide enough information to determine what the appropriate permissions are for the file.
There is nothing in the file name that says a file is even a script. Common practice perhaps says that .sh and .csh are scripts but there is no such rule.
What makes a file an executable script is the magic number in the beginning of the file. The magic number #! (the shebang, which has many other names as well) means the file is a script. For example
#!/bin/bash
When you try to execute the file (it must then also be set to executable in the permissions), the Linux kernel looks at the first two bytes of the file and finds the magic number (#!). It then knows it is supposed to execute whatever comes after the Shebang with the name of the file as argument followed by the rest of the arguments passed.
If you type the following in the shell
blablabla.sh lol1 lol2
The shell recognizes that you are trying to execute something so it invokes the kernel
exec blablabla.sh lol1 lol2
The kernel finds the shebang, and it becomes
exec /bin/bash blablabla.sh lol1 lol2
By exec i mean one of the exec family system calls.
Other fun names for #! include sha-bang, hashbang, pound-bang, hash-exclam and hash-pling
Because the .sh script extension is completely arbitrary in Unix environments. Its only a convention. You can name your script whatever you like so long as it has an executable bit set. This is unlike Windows where I believe its required (.com, .exe, and I think .dll).
touch just changes the timestamp of the file. It again does not care what the file extension of the file is. In fact most tools in Unix do not care about file extension.
I use various 3rd party libraries to convert files on my Linux server. For instance, ImageMagick/convert for images, libreoffice3.5/convert-to for Microsoft Office documents, etc.
Is it possible that these applications require the pre-converted file to have the proper extension for the type of file? For instance, if the file was a png file, it would need to be called whatever.png and not just whatever.
Thank you
your question sounds general, and in general linux apps do not require extensions. bash will execute a .png file with shell commands happily and vi will open a text file called a.exe. extensions are in general not a unix/linux concept to begin with and . is just an allowed character in the file name.
this being said, some particular application may interpret or even require correct extensions.
Given a text file in ubuntu (or debian unix in general), how do I find out the file encoding of the file ? Can I run od or hexdump on it to fingerprint its encoding ? What should I be looking out for ?
There are many tools to do this. Try a web search for "detect encoding". Here are some of the tools I found:
The Internationalizations Classes for Unicode (ICU) are a great place to start. See especially their page on Character Set Detection.
Chardet is a Python module to guess the encoding
of a file. See chardet.feedparser.org
The *nix command-line tool file detects file types, but might also detect encodings if mentioned in the file (e.g. if there's a mime-type notation in
the file). See man file
Perl modules Encode::Detect and Encode::Guess .
Someone asked a similar question in StackOverflow. Search for the question, PHP: Detect encoding and make everything UTF-8. That's in the context of fetching files from the net and using PHP, but you could write a command-line PHP script.
Note well what the ICU page says about character set detection: "Character set detection is ..., at best, an imprecise operation using statistics and heuristics...." In my experience the problem domain makes a big difference in how easy or difficult the job is. Don't forget that it's possible that the octets in a file can be of ambiguous encoding, i.e. sensibly interpreted using multiple different encodings. They can also be of mixed encoding, i.e. different subsets of the octets make sense interpreted in different encodings. This is why there's not a single command-line tool I can recommend which always does the job.
If you have a single file and you just want to get it into a known encoding, my trick is to open the file with a text editor which can import using a bunch of different encodings, such as TextWrangler or OpenOffice.org. First, open the file and let the editor guess the encoding. Take a look at the result. If you aren't satisfied with it, guess an encoding, open the file with the editor specifying that encoding, and take a look at the result. Then save as a known encoding, e.g. UTF-16.
You can use enca. Enca is a small command line tool for encoding detection and convertion.
You can install it at debian / ubuntu by:
apt-get install enca
In order to use it, just call
enca FILENAME
Also see the manpage for more information.
On Gnome/KDE you can select in which application you want to open file (Right click on file -> Open With -> Other). Is it possible open file that way, but from console?
For example: you print " file.ext" and instead of opening in concrete application, there are that application selection window forced and then users chooses - starts selected program.
I tried to figure out that myself, but not found anything like that.
"edit file.ext" doesn't fits my needs, because it starts preferred application and you cannot choose which. And also on my desktop it says:
"Error: no "edit" mailcap rules found for type "image/jpeg"
So, am I able to forse that "open with" window from console? If yes, can you say how?
Both on windows and mac you can do such things.
//edit at 2009-02-10 14:17
Thank you very much for answers. Command will be used in program code, so unfortunately probably I would not be able to make some extra bash scripts.
For GNOME:
gnome-open <file>
For KDE:
kfmclient exec <file>
These commands should open up the <file> in the preferred application in GNOME or KDE respectively, although I don't have an installation of either to test on.
Take a look at man run-mailcap, you can change or add selected applications for each mimetype modifying the /etc/mailcap, ~/.mailcap files and some others.
Traditionally, on Unix systens (and therefore Linux, too), you start applications from the console (and not from a UI). The command line (or console) expects you to enter the name of the application and then the filename (plus some options).
This allows to use applications (or commands) in shell scripts.
On Windows, there is no real console (the DOS box is just a reminiscence of the dark ages of MS DOS). So the MS developers came up with the idea to have the OS treat anything as a command. If it's not a real command or application, the OS will determine the file type (by extension on Windows and by some header information on Mac). For each file type, there will be an associated application in a look up table.
This is why on Windows, it appears that you can enter the name of a file on the console and you will get the application to edit that file.
If you want a quick way to fix this in the Unix console, create a script called "open" or "o" and use the file command with the option --mime to identify the file type. You can then use a case statement to launch your favorite editor.
As for the error about "mailcap rules": There is a file called "mailcap" on Unix where you can define abstract "commands" (open, edit, view, print) for file types. See the mailcap man page.