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diff --git a/doc/cd-after-exit.txt b/doc/cd-after-exit.txt new file mode 100644 index 00000000..18598cd3 --- /dev/null +++ b/doc/cd-after-exit.txt @@ -0,0 +1,132 @@ +The "cd-after-exit" Feature + +== Abstract + +This document explains the troublesome implementation of the "cd-after-exit" +feature. + + +== Specification + +When the feature is enabled, ranger will attempt to change the directory of +the parent shell (from which ranger is run) to the last visited directory +when ranger is exited. + +This task is, by its nature, shell dependent. As a bash or zsh user, +I focused on the implementation for those two shells and left the +addition of support for csh, ksh, and other shells to those who actually use +those shells. + + +== What's the problem? + +Shells have several limitations, the implementation could not be done easily +because: + +1. It is not possible to use something like system('cd xyz') at the end. +This command would run in a new shell and wouldn't change the directory +of the parent shell at all. + +2. Using exec('cd xyz') is not possible either, since 'cd' is a command +which is directly integrated in to the shell and can not be run this way. + + +== Redirection of streams + +The only way I found is using cd `program` from inside the shell to change +the directory to whatever `program` prints to the stdout: + + bash$ cd `echo ..` + +Since the user interface still has to be printed, we simply redirect it to +the stderr. It is not sufficient however to change sys.stdout to sys.stderr, +since curses seems not to be aware of sys.stdout and continues to print out +the interface to the actual stdout. + +So what I did was swap the stdout and stderr of the whole ranger process on +the shell command line by using: + + bash$ cd `ranger 3>&1 1>&2 2>&3 3>&-` + +Since errors are now printed to the stdout, we have do this in ranger: + sys.stderr = sys.__stdout__ + +And at the end, write the current directory to the stdout, which is now +reachable via sys.__stderr__ due to the redirections: + sys.__stderr__.write(last_visited_directory) + +To inform the ranger process about these changes, we add a --cd-after-exit +switch which: + bash$ cd `ranger --cd-after-exit 3>&1 1>&2 2>&3 3>&-` + + +== Argument passing + +This works well enough, but there are two remaining problems: + +1. How to pass arguments to ranger? + +2. How to memorize that line? Although you can just copy+paste it +into your bashrc and create an alias, the complexity of the line +could lead to errors. + +Both problems are solved by putting the command in a file: + +run.sh: + cd "`ranger --cd-after-exit \"$@\" 3>&1 1>&2 2>&3 3>&-`" + +The $@ is responsible for argument passing. By using the source command, +the file will be evaluated without creating a distinct new shell. + + bash$ source run.sh arg1 ... argN + +To add flexibility, replace the name "ranger" in the command to the first +argument. Now it requires you to pass the name of the ranger command to +the script as the first argument: + +run.sh: + RANGER="$1" + shift + cd "`$RANGER --cd-after-exit \"$@\" 3>&1 1>&2 2>&3 3>&-`" + + +== Put it in a nutshell + +I didn't want to have 2 files for the main program and wanted just one +file at /usr/bin/ranger. So I used this trick to merge both files into one: + + #!/usr/bin/python + """": + <shell code> + """ + <python code> + +If you run this file with python, or simply by typing ranger, the program will +run normally. If you, however, run this file by sourcing it into the shell, +like you did with run.sh, the cd-after-exit mode will be activated. + +Now the way of running ranger with the cd-after-exit feature is: + + bash$ source /path/to/ranger.py /path/to/ranger.py + +or, if properly installed: + + bash$ source ranger ranger + + +== Open issues + +Unfortunately there is some redundancy: you have to type the path to ranger +twice. I know of no way to fix this, because it is not possible to get the +filename of the file currently being sourced. + +Example: + + bash$ echo 'source sourced.sh' > main.sh + bash$ echo 'echo $0 $@' > sourced.sh + bash$ bash main.sh + main.sh + +If you find a way to make this print out 'sourced.sh', let me know. :) + +Dec 25, 2009 |