Haskell Implement Unix "touch" command - unix

I'm trying to implement the touch command from the unix command line, but it seems that my last line throws an exception: ** Exception: ~/.todo: openFile: does not exist (No such file or directory)
main = touch "~/.todo"
touch :: FilePath -> IO ()
touch name = do
exists <- doesFileExist name
unless exists $ appendFile name ""
If there is any OS specific behavior, I'm testing from macOS Sierra.
I feel as if this error is strange in that the documentation for openFile states that
If the file does not exist and it is opened for output, it should be created as a new file.
Any suggestions as to how to fix this?
Edit: According to #chi, the touch command should always open the file, even if it already exists, because it will then update the file's last modified date.
touch :: FilePath -> IO ()
touch name = appendFile name ""

Use touchFile from the unix package (System.Posix.Files.ByteString).
appendFile name "" does not work like touch; appendFile is a no-op when the string to append is empty.
You can confirm this by running stat on the file before and after and comparing the modification times.

In the future please paste all the code you are using that creates the error. This includes both the imports and the invocation. In your case it seems you are running something with a shell expansion character:
*Main> touch "~/foobar"
*** Exception: ~/foobar: openFile: does not exist (No such file or directory)
The ~ is typically expanded by a shell (there also exists a C library that can do that rewriting for you). Most languages actually interpret that as a literal part of the path... but the ~ directory probably doesn't exist or that symbol might not even be valid depending on your platform.
Instead try a valid file path:
*Main> touch "/tmp/thisfile"
*Main>
Leaving GHCi.
% ls -l /tmp/thisfile
-rw-rw-r--. 1 theuser theuser 0 Feb 3 12:51 /tmp/thisfile

Related

can you make make a function to handle permission denied on zsh like command not found

so I am using zsh. I have a bunch of functions that move me around the place - like if I type "bin" anywhere - I go to ~/bin etc. I do this by hooking into the command_not_found_handler as so:
command_not_found_handler() {
if [ -f ~/bin/marked/$1 ]; then
directory=$(<~/bin/marked/$1)"
echo cd \"$directory\" >~/source_function
return 0
...
and this works fantastically - anywhere I am, I can just type marker blah - it creates a marker, and from then on anywhere I am, if I type blah it will just go back to that directory I marked.
Except.
I have "." in my path. (Yes I know you think I shouldn't do that)
and if there happens to be a "blah" file in the current directory - instead of going to the command not found handler - it tries to execute that, and its of course not an executable script, so I get "Permission Denied"
Is there any way to trap this permission denied, like I trap the command not found? It really hits me a lot with the word "scripts" - because I like typing scripts to take me to my personal scripts directory - but every program I write also has a scripts directory in the git repo for scripts related to that repository.
Aside from removing . from your path (which you don't want to do), I don't see a way to configure zsh to avoid executing (or attempting to execute) files that match the given command in the current directory. zsh has lots of options, but I don't see documentation describing any relevant ones, nor do I see source code support for one.
I make this claim based on reading the source code for zsh's handling in the execute() function at https://sourceforge.net/p/zsh/code/ci/master/tree/Src/exec.c. Here, when zsh sees dot (.) in the path, it attempts to execute a file by that name in that directory:
for (pp = path; *pp; pp++)
if (!(*pp)[0] || ((*pp)[0] == '.' && !(*pp)[1])) {
ee = zexecve(arg0, argv, newenvp);
if (isgooderr(ee, *pp))
eno = ee;
} else {
z = buf;
strucpy(&z, *pp);
*z++ = '/';
strcpy(z, arg0);
ee = zexecve(buf, argv, newenvp);
if (isgooderr(ee, *pp))
eno = ee;
}
After that, the execute() function reaches the code below and calls zerr(), which produces the "permission denied" error message:
if (eno)
zerr("%e: %s", eno, arg0);
else if (commandnotfound(arg0, args) == 0)
_realexit();
else
zerr("command not found: %s", arg0);
... and there is no logic in the code to intercept zsh's behavior in that case.
My best suggestion to achieve the desired result is to remove dot (.) from your path.

Fortran90: Scripting of Standard In not working as expected

Working with Fortran90 in Unix...
I have a programme which needs to read in the input parameters from a file "input-deck.par". This filename is currently hard-coded but I want to run a number of runs using different input-deck files (input-deck01.par, input-deck02.par, input-deck03.par etc.) so I've set-up the code to do a simple "read(*,*) inpfile" to allow the user to input the name of this file directly on run-time with a view to scripting this later.
This works fine interactively. If I execute the programme it asks for the file name, you type it in and the filename is accepted, the file is opened and the programme picks up the parameters from that file.
The issue lies in scripting this. I've tried scripting using the "<" pipe command so:
myprog.e < input-deck01.par
But I get an error saying:
Fortran runtime error: Cannot open file '----------------': No such file or directory
If I print the filename right after the input line, it prints that the filename is '----------------' (I initialise the variable as having 16 characters hence the 16 hyphens I think)
It seems like the "<" pipe is not passing the keyboard input in correctly. I've tried with spaces and quotes around the filename (various combinations) but the errors are the same.
Can anyone help?
(Please be gentle- this is my first post on SO and Fortran is not my primary language....)
Fortran has the ability to read the command line arguments. A simple example is
program foo
implicit none
character(len=80) name
logical available
integer fd
if (command_argument_count() == 1) then
call get_command_argument(1, name)
else
call usage
end if
inquire(file=name, exist=available)
if (.not. available) then
call usage
end if
open(newunit=fd, file=name, status='old')
! Read file
contains
subroutine usage
write(*,'(A)') 'Usage: foo filename'
write(*,'(A)') ' filename --> file containing input info'
stop
end subroutine usage
end program foo
Instead of piping the file into the executable you simply do
% foo input.txt

Why can't I open a *.w file in the appBuilder?

I have a *.w file, referring to two include files ({incl\include_file.i}, {incl\do_something_file.i}). That first include-file contains the definition of a RECID variable "recordid":
DEF INPUT-OUTPUT PARAMETER recordid AS RECID.
I am capable to compile the *.w file, the listing file looks as follows: (just a fragment)
Prompt>findstr "recordid do_something" listing.txt
...
1 x DEF INPUT-OUTPUT PARAMETER recordid AS RECID.
...
1 x 1 {incl\do_something_file.i
2 x 1 INPUT-OUTPUT recordid
So, the compilation works. In top of that, I've checked the pairs of "&ANALYZE-SUSPEND" and "&ANALYZE-RESUME" clauses and everything is fine.
Nevertheless, I can't open the *.w file, as the mentioned RECID seems not to be known (errors 201 and 196).
Edit after first comments
This the exact error message I get while opening the *.w file, using the AppBuilder (I'm working with a Dutch version of the tool, hence the Dutch words in between):
---------------------------
Fout
---------------------------
This file cannot be analyzed by the AppBuilder.
Please check these problems in your file or environment:
** Onbekende veld- of variabelenaam - recordid. (201)
** .\incl\<do_something_file>.i Compilatiefout op regel 7. (196)
---------------------------
OK
---------------------------
Edit with more information on ANALYZE- clauses
I've launched following findstr command on my code with the following results:
Prompt>findstr /I "ANALYZE-RESUME ANALYZE-SUSPEND" <filename>.w
&ANALYZE-SUSPEND _VERSION-NUMBER ... GUI
&ANALYZE-RESUME
&ANALYZE-SUSPEND _UIB-CODE-BLOCK _CUSTOM _DEFINITIONS ...
&ANALYZE-RESUME
...
I confirm that the number of &ANALYZE-SUSPEND clauses equals the number of &ANALYZE-RESUME clauses, they are in the right sequence (first a SUSPEND and then a RESUME) and none of them is commented out.
Does anybody have an idea what's going wrong?
The problem was caused by an include, being outside of an suspend resume clause, in order to solve such a situation the following command might be useful:
findstr /I "ANALYZE {incl" <source_file>.w
The result should look like the following:
...
&ANALYZE-SUSPEND _UIB-CODE-BLOCK _CONTROL C-Win
&ANALYZE-RESUME
&ANALYZE-SUSPEND _UIB-CODE-BLOCK _CUSTOM _MAIN-BLOCK C-Win
{incl\something.i}
{incl\something_else.i}
&ANALYZE-RESUME
...
You see following rules:
The number of suspends and resumes must be equal.
Every suspend is to be closed by a resume.
Not one of those can be commented out.
It is advised to have includes between the suspend and the resume.

Unix SQLLDR scipt gives 'Unexpected End of File' error

All, I am running the following script to load the data on to the Oracle Server using unix box and sqlldr. Earlier it gave me an error saying sqlldr: command not found. I added "SQLPLUS < EOF", it still gives me an error for unexpected end of file syntax error on line 12 but it is only 11 line of code. What seems to be the problem according to you.
#!/bin/bash
FILES='ls *.txt'
CTL='/blah/blah1/blah2/name/filename.ctl'
for f in $FILES
do
cat $CTL | sed "s/:FILE/$f/g" >$f.ctl
sqlplus ID/'PASSWORD'#SERVERNAME << EOF sqlldr SCHEMA_NAME/SCHEMA_PASSWORD control=$f.ctl data=$f EOF
done
sqlplus will never know what to do with the command sqlldr. They are two complementary cmd-line utilities for interfacing with Oracle DB.
Note NO sqlplus or EOF etc required to load data into a schema:
#!/bin/bash
#you dont want this FILES='ls *.txt'
CTL_PATH=/blah/blah1/blah2/name/'
CTL_FILE="$CTL_PATH/filename.ctl"
SCHEMA_NM=SCHEMA_NAME
SCHEMA_PSWD=SCHEMA_PASSWORD
for f in *.txt
do
# don't need cat! cat $CTL | sed "s/:FILE/$f/g" >"$f".ctl
sed "s/:FILE/$f/g" "$CTL_FILE" > "$CTL_PATH/$f.ctl"
#myBad sqlldr "$SCHEMA_NAME/$SCHEMA_PASSWORD" control="$CTL_PATH/$f.ctl" data="$f"
sqlldr $SCHEMA_USER/$SCHEMA_PASSWORD#$SERVER_NAME control="$CTL_PATH/$f.ctl" data="$f" rows=10000 direct=true errors=999
done
Without getting too philosophical, using assignments like FILES=$(ls *.txt) is a bad habit to get into. By contrast, for f in *.txt will deal correctly for files with odd characters in them (like spaces or other syntax breaking values). BUT the other habit you do want to get into is to quote all variable references (like $f), with dbl-quotes : "$f", OK? ;-) This is the otherside of protection for files with spaces etc embedded in them.
In the edit update, I've varibalized your CTL_PATH and CTL_FILE. I think I understand your intent, that you have 1 std CTL_FILE that you pass thru sed to create a table specific .ctl file (a good approach in my experience). Note that you don't need to use cat to send a file to sed, but your use to create a altered file via redirection (> $f.ctl) is very shell-like too.
In 2nd edit update, I looked here on S.O. and found an example sqlldr cmdline that has the correct syntax and have modified to work with your variable names.
To finish up,
A. Are you sure the Oracle Client package is installed on the machine
that you are running your script on?
B. Is the /path/to/oracle/client/tools/bin included in your working
$PATH?
C. try which sqlldr. If you don't get anything, either its not
installed or its not in the path.
D. If not installed, you'll have to get it installed.
E. Once installed, note the directory that contains the sqlldr cmd.
find / -name 'sqlldr*' will take a long time to run, but it will
print out the path you want to use.
F. Take the "path" part of what is returned (like
/opt/oracle/11.2/client/bin/ (but not the sqlldr at the end), and
edit script at 2nd line with
(Txt added to appease the S.O. Formatter ;-) )
export ORCL_PATH="/path/you/found/to/oracle/client"
export PATH="$ORCL_PATH:$PATH"
These steps should solve any remaining issues. If this doesn't work, see if there is someone where you work that understands your local computing environment that can help explain any missing or different steps.
IHTH

Has there ever been a unix system call to create a link from an open file descriptor? [duplicate]

In Unix, it's possible to create a handle to an anonymous file by, e.g., creating and opening it with creat() and then removing the directory link with unlink() - leaving you with a file with an inode and storage but no possible way to re-open it. Such files are often used as temp files (and typically this is what tmpfile() returns to you).
My question: is there any way to re-attach a file like this back into the directory structure? If you could do this it means that you could e.g. implement file writes so that the file appears atomically and fully formed. This appeals to my compulsive neatness. ;)
When poking through the relevant system call functions I expected to find a version of link() called flink() (compare with chmod()/fchmod()) but, at least on Linux this doesn't exist.
Bonus points for telling me how to create the anonymous file without briefly exposing a filename in the disk's directory structure.
A patch for a proposed Linux flink() system call was submitted several years ago, but when Linus stated "there is no way in HELL we can do this securely without major other incursions", that pretty much ended the debate on whether to add this.
Update: As of Linux 3.11, it is now possible to create a file with no directory entry using open() with the new O_TMPFILE flag, and link it into the filesystem once it is fully formed using linkat() on /proc/self/fd/fd with the AT_SYMLINK_FOLLOW flag.
The following example is provided on the open() manual page:
char path[PATH_MAX];
fd = open("/path/to/dir", O_TMPFILE | O_RDWR, S_IRUSR | S_IWUSR);
/* File I/O on 'fd'... */
snprintf(path, PATH_MAX, "/proc/self/fd/%d", fd);
linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file", AT_SYMLINK_FOLLOW);
Note that linkat() will not allow open files to be re-attached after the last link is removed with unlink().
My question: is there any way to re-attach a file like this back into the directory structure? If you could do this it means that you could e.g. implement file writes so that the file appears atomically and fully formed. This appeals to the my compulsive neatness. ;)
If this is your only goal, you can achieve this in a much simpler and more widely used manner. If you are outputting to a.dat:
Open a.dat.part for write.
Write your data.
Rename a.dat.part to a.dat.
I can understand wanting to be neat, but unlinking a file and relinking it just to be "neat" is kind of silly.
This question on serverfault seems to indicate that this kind of re-linking is unsafe and not supported.
Thanks to #mark4o posting about linkat(2), see his answer for details.
I wanted to give it a try to see what actually happened when trying to actually link an anonymous file back into the filesystem it is stored on. (often /tmp, e.g. for video data that firefox is playing).
As of Linux 3.16, there still appears to be no way to undelete a deleted file that's still held open. Neither AT_SYMLINK_FOLLOW nor AT_EMPTY_PATH for linkat(2) do the trick for deleted files that used to have a name, even as root.
The only alternative is tail -c +1 -f /proc/19044/fd/1 > data.recov, which makes a separate copy, and you have to kill it manually when it's done.
Here's the perl wrapper I cooked up for testing. Use strace -eopen,linkat linkat.pl - </proc/.../fd/123 newname to verify that your system still can't undelete open files. (Same applies even with sudo). Obviously you should read code you find on the Internet before running it, or use a sandboxed account.
#!/usr/bin/perl -w
# 2015 Peter Cordes <peter#cordes.ca>
# public domain. If it breaks, you get to keep both pieces. Share and enjoy
# Linux-only linkat(2) wrapper (opens "." to get a directory FD for relative paths)
if ($#ARGV != 1) {
print "wrong number of args. Usage:\n";
print "linkat old new \t# will use AT_SYMLINK_FOLLOW\n";
print "linkat - <old new\t# to use the AT_EMPTY_PATH flag (requires root, and still doesn't re-link arbitrary files)\n";
exit(1);
}
# use POSIX qw(linkat AT_EMPTY_PATH AT_SYMLINK_FOLLOW); #nope, not even POSIX linkat is there
require 'syscall.ph';
use Errno;
# /usr/include/linux/fcntl.h
# #define AT_SYMLINK_NOFOLLOW 0x100 /* Do not follow symbolic links. */
# #define AT_SYMLINK_FOLLOW 0x400 /* Follow symbolic links. */
# #define AT_EMPTY_PATH 0x1000 /* Allow empty relative pathname */
unless (defined &AT_SYMLINK_NOFOLLOW) { sub AT_SYMLINK_NOFOLLOW() { 0x0100 } }
unless (defined &AT_SYMLINK_FOLLOW ) { sub AT_SYMLINK_FOLLOW () { 0x0400 } }
unless (defined &AT_EMPTY_PATH ) { sub AT_EMPTY_PATH () { 0x1000 } }
sub my_linkat ($$$$$) {
# tmp copies: perl doesn't know that the string args won't be modified.
my ($oldp, $newp, $flags) = ($_[1], $_[3], $_[4]);
return !syscall(&SYS_linkat, fileno($_[0]), $oldp, fileno($_[2]), $newp, $flags);
}
sub linkat_dotpaths ($$$) {
open(DOTFD, ".") or die "open . $!";
my $ret = my_linkat(DOTFD, $_[0], DOTFD, $_[1], $_[2]);
close DOTFD;
return $ret;
}
sub link_stdin ($) {
my ($newp, ) = #_;
open(DOTFD, ".") or die "open . $!";
my $ret = my_linkat(0, "", DOTFD, $newp, &AT_EMPTY_PATH);
close DOTFD;
return $ret;
}
sub linkat_follow_dotpaths ($$) {
return linkat_dotpaths($_[0], $_[1], &AT_SYMLINK_FOLLOW);
}
## main
my $oldp = $ARGV[0];
my $newp = $ARGV[1];
# link($oldp, $newp) or die "$!";
# my_linkat(fileno(DIRFD), $oldp, fileno(DIRFD), $newp, AT_SYMLINK_FOLLOW) or die "$!";
if ($oldp eq '-') {
print "linking stdin to '$newp'. You will get ENOENT without root (or CAP_DAC_READ_SEARCH). Even then doesn't work when links=0\n";
$ret = link_stdin( $newp );
} else {
$ret = linkat_follow_dotpaths($oldp, $newp);
}
# either way, you still can't re-link deleted files (tested Linux 3.16 and 4.2).
# print STDERR
die "error: linkat: $!.\n" . ($!{ENOENT} ? "ENOENT is the error you get when trying to re-link a deleted file\n" : '') unless $ret;
# if you want to see exactly what happened, run
# strace -eopen,linkat linkat.pl
Clearly, this is possible -- fsck does it, for example. However, fsck does it with major localized file system mojo and will clearly not be portable, nor executable as an unprivileged user. It's similar to the debugfs comment above.
Writing that flink(2) call would be an interesting exercise. As ijw points out, it would offer some advantages over current practice of temporary file renaming (rename, note, is guaranteed atomic).
Kind of late to the game but I just found http://computer-forensics.sans.org/blog/2009/01/27/recovering-open-but-unlinked-file-data which may answer the question. I haven't tested it, though, so YMMV. It looks sound.

Resources