I am trying to figure out in what condition would two different cd commands lead to the same directory.
For example: Under what condition do both cd /b/c/d/e and cd d/e change to the same directory?
Could someone help me with this as I do not quite understand how to figure this out.
If you create a symbolic link to directory "d" in the directory below b:
e.g.
/temp/b/c/d/e is the directory structure
cd /temp
ln -s /temp/b/c/d
now inside of temp you will have two directories, one real and one a symbolic link to d.
/temp$ ls -al
total 12
drwxrwxr-x 3 temp temp 4096 Oct 5 18:25 .
drwxr-xr-x 34 temp temp 4096 Oct 5 18:24 ..
drwxrwxr-x 3 temp temp 4096 Oct 5 18:24 b
lrwxrwxrwx 1 temp temp 20 Oct 5 18:25 d -> /temp/b/c/d
and you can do
cd b/c/d/e
or
cd d/e
and get to the same directory.
Say you are at the pwd (present working directory) of /b/c
Then cd /b/c/d/e will bring you to the same location as cd d/e
In the first you are defining the absolute path, whereas in the second you are defining the relative path.
Let's see the examples:
cd b/c/d/e
cd d/e
Both of them are using relative paths to e/
In fact, we can assume that in the seconds one, you are in the directory c
When I say relative path, it's because you don't have a / at the beggining of the path, and it means that cd will look for the path you're providing, from your current directory (. or $PWD).
cd ./b/c/d/e
cd ./d/e
or
cd "$PWD"/b/c/d/e
cd "$PWD"/d/e
You can get your current directory with the pwd command. It will give you the value of the PWD environment variable
Related
I connect to a remote computer thru VNC and use Konsole for my work. Konsole is version 2.3.3 using KDE 4.3.4. I have these two aliases:
alias ll 'ls -haltr; pwd'
alias cd 'cd \!*; ll'
which I observed to have the following behavior:
When path exists, it will cd to it and also do the ll alias
When path doesn't exist, it will simply say the path doesn't exist and won't do the ll anymore
Example:
Path exists
[10] % cd foo
total 14K
-rw-r----- 1 user group 913 Jun 3 2014 readme
-rw-r----- 1 user group 1.2K Dec 3 2020 report.txt
drwxr-x--- 2 user group 4.0K Jan 12 17:50 ./
drwx------ 77 user group 8.0K Jun 24 11:57 ../
/home/user/foo
[11] %
Path doesn't exist
[10] % cd nowhere
nowhere: No such file or directory.
[11] %
Now our department has transferred to another division and we've just started to connect remotely thru Exceed TurboX. I still use Konsole but the version is now 2.10.5 using KDE 4.10.5. I copied over the same two aliases, but I'm now observing a different behavior:
When path exists, it will cd to it and also do the ll alias (basically same as #1 above)
When path doesn't exist, it will attempt to cd AND still do the ll (different as #2 above)
So for #2, here's how it looks like:
[10] % cd nowhere
nowhere: No such file or directory.
total 120K
-rw-r----- 1 user group 272 Jan 6 2021 .cshrc
-rw-r----- 1 user group 1.2K Jan 6 2021 .alias
drwxr-x--- 2 user group 4.0K Jan 12 17:50 ./
drwx------ 77 user group 8.0K Jun 24 11:57 ../
/home/user
[11] %
I would like to know how to get behavior #2 of the previous working environment to this current working environment.
I've added the information on the Konsole and KDE versions because if the behavior is due to the version and there's no workaround, then I'll just be sad for the rest of my life working in this new remote desktop env. ^_^
I'm currently exploring a "check first if the path exists before doing ll" but to no avail. :'(
Edit:
The shell I'm using is tcsh
% printenv SHELL in both environments showed /bin/tcsh
And in addition:
Old environment
% echo $version
tcsh 6.17.00 (Astron) 2009-07-10 (x86_64-unknown-linux) options wide,nls,dl,al,kan,sm,rh,color,filec
New environment
% echo $version
3
The information in the question is still not sufficient to find out why tcsh behaves differently in the two working environments.
The alias defines a sequence of commands (cd followed by ll) without any condition.
alias cd 'cd \!*; ll'
On one system the execution of the alias seems to stop if the cd command reports an error for currently unknown reasons.
The correct way to prevent the execution of ll after a failed cd command would be the conditional execution of the ll command, e.g.
alias cd 'cd \!* && ll'
The decade long advice is: do not use aliases, use functions.
ll() { ls -haltr "$#" && pwd; }
cd() { cd "$#" && ll; }
Can someone please help me with this:
As seen below I have a file and directory with the same name as "sp"
How do I delete the file "sp" the one with 44673Bytes size
opxnyd#opxzone1d:/opt/opxnyd/packages/OPXPNY3DB/src/OPXPNYP>ls -alrt
-rwxr-xr-x 1 opxnyd opics 44673 Sep 7 2011 sp
drwxr-xr-x 4 opxnyd opics 1974 May 10 10:22 sp
The trick is that they don't actually have the same name. one of them has a blank or non-printing character in the name. Try ls --escape to see.
Like Charlie Martin said, they dont actually have the same name. But you could do a rm sp* without -r option (directory), deleting only the file.
Try renaming the file (any of them, then) delete the one you don't want (and rename again, if you happed to rename the folder)
You cannot have a directory and file with the same name. One of them would be probably having a white-space or some other non printable character in it.
Take this example:
$ touch "sp"
$ mkdir "sp "
$ ls -lrt
total 2
-rw-r--r-- 1 user staff 0 May 18 15:47 sp
drwxr-xr-x 2 user staff 68 May 18 15:47 sp
To delete just the file you can use following find command:
find -E . -depth 1 -type f -regex "\./sp[ \t]*" -exec rm {} \;
OR following rm command:
\rm -i sp\ *
See:
mkdir sym
cd sym
mkdir one
//Create the symlink
ln -s one two
ls -l
drwxr-xr-x 2 lola lola 4096 2012-02-14 07:58 one
lrwxrwxrwx 1 lola lola 3 2012-02-14 07:58 two -> one
Now, if I put something in one I could reach it in two. For what I understand two is the name of the symlink, and creates a directory to it (namely, two) [is this correct?].
Question: Is two is a directory that points to one?
But if I do:
(assuming a clean configuration)
mkdir sym
cd sym
mkdir one
mkdir two <--- notice the creation of two!!
//Create the symlink
ln -s one two
drwxr-xr-x 2 lola lola 4096 2012-02-14 07:59 one
lrwxrwxrwx 1 lola lola 3 2012-02-14 07:59 two
but in two/
lrwxrwxrwx 1 lola lola 3 2012-02-14 07:59 one -> one
If I put something in one I cannot reach it in two.
But from man ln:
SYNOPSIS
ln [OPTION]... [-T] TARGET LINK_NAME (1st form)
ln [OPTION]... TARGET (2nd form)
ln [OPTION]... TARGET... DIRECTORY (3rd form)
ln [OPTION]... -t DIRECTORY TARGET... (4th form)
I'm trying to do the 3rd form, that is: create a symlink from one directory to another directory.
Could you give me a hint about my mistake? I think is conceptual (and technical).
In your first example, "two" is not a directory. It is a simlink (basically a small label that says "if someone asks for me, look in "one" in stead.
In the second case, you indeed use the 3rd form. However what this form does is "Make a simlink to TARGET inside directory DIRECTORY. Because "two" is a directory, the ln command recognizes the second example as the 3rd form.
A symbolic link is a kind of file. It is not a directory. The readlink() system call reads the characters in the symbolic link file. That is where the "-> one" comes from in the ls display.
In order for your example to work ls -l two should show "../one" because the two symlink is inside the one directory, so for it to correctly reference the directory is has to go "up" to find one.
I am somewhat confused how soft links work in unix. See the example.
% cd /usr/local/
% ls -la
total 6
drwxr-xr-x 2 root root 512 Jan 19 15:03 .
drwxr-xr-x 41 root sys 1024 Jan 20 16:24 ..
lrwxrwxrwx 1 root root 38 Jan 19 15:03 java -> /otherDir/java/jdk1.6.0_17 **<- this is a soft link**
% cd java **<- move to the softlink**
% pwd
/usr/local/java **<- the current location, say LOCATION_A**
% cd /otherDir/java/jdk1.6.0_17/ **<-move to the location of the softlink**
% pwd
/otherDir/java/jdk1.6.0_17 **<- the new current location, say LOCATION_B**
Isn't this problematic that even though LOCATION_A is LOCATION_B, they have different paths?
Is there a command (other than pwd) that will give the real location of a file (not just how the user go there).
It seems to me like pwd is just the sum of a user's cd. NOT their current location.
Try pwd -P. It's not "other than pwd" but it does the trick, at least on my bash 4.0.35 on Fedora 12. YMMV.
Update: Even works with sh, so it seems to be portable.
This behaves like this with a purpose. If you cd to /a/b/c/d and then cd to .. then you realistically expect to be in /a/b/c. If c happens to be a symbolic link (or symlink in unix terms - but not soft link) that takes you to /f/g/h, with the behaviour you would like to have you would end up in /f/g and then you (or any program) would not understand how it got there.
You can use readlink on the current working directory to get the true directory name:
readlink `pwd`
Normally, pwd should return /usr/local/java in the last line, if i understand your example. But some shells have a build in pwd command that tries to be more "intelligent" handling symlinks in the current working directory.
Try /bin/pwd, do you get other results?
realpath does what you want.
It is not possible to absolutely get your path under all circumstances. This is a bit odd, but a variation of this (plus chroot and setuid) is sometimes used for locking down a process.
$ mkdir -p /tmp/a/b
$ cd /tmp/a/b
$ rmdir /tmp/a/b
$ chmod 0 /tmp/a
$ rmdir /tmp/a
$ ls ..
ls: cannot open directory ..: Permission denied
$ ls -al
total 0
$ pwd -P
pwd: error retrieving current directory: getcwd: cannot access parent directories: No such file or directory
Does any operating system provide a mechanism (system call — not command line program) to change the pathname referenced by a symbolic link (symlink) — other than by unlinking the old one and creating a new one?
The POSIX standard does not. Solaris 10 does not. MacOS X 10.5 (Leopard) does not. (I'm tolerably certain neither AIX nor HP-UX does either. Judging from this list of Linux system calls, Linux does not have such a system call either.)
Is there anything that does?
(I'm expecting that the answer is "No".)
Since proving a negative is hard, let's reorganize the question.
If you know that some (Unix-like) operating system not already listed has no system call for rewriting the value of a symlink (the string returned by readlink()) without removing the old symlink and creating a new one, please add it — or them — in an answer.
Yes, you can!
$ ln -sfn source_file_or_directory_name softlink_name
AFAIK, no, you can't. You have to remove it and recreate it. Actually, you can overwrite a symlink and thus update the pathname referenced by it:
$ ln -s .bashrc test
$ ls -al test
lrwxrwxrwx 1 pascal pascal 7 2009-09-23 17:12 test -> .bashrc
$ ln -s .profile test
ln: creating symbolic link `test': File exists
$ ln -s -f .profile test
$ ls -al test
lrwxrwxrwx 1 pascal pascal 8 2009-09-23 17:12 test -> .profile
EDIT: As the OP pointed out in a comment, using the --force option will make ln perform a system call to unlink() before symlink(). Below, the output of strace on my linux box proving it:
$ strace -o /tmp/output.txt ln -s -f .bash_aliases test
$ grep -C3 ^unlink /tmp/output.txt
lstat64("test", {st_mode=S_IFLNK|0777, st_size=7, ...}) = 0
stat64(".bash_aliases", {st_mode=S_IFREG|0644, st_size=2043, ...}) = 0
symlink(".bash_aliases", "test") = -1 EEXIST (File exists)
unlink("test") = 0
symlink(".bash_aliases", "test") = 0
close(0) = 0
close(1) = 0
So I guess the final answer is "no".
EDIT: The following is copied from Arto Bendiken's answer over on unix.stackexchange.com, circa 2016.
This can indeed be done atomically with rename(2), by first creating the new symlink under a temporary name and then cleanly overwriting the old symlink in one go. As the man page states:
If newpath refers to a symbolic link the link will be overwritten.
In the shell, you would do this with mv -T as follows:
$ mkdir a b
$ ln -s a z
$ ln -s b z.new
$ mv -T z.new z
You can strace that last command to make sure it is indeed using rename(2) under the hood:
$ strace mv -T z.new z
lstat64("z.new", {st_mode=S_IFLNK|0777, st_size=1, ...}) = 0
lstat64("z", {st_mode=S_IFLNK|0777, st_size=1, ...}) = 0
rename("z.new", "z") = 0
Note that in the above, both mv -T and strace are Linux-specific.
On FreeBSD, use mv -h alternately.
Editor's note: This is how Capistrano has done it for years now, ever since ~2.15. See this pull request.
It is not necessary to explicitly unlink the old symlink. You can do this:
ln -s newtarget temp
mv temp mylink
(or use the equivalent symlink and rename calls). This is better than explicitly unlinking because rename is atomic, so you can be assured that the link will always point to either the old or new target. However this will not reuse the original inode.
On some filesystems, the target of the symlink is stored in the inode itself (in place of the block list) if it is short enough; this is determined at the time it is created.
Regarding the assertion that the actual owner and group are immaterial, symlink(7) on Linux says that there is a case where it is significant:
The owner and group of an existing symbolic link can be changed using
lchown(2). The only time that the ownership of a symbolic link matters is
when the link is being removed or renamed in a directory that has the sticky
bit set (see stat(2)).
The last access and last modification timestamps of a symbolic link can be
changed using utimensat(2) or lutimes(3).
On Linux, the permissions of a symbolic link are not used in any operations;
the permissions are always 0777 (read, write, and execute for all user
categories), and can't be changed.
Just a warning to the correct answers above:
Using the -f / --force Method provides a risk to lose the file if you mix up source and target:
mbucher#server2:~/test$ ls -la
total 11448
drwxr-xr-x 2 mbucher www-data 4096 May 25 15:27 .
drwxr-xr-x 18 mbucher www-data 4096 May 25 15:13 ..
-rw-r--r-- 1 mbucher www-data 4109466 May 25 15:26 data.tar.gz
-rw-r--r-- 1 mbucher www-data 7582480 May 25 15:27 otherdata.tar.gz
lrwxrwxrwx 1 mbucher www-data 11 May 25 15:26 thesymlink -> data.tar.gz
mbucher#server2:~/test$
mbucher#server2:~/test$ ln -s -f thesymlink otherdata.tar.gz
mbucher#server2:~/test$
mbucher#server2:~/test$ ls -la
total 4028
drwxr-xr-x 2 mbucher www-data 4096 May 25 15:28 .
drwxr-xr-x 18 mbucher www-data 4096 May 25 15:13 ..
-rw-r--r-- 1 mbucher www-data 4109466 May 25 15:26 data.tar.gz
lrwxrwxrwx 1 mbucher www-data 10 May 25 15:28 otherdata.tar.gz -> thesymlink
lrwxrwxrwx 1 mbucher www-data 11 May 25 15:26 thesymlink -> data.tar.gz
Of course this is intended, but usually mistakes occur. So, deleting and rebuilding the symlink is a bit more work but also a bit saver:
mbucher#server2:~/test$ rm thesymlink && ln -s thesymlink otherdata.tar.gz
ln: creating symbolic link `otherdata.tar.gz': File exists
which at least keeps my file.
Wouldn't unlinking it and creating the new one do the same thing in the end anyway?
Just in case it helps: there is a way to edit a symlink with midnight commander (mc).
The menu command is (in French on my mc interface):
Fichier / Éditer le lien symbolique
which may be translated to:
File / Edit symbolic link
The shortcut is C-x C-s
Maybe it internally uses the ln --force command, I don't know.
Now, I'm trying to find a way to edit a whole lot of symlinks at once (that's how I arrived here).
Technically, there's no built-in command to edit an existing symbolic link. It can be easily achieved with a few short commands.
Here's a little bash/zsh function I wrote to update an existing symbolic link:
# -----------------------------------------
# Edit an existing symbolic link
#
# #1 = Name of symbolic link to edit
# #2 = Full destination path to update existing symlink with
# -----------------------------------------
function edit-symlink () {
if [ -z "$1" ]; then
echo "Name of symbolic link you would like to edit:"
read LINK
else
LINK="$1"
fi
LINKTMP="$LINK-tmp"
if [ -z "$2" ]; then
echo "Full destination path to update existing symlink with:"
read DEST
else
DEST="$2"
fi
ln -s $DEST $LINKTMP
rm $LINK
mv $LINKTMP $LINK
printf "Updated $LINK to point to new destination -> $DEST"
}
You can modify the softlink created once in one of the two ways as below in Linux
one is where you can remove existing softlink with rm and again create new softlink with ln -s command .
However this can be done in one step , you can replace existing softlink with updated path with "ln -vfns Source_path Destination_path" command.
Listing initial all files in directory
$ ls -lrt
drwxrwxr-x. 3 root root 110 Feb 27 18:58 test_script
$
Create softlink test for test_script with ln -s command.
$ ln -s test_script test
$ ls -lrt
drwxrwxr-x. 3 root root 110 Feb 27 18:58 test_script
lrwxrwxrwx. 1 root root 11 Feb 27 18:58 test -> test_script
$
Update softlink test with new directory test_script/softlink with single command
$ ln -vfns test_script/softlink/ test
'test' -> 'test_script/softlink/'
$
List new softlink location
$ ls -lrt
lrwxrwxrwx. 1 root root 21 Feb 27 18:59 test -> test_script/softlink/
$
ln --help
-v, --verbose print name of each linked file
-f, --force remove existing destination files
-n, --no-dereference treat LINK_NAME as a normal file if it is a symbol
-s, --symbolic make symbolic links instead of hard links