Friday 14 October 2022

Symbolic Links (symlinks)


linux - Find out symbolic link target via command line - Server Fault

What are they?

  • files that contain a reference to another file or directory on the same system
  • like shortcuts on Windows OS

What is their purpose?

  • to avoid copying the same binary (usually a library) at multiple locations but simply creating a symlink where file is required to be
  • various clients might require the same file but with name in different format so instead of having multiple copies of the same file but with different names we'd have multiple symlink, each with the name that satisfies requirements of each service

How do they work?

  • opening/running the symlink would open/run the target file
  • editing the content of the symlink edits the content of the target file
  • if target file is deleted symlink becomes a dangling symlink
  • if symlink is deleted, target file remains unaffected
  • it is possible to create symlink that refers to another symlink [How can I create a symlink which points to another symlink?]

How to create them?

How to: Linux / UNIX create soft link with ln command

Use ln command:

       ln - make links between files

       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)

       In the 1st form, create a link to TARGET with the name LINK_NAME.  In the 2nd form, create a link to TARGET in the current directory.  In the 3rd and 4th forms, create links to each TARGET in DIRECTORY.  Create hard links by default, symbolic links with --symbolic.  By default, each destination (name of new link) should not already exist.  When creating hard links,  each  TARGET  must  exist.
       Symbolic links can hold arbitrary text; if later resolved, a relative link is interpreted in relation to its parent directory.

       Mandatory arguments to long options are mandatory for short options too.

              make a backup of each existing destination file

       -b     like --backup but does not accept an argument

       -d, -F, --directory
              allow the superuser to attempt to hard link directories (note: will probably fail due to system restrictions, even for the superuser)

       -f, --force
              remove existing destination files

       -i, --interactive
              prompt whether to remove destinations

       -L, --logical
              dereference TARGETs that are symbolic links

       -n, --no-dereference
              treat LINK_NAME as a normal file if it is a symbolic link to a directory

       -P, --physical
              make hard links directly to symbolic links

       -r, --relative
              create symbolic links relative to link location

       -s, --symbolic
              make symbolic links instead of hard links

       -S, --suffix=SUFFIX
              override the usual backup suffix

       -t, --target-directory=DIRECTORY
              specify the DIRECTORY in which to create the links

       -T, --no-target-directory
              treat LINK_NAME as a normal file always

       -v, --verbose
              print name of each linked file

       --help display this help and exit

              output version information and exit

       The  backup  suffix is '~', unless set with --suffix or SIMPLE_BACKUP_SUFFIX.  The version control method may be selected via the --backup option or through the VERSION_CONTROL environment variable.
       Here are the values:

       none, off
              never make backups (even if --backup is given)

       numbered, t
              make numbered backups

       existing, nil
              numbered if numbered backups exist, simple otherwise

       simple, never
              always make simple backups

       Using -s ignores -L and -P.  Otherwise, the last option specified controls behavior when a TARGET is a symbolic link, defaulting to -P.


$ sudo ln -s /usr/local/go/bin/go /usr/local/bin/go

Creating a symlink from one folder to another with different names?

Types of symlinks:

(!) Important: at the time the symlink is being used and resolved, target path (in ls command) is understood as a relative path to the parent directory of the symlink (when it doesn't start with /).

$ pwd
$ ln -s foo/bar.txt bar.txt
$ readlink -f /home/beau/bar.txt


$ cd foo
$ ln -s foo/bar.txt ../bar.txt

How to list all symbolic links in the current directory?

$ find -type l

[man find]: If no paths are given, the current directory is used.
[How to list all symbolic links in a directory]

# save symlinks and their targets (relative to ./path/to/dir/) in csv file which will also be pushed to S3
cd ./path/to/dir/
find . -type l -ls | awk '{print $11,",",$13}' > symlinks.csv

How do I tell if a folder is actually a symlink and how do I fix it if it's broken?

Here are some ways that can be used to verify symlink:

$ stat ./data-vol/content/app/74.0.1365.76 
  File: ./data-vol/content/app/74.0.1365.76 -> data-vol/content/app/win/x86/74.0.1365.76
  Size: 45              Blocks: 0          IO Block: 4096   symbolic link
Device: fd01h/64769d    Inode: 26479224    Links: 1
Access: (0777/lrwxrwxrwx)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2019-07-12 17:17:09.278071996 +0100
Modify: 2019-07-12 17:17:08.666073171 +0100
Change: 2019-07-12 17:17:08.666073171 +0100
 Birth: -

$ stat -L ./data-vol/content/app/74.0.1365.76 
stat: cannot stat './data-vol/content/app/74.0.1365.76': No such file or directory

$ file -L ./data-vol/content/app/74.0.1365.76 
./data-vol/content/app/74.0.1365.76: cannot open `./data-vol/content/app/74.0.1365.76' (No such file or directory)

$ ls ./data-vol/content/app/74.0.1365.76 

$ ll ./data-vol/content/app/74.0.1365.76 
lrwxrwxrwx 1 root root 45 Jul 12 17:17 ./data-vol/content/app/74.0.1365.76 -> data-vol/content/app/win/x86/74.0.1365.76

How to see full symlink path

$ readlink -f symlinkName

Hard links

How to create hardlink of one file in different directories in linux

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