Navigating#

This chapter serves as a guide to navigating within a Linux shell, giving users essential techniques to traverse directories. A very important skill.

Current directory: "pwd" and "$PWD"#

To print the current directory, use pwd or \$PWD:

$ cd $HOME 
$ pwd
/user/home/gent/vsc400/vsc40000 
$ echo "The current directory is: $PWD" 
The current directory is: /user/home/gent/vsc400/vsc40000

Listing files and directories: "ls"#

A very basic and commonly used command is ls, which can be used to list files and directories.

In its basic usage, it just prints the names of files and directories in the current directory. For example:

$ ls
afile.txt some_directory

When provided an argument, it can be used to list the contents of a directory:

$ ls some_directory 
one.txt two.txt

A couple of commonly used options include:

detailed listing using ls -l:

$ ls -l
    total 4224 
    -rw-rw-r-- 1 vsc40000 vsc40000 2157404 Apr 12 13:17 afile.txt 
    drwxrwxr-x 2 vsc40000 vsc40000 512 Apr 12 12:51 some_directory

To print the size information in human-readable form, use the -h flag:

$ ls -lh
    total 4.1M 
    -rw-rw-r-- 1 vsc40000 vsc40000 2.1M Apr 12 13:16 afile.txt
    drwxrwxr-x 2 vsc40000 vsc40000 512 Apr 12 12:51 some_directory

also listing hidden files using the -a flag:

$ ls -lah
    total 3.9M 
    drwxrwxr-x 3 vsc40000 vsc40000 512 Apr 12 13:11 .
    drwx------ 188 vsc40000 vsc40000 128K Apr 12 12:41 .. 
    -rw-rw-r-- 1 vsc40000 vsc40000 1.8M Apr 12 13:12 afile.txt 
    -rw-rw-r-- 1 vsc40000 vsc40000 0 Apr 12 13:11 .hidden_file.txt 
    drwxrwxr-x 2 vsc40000 vsc40000 512 Apr 12 12:51 some_directory

ordering files by the most recent change using -rt:

$ ls -lrth
    total 4.0M 
    drwxrwxr-x 2 vsc40000 vsc40000 512 Apr 12 12:51 some_directory 
    -rw-rw-r-- 1 vsc40000 vsc40000 2.0M Apr 12 13:15 afile.txt

If you try to use ls on a file that doesn't exist, you will get a clear error message:

$ ls nosuchfile 
ls: cannot access nosuchfile: No such file or directory

Changing directory: "cd"#

To change to a different directory, you can use the cd command:

$ cd some_directory

To change back to the previous directory you were in, there's a shortcut: cd -

Using cd without an argument results in returning back to your home directory:

$ cd 
$ pwd
/user/home/gent/vsc400/vsc40000

Inspecting file type: "file"#

The file command can be used to inspect what type of file you're dealing with:

$ file afile.txt
afile.txt: ASCII text

$ file some_directory 
some_directory: directory

Absolute vs relative file paths#

An absolute filepath starts with / (or a variable which value starts with /), which is also called the root of the filesystem.

Example: absolute path to your home directory: /user/home/gent/vsc400/vsc40000.

A relative path starts from the current directory, and points to another location up or down the filesystem hierarchy.

Example: some_directory/one.txt points to the file one.txt that is located in the subdirectory named some_directory of the current directory.

There are two special relative paths worth mentioning:

. is a shorthand for the current directory
.. is a shorthand for the parent of the current directory

You can also use .. when constructing relative paths, for example:

$ cd $HOME/some_directory 
$ ls ../afile.txt 
../afile.txt

Permissions#

Each file and directory has particular permissions set on it, which can be queried using ls -l.

For example:

$ ls -l afile.txt 
-rw-rw-r-- 1 vsc40000 agroup 2929176 Apr 12 13:29 afile.txt

The -rwxrw-r-- specifies both the type of file (- for files, d for directories (see first character)), and the permissions for user/group/others:

each triple of characters indicates whether the read (r), write (w), execute (x) permission bits are set or not
the 1st part rwx indicates that the owner "vsc40000" of the file has all the rights
the 2nd part rw- indicates the members of the group "agroup" only have read/write permissions (not execute)
the 3rd part r-- indicates that other users only have read permissions

The default permission settings for new files/directories are determined by the so-called umask setting, and are by default:

read-write permission on files for user/group (no execute), read-only for others (no write/execute)
read-write-execute permission for directories on user/group, read/execute-only for others (no write)

See also the chmod command later in this manual.

Finding files/directories: "find"#

find will crawl a series of directories and lists files matching given criteria.

For example, to look for the file named one.txt:

$ cd $HOME 
$ find . -name one.txt
./some_directory/one.txt

To look for files using incomplete names, you can use a wildcard *; note that you need to escape the * to avoid that Bash expands it into afile.txt by adding double quotes:

$ find . -name "*.txt"
./.hidden_file.txt 
./afile.txt 
./some_directory/one.txt
./some_directory/two.txt

A more advanced use of the find command is to use the -exec flag to perform actions on the found file(s), rather than just printing their paths (see man find).

Exercises#

Go to /tmp, then back to your home directory. How many different ways to do this can you come up with?
When was your home directory created or last changed?
Determine the name of the last changed file in /tmp.
See how home directories are organised. Can you access the home directory of other users?

The next chapter will teach you how to interact with files and directories.