Absolute and relative paths, . and ..

A path is a set of directions to a file. There are two kinds, and the difference is the first character. An absolute path starts with /: directions from the root of the tree, valid from anywhere, like a full postal address. A relative path starts with anything else: directions from the directory you are standing in, like "second door on the left". The same relative path points at different things from different places; an absolute path never changes meaning.

~% cd /root/lab/l6
l6% ls a/b              (relative: /root/lab/l6/a/b)
note.txt
l6% ls /root/lab/l6/a/b   (absolute: same place)
note.txt
l6% cd a/b
b% ls a/b               (relative again, from a new place)
ls: a/b: No such file or directory

The two entries every directory has

Every directory contains two special names. . (one dot) is the directory itself. .. (two dots) is its parent. ls -a shows them at the top of every listing. They are real entries, not shell tricks, which is why they work inside any path for any command: cp ../config.txt . copies a file from the parent into here; ls ../../etc lists a directory two levels up and over; ./run.sh means "the run.sh in this very directory".

Resolving a path by hand

To work out where a relative path leads, start from your working directory and apply each component in turn: a name enters it, .. goes up, . stays. From /root/lab/l6/a/b, the path ../../c/x.txt goes up to a, up to l6, into c, to x.txt: /root/lab/l6/c/x.txt. When you would rather not do the arithmetic, realpath does it for you and prints the absolute result: realpath ../../c prints /root/lab/l6/c. It also resolves symbolic links, which pwd and cd do not always do.

b% pwd
/root/lab/l6/a/b
b% realpath ../../c
/root/lab/l6/c
b% realpath .
/root/lab/l6/a/b
b% touch ../../c/hop.txt
b% ls /root/lab/l6/c
hop.txt

Which kind to use

Relative paths are shorter and move with you: a project you copy elsewhere keeps working if its internal references are relative. Absolute paths are unambiguous and safe in scripts, scheduled jobs and anything that might run from an unknown directory. A fair rule: type relative paths while exploring interactively, write absolute paths — or paths relative to a known anchor — into anything you save. The home shortcut ~ behaves like an absolute path: ~/lab is /root/lab from anywhere.