layout | title | subtitle | minutes |
---|---|---|---|
page |
The Unix Shell |
Navigating Files and Directories |
15 |
- Explain the similarities and differences between a file and a directory.
- Translate an absolute path into a relative path and vice versa.
- Construct absolute and relative paths that identify specific files and directories.
- Explain the steps in the shell's read-run-print cycle.
- Identify the actual command, flags, and filenames in a command-line call.
- Demonstrate the use of tab completion, and explain its advantages.
The part of the operating system responsible for managing files and directories is called the file system. It organizes our data into files, which hold information, and directories (also called "folders"), which hold files or other directories.
Several commands are frequently used to create, inspect, rename, and delete files and directories. To start exploring them, let's open a shell window:
If you type the command:
PS1='$ '
into your shell, your window should look like our example in this lesson.
This isn't necessary to follow along (in fact, your prompt may have other helpful information you want to know about). This is up to you!
$
The dollar sign is a prompt, which shows us that the shell is waiting for input; your shell may use a different character as a prompt and may add information before the prompt. When typing commands, either from these lessons or from other sources, do not type the prompt, only the commands that follow it.
Type the command whoami
,
then press the Enter key (sometimes marked Return) to send the command to the shell.
The command's output is the ID of the current user,
i.e.,
it shows us who the shell thinks we are:
$ whoami
nelle
More specifically, when we type whoami
the shell:
- finds a program called
whoami
, - runs that program,
- displays that program's output, then
- displays a new prompt to tell us that it's ready for more commands.
In this lesson, we have used the username
nelle
(associated with our hypothetical scientist Nelle) in example input and output throughout.
However, when you type this lesson's commands on your computer, you should see and use something different, namely, the username associated with the user account on your computer. This username will be the output fromwhoami
. In what follows,nelle
should always be replaced by that username.
Next,
let's find out where we are by running a command called pwd
(which stands for "print working directory").
At any moment,
our current working directory
is our current default directory,
i.e.,
the directory that the computer assumes we want to run commands in
unless we explicitly specify something else.
Here,
the computer's response is /Users/nelle
,
which is Nelle's home directory:
$ pwd
/Users/nelle
The home directory path will look different on different operating systems. On Linux it may look like
/home/nelle
, and on Windows it will be similar toC:\Documents and Settings\nelle
orC:\Users\nelle
.
(Note that it may look slightly different for different versions of Windows.) In future examples, we've used Mac output as the default - Linux and Windows output may differ slightly, but should be generally similar.
To understand what a "home directory" is, let's have a look at how the file system as a whole is organized. For the sake of example, we'll be illustrating the filesystem on our scientist Nelle's computer. After this illustration, you'll be learning commands to explore your own filesystem, which will be constructed in a similar way, but not be exactly identical.
On Nelle's computer, the filesystem looks like this:
At the top is the root directory
that holds everything else.
We refer to it using a slash character /
on its own;
this is the leading slash in /Users/nelle
.
Inside that directory are several other directories:
bin
(which is where some built-in programs are stored),
data
(for miscellaneous data files),
Users
(where users' personal directories are located),
tmp
(for temporary files that don't need to be stored long-term),
and so on.
We know that our current working directory /Users/nelle
is stored inside /Users
because /Users
is the first part of its name.
Similarly,
we know that /Users
is stored inside the root directory /
because its name begins with /
.
Notice that there are two meanings for the
/
character. When it appears at the front of a file or directory name, it refers to the root directory. When it appears inside a name, it's just a separator.
Underneath /Users
,
we find one directory for each user with an account on Nelle's machine,
her colleagues the Mummy and Wolfman.
The Mummy's files are stored in /Users/imhotep
,
Wolfman's in /Users/larry
,
and Nelle's in /Users/nelle
. Because Nelle is the user in our
examples here, this is why we get /Users/nelle
as our home directory.
Typically, when you open a new command prompt you will be in
your home directory to start.
Now let's learn the command that will let us see the contents of our
own filesystem. We can see what's in our home directory by running ls
,
which stands for "listing":
$ ls
Applications Documents Library Music Public
Desktop Downloads Movies Pictures
(Again, your results may be slightly different depending on your operating system and how you have customized your filesystem.)
ls
prints the names of the files and directories in the current directory in
alphabetical order,
arranged neatly into columns.
We can make its output more comprehensible by using the flag -F
,
which tells ls
to add a trailing /
to the names of directories:
$ ls -F
Applications/ Documents/ Library/ Music/ Public/
Desktop/ Downloads/ Movies/ Pictures/
ls
has lots of other options. To find out what they are, we can type:
$ ls --help
Usage: ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).
Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.
Mandatory arguments to long options are mandatory for short options too.
-a, --all do not ignore entries starting with .
-A, --almost-all do not list implied . and ..
--author with -l, print the author of each file
-b, --escape print C-style escapes for nongraphic characters
--block-size=SIZE scale sizes by SIZE before printing them; e.g.,
'--block-size=M' prints sizes in units of
1,048,576 bytes; see SIZE format below
-B, --ignore-backups do not list implied entries ending with ~
-c with -lt: sort by, and show, ctime (time of last
modification of file status information);
with -l: show ctime and sort by name;
otherwise: sort by ctime, newest first
-C list entries by columns
--color[=WHEN] colorize the output; WHEN can be 'always' (default
if omitted), 'auto', or 'never'; more info below
-d, --directory list directories themselves, not their contents
-D, --dired generate output designed for Emacs' dired mode
-f do not sort, enable -aU, disable -ls --color
-F, --classify append indicator (one of */=>@|) to entries
--file-type likewise, except do not append '*'
--format=WORD across -x, commas -m, horizontal -x, long -l,
single-column -1, verbose -l, vertical -C
--full-time like -l --time-style=full-iso
-g like -l, but do not list owner
--group-directories-first
group directories before files;
can be augmented with a --sort option, but any
use of --sort=none (-U) disables grouping
-G, --no-group in a long listing, don't print group names
-h, --human-readable with -l and/or -s, print human readable sizes
(e.g., 1K 234M 2G)
--si likewise, but use powers of 1000 not 1024
-H, --dereference-command-line
follow symbolic links listed on the command line
--dereference-command-line-symlink-to-dir
follow each command line symbolic link
that points to a directory
--hide=PATTERN do not list implied entries matching shell PATTERN
(overridden by -a or -A)
--indicator-style=WORD append indicator with style WORD to entry names:
none (default), slash (-p),
file-type (--file-type), classify (-F)
-i, --inode print the index number of each file
-I, --ignore=PATTERN do not list implied entries matching shell PATTERN
-k, --kibibytes default to 1024-byte blocks for disk usage
-l use a long listing format
-L, --dereference when showing file information for a symbolic
link, show information for the file the link
references rather than for the link itself
-m fill width with a comma separated list of entries
-n, --numeric-uid-gid like -l, but list numeric user and group IDs
-N, --literal print raw entry names (don't treat e.g. control
characters specially)
-o like -l, but do not list group information
-p, --indicator-style=slash
append / indicator to directories
-q, --hide-control-chars print ? instead of nongraphic characters
--show-control-chars show nongraphic characters as-is (the default,
unless program is 'ls' and output is a terminal)
-Q, --quote-name enclose entry names in double quotes
--quoting-style=WORD use quoting style WORD for entry names:
literal, locale, shell, shell-always,
shell-escape, shell-escape-always, c, escape
-r, --reverse reverse order while sorting
-R, --recursive list subdirectories recursively
-s, --size print the allocated size of each file, in blocks
-S sort by file size, largest first
--sort=WORD sort by WORD instead of name: none (-U), size (-S),
time (-t), version (-v), extension (-X)
--time=WORD with -l, show time as WORD instead of default
modification time: atime or access or use (-u);
ctime or status (-c); also use specified time
as sort key if --sort=time (newest first)
--time-style=STYLE with -l, show times using style STYLE:
full-iso, long-iso, iso, locale, or +FORMAT;
FORMAT is interpreted like in 'date'; if FORMAT
is FORMAT1<newline>FORMAT2, then FORMAT1 applies
to non-recent files and FORMAT2 to recent files;
if STYLE is prefixed with 'posix-', STYLE
takes effect only outside the POSIX locale
-t sort by modification time, newest first
-T, --tabsize=COLS assume tab stops at each COLS instead of 8
-u with -lt: sort by, and show, access time;
with -l: show access time and sort by name;
otherwise: sort by access time, newest first
-U do not sort; list entries in directory order
-v natural sort of (version) numbers within text
-w, --width=COLS set output width to COLS. 0 means no limit
-x list entries by lines instead of by columns
-X sort alphabetically by entry extension
-Z, --context print any security context of each file
-1 list one file per line. Avoid '\n' with -q or -b
--help display this help and exit
--version output version information and exit
The SIZE argument is an integer and optional unit (example: 10K is 10*1024).
Units are K,M,G,T,P,E,Z,Y (powers of 1024) or KB,MB,... (powers of 1000).
Using color to distinguish file types is disabled both by default and
with --color=never. With --color=auto, ls emits color codes only when
standard output is connected to a terminal. The LS_COLORS environment
variable can change the settings. Use the dircolors command to set it.
Exit status:
0 if OK,
1 if minor problems (e.g., cannot access subdirectory),
2 if serious trouble (e.g., cannot access command-line argument).
GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
Full documentation at: <http://www.gnu.org/software/coreutils/ls>
or available locally via: info '(coreutils) ls invocation'
Many bash commands, and programs that people have written that can be run from within bash, support a --help
flag to display more information on how to use the commands or programs.
For more information on how to use ls
we can type man ls
.
man
is the Unix "manual" command:
it prints a description of a command and its options,
and (if you're lucky) provides a few examples of how to use it.
The bash shell provided by Git for Windows does not support the
man
command. Doing a web search forunix man page COMMAND
(e.g.unix man page grep
) provides links to numerous copies of the Unix manual pages online. For example, GNU provides links to its manuals: these include grep, and the core GNU utilities, which covers many commands introduced within this lesson.
To navigate through the man
pages,
you may use the up and down arrow keys to move line-by-line,
or try the "b" and spacebar keys to skip up and down by full page.
Quit the man
pages by typing "q".
Here,
we can see that our home directory contains mostly sub-directories.
Any names in your output that don't have trailing slashes,
are plain old files.
And note that there is a space between ls
and -F
:
without it,
the shell thinks we're trying to run a command called ls-F
,
which doesn't exist.
According to Wikipedia, the terms argument and parameter mean slightly different things. In practice, however, most people use them interchangeably or inconsistently, so we will too.
We can also use ls
to see the contents of a different directory. Let's take a
look at our Desktop
directory by running ls -F Desktop
,
i.e.,
the command ls
with the arguments -F
and Desktop
.
The second argument --- the one without a leading dash --- tells ls
that
we want a listing of something other than our current working directory:
$ ls -F Desktop
data-shell/
Your output should be a list of all the files and sub-directories on your
Desktop, including the data-shell
directory you downloaded at
the start of the lesson. Take a look at your Desktop to confirm that
your output is accurate.
As you may now see, using a bash shell is strongly dependent on the idea that
your files are organized in an hierarchical file system.
Organizing things hierarchically in this way helps us keep track of our work:
it's possible to put hundreds of files in our home directory,
just as it's possible to pile hundreds of printed papers on our desk,
but it's a self-defeating strategy.
Now that we know the data-shell
directory is located on our Desktop, we
can do two things.
First, we can look at its contents, using the same strategy as before, passing
a directory name to ls
:
$ ls -F Desktop/data-shell
creatures/ molecules/ notes.txt solar.pdf
data/ north-pacific-gyre/ pizza.cfg writing/
Second, we can actually change our location to a different directory, so we are no longer located in our home directory.
The command to change locations is cd
followed by a
directory name to change our working directory.
cd
stands for "change directory",
which is a bit misleading:
the command doesn't change the directory,
it changes the shell's idea of what directory we are in.
Let's say we want to move to the data
directory we saw above. We can
use the following series of commands to get there:
$ cd Desktop
$ cd data-shell
$ cd data
These commands will move us from our home directory onto our Desktop, then into
the data-shell
directory, then into the data
directory. cd
doesn't print anything,
but if we run pwd
after it, we can see that we are now
in /Users/nelle/Desktop/data-shell/data
.
If we run ls
without arguments now,
it lists the contents of /Users/nelle/Desktop/data-shell/data
,
because that's where we now are:
$ pwd
/Users/nelle/Desktop/data-shell/data
$ ls -F
amino-acids.txt elements/ pdb/ salmon.txt
animals.txt morse.txt planets.txt sunspot.txt
We now know how to go down the directory tree: how do we go up? We might try the following:
cd data-shell
-bash: cd: data-shell: No such file or directory
But we get an error! Why is this?
With our methods so far,
cd
can only see sub-directories inside your current directory. There are
different ways to see directories above your current location; we'll start
with the simplest.
There is a shortcut in the shell to move up one directory level that looks like this:
$ cd ..
..
is a special directory name meaning
"the directory containing this one",
or more succinctly,
the parent of the current directory.
Sure enough,
if we run pwd
after running cd ..
, we're back in /Users/nelle/Desktop/data-shell
:
$ pwd
/Users/nelle/Desktop/data-shell
The special directory ..
doesn't usually show up when we run ls
. If we want
to display it, we can give ls
the -a
flag:
$ ls -F -a
./ creatures/ notes.txt
../ data/ pizza.cfg
.bash_profile molecules/ solar.pdf
Desktop/ north-pacific-gyre/ writing/
-a
stands for "show all";
it forces ls
to show us file and directory names that begin with .
,
such as ..
(which, if we're in /Users/nelle
, refers to the /Users
directory)
As you can see,
it also displays another special directory that's just called .
,
which means "the current working directory".
It may seem redundant to have a name for it,
but we'll see some uses for it soon.
Other Hidden Files {.callout}
In addition to the hidden directories
..
and.
, you may also see a file called.bash_profile
. This file usually contains shell configuration settings. You may also see other files and directories beginning with.
. These are usually files and directories that are used to configure different programs on your computer. The prefix.
is used to prevent these configuration files from cluttering the terminal when a standardls
command is used.
The special names
.
and..
don't belong tols
; they are interpreted the same way by every program. For example, if we are in/Users/nelle/data
, the commandls ..
will give us a listing of/Users/nelle
. When the meanings of the parts are the same no matter how they're combined, programmers say they are orthogonal: Orthogonal systems tend to be easier for people to learn because there are fewer special cases and exceptions to keep track of.
These then, are the basic commands for navigating the filesystem on your computer:
pwd
, ls
and cd
. Let's explore some variations on those commands. What happens
if you type cd
on its own, without giving
a directory?
$ cd
How can you check what happened? pwd
gives us the answer!
$ pwd
/Users/nelle
It turns out that cd
without an argument will return you to your home directory,
which is great if you've gotten lost in your own filesystem.
Let's try returning to the data
directory from before. Last time, we used
three commands, but we can actually string together the list of directories
to move to data
in one step:
$ cd Desktop/data-shell/data
Check that we've moved to the right place by running pwd
and ls -F
.
If we want to move up one level from the shell directory, we could use cd ..
. But
there is another way to move to any directory, regardless of your
current location.
So far, when specifying directory names, or even a directory path (as above),
we have been using relative paths. When you use a relative path with a command
like ls
or cd
, it tries to find that location from where we are,
rather than from the root of the file system.
However, it is possible to specify the absolute path to a directory by
including its entire path from the root directory, which is indicated by a
leading slash. The leading /
tells the computer to follow the path from
the root of the file system, so it always refers to exactly one directory,
no matter where we are when we run the command.
This allows us to move to our data-shell
directory from anywhere on
the filesystem (including from inside data
). To find the absolute path
we're looking for, we can use pwd
and then extract the piece we need
to move to data-shell
.
$ pwd
/Users/nelle/Desktop/data-shell/data
$ cd /Users/nelle/Desktop/data-shell
Run pwd
and ls -F
to ensure that we're in the directory we expect.
The shell interprets the character
~
(tilde) at the start of a path to mean "the current user's home directory". For example, if Nelle's home directory is/Users/nelle
, then~/data
is equivalent to/Users/nelle/data
. This only works if it is the first character in the path:here/there/~/elsewhere
is nothere/there/Users/nelle/elsewhere
.Another shortcut is the
-
(dash) character.cd
will translate-
into the previous directory I was in, which is faster than having to remember, then type, the full path. This is a very efficient way of moving back and forth between directories. The difference betweencd ..
andcd -
is that the former brings you up, while the latter brings you back.
Knowing just this much about files and directories,
Nelle is ready to organize the files that the protein assay machine will create.
First,
she creates a directory called north-pacific-gyre
(to remind herself where the data came from).
Inside that,
she creates a directory called 2012-07-03
,
which is the date she started processing the samples.
She used to use names like conference-paper
and revised-results
,
but she found them hard to understand after a couple of years.
(The final straw was when she found herself creating
a directory called revised-revised-results-3
.)
Nelle names her directories "year-month-day", with leading zeroes for months and days, because the shell displays file and directory names in alphabetical order. If she used month names, December would come before July; if she didn't use leading zeroes, November ('11') would come before July ('7'). Similarly, putting the year first means that June 2012 will come before June 2013.
Each of her physical samples is labelled according to her lab's convention
with a unique ten-character ID,
such as "NENE01729A".
This is what she used in her collection log
to record the location, time, depth, and other characteristics of the sample,
so she decides to use it as part of each data file's name.
Since the assay machine's output is plain text,
she will call her files NENE01729A.txt
, NENE01812A.txt
, and so on.
All 1520 files will go into the same directory.
If she is in her home directory, Nelle can see what files she has using the command:
$ ls north-pacific-gyre/2012-07-03/
This is a lot to type, but she can let the shell do most of the work through what is called tab completion. If she types:
$ ls nor
and then presses tab (the tab key on her keyboard), the shell automatically completes the directory name for her:
$ ls north-pacific-gyre/
If she presses tab again,
Bash will add 2012-07-03/
to the command,
since it's the only possible completion.
Pressing tab again does nothing,
since there are 19 possibilities;
pressing tab twice brings up a list of all the files,
and so on.
This is called tab completion,
and we will see it in many other tools as we go on.
Starting from a filesystem location of
/Users/amanda/data/
, which of the following commands could Amanda use to navigate to her home directory, which is/Users/amanda
?
cd .
cd /
cd /home/amanda
cd ../..
cd ~
cd home
cd ~/data/..
cd
cd ..
Using the filesystem diagram below, if
pwd
displays/Users/thing
, what willls ../backup
display?
../backup: No such file or directory
2012-12-01 2013-01-08 2013-01-27
2012-12-01/ 2013-01-08/ 2013-01-27/
original pnas_final pnas_sub
Assuming a directory structure as in the above Figure (File System for Challenge Questions), if
pwd
displays/Users/backup
, and-r
tellsls
to display things in reverse order, what command will display:pnas_sub/ pnas_final/ original/
ls pwd
ls -r -F
ls -r -F /Users/backup
- Either #2 or #3 above, but not #1.
What does the command
ls
do when used with the-l
and-h
arguments?Some of its output is about properties that we do not cover in this lesson (such as file permissions and ownership), but the rest should be useful nevertheless.
The command ls -R lists the contents of directories recursively, i.e., lists their sub-directories, sub-sub-directories, and so on in alphabetical order at each level. The command ls -t lists things by time of last change, with most recently changed files or directories first. In what order does ls -R -t display things? Hint: ls -l uses a long listing format to view timestamps.