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introduction to the unix command-line


UNIX was an operating system developed at AT&T Bell Labs in the 1960s through the 1980s.

GNU/Linux, MacOSX, and Android are all based on ideas and specifications created by UNIX.


UNIX was originally built for large mainframe computers that many people would use at the same time.

terminals and teleprinters

  • teleprinters printed program output on paper
  • terminals displayed output on a CRT monitor

Neither device had processing power of their own.

Connected to the mainframe over cables or by telephone.

teletype legacy: standard input and output

Every program on a UNIX system can read input from the standard input device (stdin) and write to standard output (stdout).

By default, stdin comes from the keyboard and stdout gets "printed" to the graphical display.


The UNIX operating system is a collection of programs, each with a special role:

  • kernel
  • shell
  • utilities


mediate access between user programs and system resources

  • CPU scheduling
  • I/O to computer hardware
  • memory

Programs request resources by making a syscall.


A shell is a computer program that can execute other programs from a text-based interface.

In a text-based interface, you interact with a program completely from the command-line with text commands and text output.

Most modern shells are strongly influenced by the first UNIX shells.

shells through the ages

  • thompson shell - Ken Thompson 1971
  • pwb (mashey) shell - John Mashey 1975
  • bourne shell - Stephen Bourne 1977
  • c shell (csh) - Bill Joy 1978
  • tcsh - Ken Greer and Mike Ellis 1983
  • korn shell - David Korn 1983
  • bourne again shell (bash) - Brian Fox 1987
  • almquist shell (ash) - Kenneth Almquist 1989
  • debian almquist shell (dash) - Herbert Xu 1997

incomplete list of popular or influential shells


Any distribution of UNIX will come with dozens of other programs that perform narrow single-purpose tasks.

The available utilities on a given system vary widely but some utilities are very common.

For example, there is a command to make new directories and another to move files.

why UNIX still matters

  • portable to many kinds of hardware
  • consistent conventions
  • vast software ecosystem
  • text!

places you can find a unix command-line

  • wifi routers
  • dsl and cable modems
  • raspberry pi, beaglebone, nvidia jetson
  • android phones
  • linux laptop or desktop
  • Mac OSX computer
  • web server

You can take your command-line skills with you to all of these platforms and more!

text interface

To remotely access a UNIX system, you can use the same command-line tools and interface that you use locally. You can remotely access devices without a display.

Text is easy to read so you can poke around more easily to figure out what's going on. Many aspects of computer programming involve shuffling text around. UNIX excels at these kinds of tasks.

unix philosophy

The unix philosophy is a set of design principles for how programs relate to each other.

  • each program should do one thing well
  • the output of a program can become the input of another

Unix Programming Environment 1984 Brian Kernighan and Rob Pike

let's learn the command line!

bash is a popular shell for UNIX-like systems.

Open up a bash shell for the next sections to follow along.

If you're not sure which shell you're in, type:

echo $SHELL

If you're in bash, you sould see something like:


list files

You can use the ls command to show all the files in the current directory.

Type ls and you should see something like:

~ $ ls
doc  media  notes.txt  projects


By default, ls lists files from the current directory.

You can list files from another directory by giving ls an argument. An argument is just another piece of text after the ls.

For example, to list the files in / (the root) we can do:

~ $ ls /
bin   etc         lib         media  proc  sbin     sys  var
boot  home        lib64       mnt    root  selinux  tmp  vmlinuz
dev   initrd.img  lost+found  opt    run   srv      usr

In this example, ls is the command and / is the argument.

Commands can have multiple arguments separated by spaces or no arguments.

print the current directory

To display the current directory, you can use the pwd command:

$ pwd

pwd stands for print working directory.

change directory

To change the current working directory, use the cd command. The cd command takes a single argument: the directory to move to.

After changing the current directory, list the files again with ls.

~ $ ls
doc  media  notes.txt  projects
~ $ cd media
~/media $ ls
3d  audio  avatars  vector  warp
~/media $ cd warp
~/media/warp $ ls*                      mac_startup.mp3  mac_warped.mp3  watch.js
Mac Startup-i9qOJqNjalE.mp4  mac_startup.wav  mac_warp.mp3

special directories

There are some special directories:

  • .. - the parent directory
  • . - the current directory
  • ~ - your home directory

To navigate back up to the parent directory, do cd ...

~/media/warp $ cd ..
~/media $ 

You can also list the parent directory without changing the current directory by doing ls ..:

~/media $ ls ..
doc  notes.txt  media  projects

You can add paths after .. too:

~/media $ ls ../projects/workshops
computers.markdown  unix.markdown

Or ls . is the same as ls:

~/media $ ls .
3d  audio  avatars  vector  warp

Jump back to your home directory at any time by typing cd with no arguments.


cat was originally written to concatenate all the files from its arguments:

~/doc $ cat beep.txt boop.txt

but it also a handy way to display single text files on the command-line:

~/doc $ cat beep.txt


Copy a file to another directory or file name. You can copy a single file to make a new duplicate file:

~/doc $ ls

We can copy a.txt to b.txt:

~/doc $ cp a.txt b.txt

Now there are 2 identical files, a.txt and b.txt:

~/doc $ ls
a.txt  b.txt


You can copy a file or a directory too. Here we'll copy a.txt to the directory called wow:

~/doc $ mkdir wow
~/doc $ ls
a.txt  b.txt  wow
~/doc $ cp a.txt wow

now wow/ has an a.txt file in it:

~/doc $ ls wow

You can copy to a specific destination file:

~/doc $ cp a.txt wow/whatever.txt
~/doc $ ls wow
a.txt  whatever.txt

cp (multiple files)

You can even copy multiple files at once to a new place:

~/doc $ mkdir xyz
~/doc $ cp a.txt b.txt xyz/
~/doc $ ls xyz
a.txt  b.txt

The last argument is the destination file or directory and the other arguments are the source files.

cp -r

If you have a directory full of files and directories you want to copy to a new place, you can use cp -r to recursively copy a directory and all its subdirectories to a new location:

~/doc $ mkdir xyz/123
~/doc $ cp a.txt xyz/123/
~/doc $ cp -r xyz newxyz
~/doc $ ls newxyz/
123  a.txt  b.txt
~/doc $ ls newxyz/123

Likewise, there is a -R for the ls command that recursively lists subdirectories:

~/doc $ ls -R newxyz
123  a.txt  b.txt



The mv command is used to rename and overwrite files and directories.

To rename a file, set the first argument to the original file name and the second argument to the new file name or destination directory.

We can rename a.txt to be pigeon.txt:

~/doc $ mv a.txt pigeon.txt
~/doc $ ls
b.txt  newxyz  pigeon.txt  xyz

Or we can move a file to a new directory:

~/doc $ mv pigeon.txt xyz
~/doc $ ls xyz
123  a.txt  b.txt  pigeon.txt

We can rename directories just the same as files:

~/doc $ mv xyz woo
~/doc $ ls
b.txt  newxyz  woo
~/doc $ ls woo
123  a.txt  b.txt  pigeon.txt


To make a new directory, just execute the mkdir command with a list of new directory names to make as arguments:

$ mkdir hooray

and now a new directory called hooray exists.

You can create multiple directories at once:

$ mkdir one two

and now two new directories, one and two, exist.

mkdir -p

Suppose we want to make the following nested directory structure:


Instead of doing:

~ $ mkdir foo foo/bar foo/bar/baz foo/bar/qrs

We can just do:

~/doc $ mkdir -p foo/bar/baz foo/bar/qrs

and the necessary parent directories foo/ and foo/bar/ will be created automatically.

brace expansion

There is a handy syntax built into bash for expanding patterns that would be repetitive to type out by hand.

Instead of doing something like:

~/doc $ mkdir -p foo/bar/baz foo/bar/qrs

we can use a list of items between curly braces:

~/doc $ mkdir -p foo/bar/{baz,qrs}

which expands to the same command as before.

To prove this you can use echo to see what the expansion is:

~ $ echo mkdir -p foo/bar/{baz,qrs}
mkdir -p foo/bar/baz foo/bar/qrs

The items that a brace expansion generates are separated by spaces as if you had typed out those words by hand.

You can have as many items as you like in a list:

~ $ echo robot-{one,two,three,four}-x
robot-one-x robot-two-x robot-three-x robot-four-x

With brace expansions, you can have multiple expansions:

~/doc $ echo robot/{c3po,r2d2}/{sound.mp3,info.txt}
robot/c3po/sound.mp3 robot/c3po/info.txt robot/r2d2/sound.mp3 robot/r2d2/info.txt

You can even nest the expansions!

~/doc $ echo x-{wing,b{ee,oo}p}
x-wing x-beep x-boop

brace expansion sequences

It can be tedious to type out numerical lists by hand.

Brace expansions can help with that:

~/doc $ echo wow{1..10}
wow1 wow2 wow3 wow4 wow5 wow6 wow7 wow8 wow9 wow10

and you can even specify an amount to skip:

~/doc $ echo img{0..100..10}
img0 img10 img20 img30 img40 img50 img60 img70 img80 img90 img100


To remove a file, just do:

~/doc $ rm b.txt

You can remove multiple files at once:

~/doc $ rm newxyz/a.txt newxyz/b.txt

and you can remove entire directories including subdirectories with:

~/doc $ rm -r newxyz

Be very careful with -r. You might accidentally delete much more than you meant to delete!


The wc command computes the number of lines, words, and bytes in a file:

~ $ wc notes.txt
 3  7 35 /home/substack/notes.txt

To see each field independently, you can use different options: arguments that start with a - or -- followed by a letter or word.

To get just the word counts, we can use -w:

~ $ wc -w notes.txt
7 notes.txt

To get just the number of lines in a file, use -l:

~ $ wc -l notes.txt
3 notes.txt

To get just the number of bytes in a file, use -c:

~ $ wc -c notes.txt
35 notes.txt

If you don't specify a file, wc will read from stdin. Type Ctrl+D (^D) to end the input.

~ $ wc -l


All of these command options are a lot to remember!

You can pull up documentation at any time in your shell by typing man foo for any command foo.

For example to read up on all the options you can give to the wc command, do:

~ $ man wc

The help page will open up in your $PAGER. Type q to exit back to your shell.

more on options

Options (also called flags or switches) are special arguments that start with a - or -- followed by a letter or word.

Generally speaking, they are distinct from other arguments in that their order usually doesn't matter. For example:

grep -i wow

is the same as

grep wow -i

where -i just informs the grep command to perform a case-insensitive search.

Sometimes options have a value that follows:

head -n 1

means that -n has the value 1.

Sometimes you can omit the space:

head -n1

but each program individually decides how to interpret its arguments.

absolute and relative paths

Paths that start with . or .. are relative paths. Paths that start with / are absolute paths.

Relative paths are resolved according to the current working directory:

~/doc $ cat ../media/warp/
youtube-dl ''
ffmpeg -i *.mp4 -vn mac_startup.wav
sox mac_startup.wav mac_warp.mp3 chorus 0.6 0.9 25 0.9 1 8 -s \
  echos 0.8 0.7 40 0.25 63 0.3 phaser 1 0.7 3 0.7 0.5 -t
  play mac_startup.wav

Absolute paths are the same no matter what the current working directory is:

~/projects/workshops $ cat /etc/issue
Debian GNU/Linux 7 \n \l


The echo command just prints text from its arguments:

~ $ echo wow cool
wow cool

This is not very useful by itself, but becomes useful when combined with redirects and pipes.

write to a file

Using the > character, you can write the output of a command to a file.

For example, to make a new file greetings.txt with the contents "ahoy thar", we can do:

~ $ echo ahoy thar > greetings.txt

and to print the contents of greetings.txt, use cat:

~ $ cat greetings.txt
ahoy thar

You can redirect the output of any program to a file:

~ $ ls / > list.txt

append to a file

The > redirect operator will overwrite a file with new contents if it already exists.

There is a >> operator that appends to the end of a file if it already exists:

~ $ echo wow > cool.txt
~ $ ls >> cool.txt
~ $ cat cool.txt

read from a file

You can read a file into the stdin of a program with <.

Remember that if wc doesn't get a file as an argument, it will read from stdin. We can load a file in to wc with < instead:

~ $ wc -c < notes.txt


The last but most important kind of redirect is the pipe operator |.

With | you can feed the output of one program to the input of the next.

For example, the ls -1 command will list files, one per line, to stdout. The wc -l command, meanwhile, will count the number of lines.

By piping these two programs together, we can count the number of files and subdirectories in a directory:

~ $ ls -1 | wc -l

and indeed, there are five files and subdirectories in this directory:

~ $ ls -1

You can chain together commands with | as much as you like.

Here's an example using two new commands curl and sed that will fetch Moby Dick from Project Gutenberg and count the number of occurences of "whale", case-insensitive:

~ $ curl -s
| sed -r 's/\s+/\n/g' | grep -i whale | wc -l

We can even save that number of a file. Just add > whale_count.txt to the end of the pipeline:

~ $ curl -s |
sed -r 's/\s+/\n/g' | grep -i whale | wc -l > whalecount.txt

pipeline breakdown: curl

Here's a breakdown of each part of the pipeline and what it does:

curl -s

fetches Moby Dick from Project Gutenberg and prints the results to stdout.

pipeline breakdown: sed

sed -r 's/\s+/\n/g'

converts all whitespace (tabs, spaces, newlines) into newlines.

This means that each word gets its own line:

~ $ echo one two three beep boop | sed -r 's/\s+/\n/g'

pipeline breakdown: grep

grep -i whale

filters the output so that only lines that contain the word "whale" will be shown. -i makes the search case-insensitive.

For example if we have a file tale.txt:


then our grep command will show:

~ $ grep -i whale < tale.txt

pipeline breakdown: wc -l

wc -l

counts the number of lines from stdin and prints the result.


The head command prints the first part of a file.

If a file isn't given, head reads from stdin.

Read the first 3 lines of a file with head -n3:

$ head -n3 mobydick.txt 
The Project Gutenberg EBook of Moby Dick; or The Whale, by Herman Melville

This eBook is for the use of anyone anywhere at no cost and with

Read the first 20 bytes of a file with head -c20:

~ $ head -c20 mobydick.txt 
The Project Guten


The tail command prints the last part of a file.

If a file isn't given, tail reads from stdin.

Read the last 4 lines of a file with tail -n4:

~ $ tail -n4 mobydick.txt 
This Web site includes information about Project Gutenberg-tm,
including how to make donations to the Project Gutenberg Literary
Archive Foundation, how to help produce our new eBooks, and how to
subscribe to our email newsletter to hear about new eBooks.

Read the last 9 bytes of a file with tail -c9:

~ $ tail -c9 mobydick.txt 


If you need a handy text calendar, just type cal:

~ $ cal
   December 2014      
Su Mo Tu We Th Fr Sa  
    1  2  3  4  5  6  
 7  8  9 10 11 12 13  
14 15 16 17 18 19 20  
21 22 23 24 25 26 27  
28 29 30 31           

You can show the current, previous, and next month:

~ $ cal -3
   November 2014         December 2014          January 2015      
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  
                   1      1  2  3  4  5  6               1  2  3  
 2  3  4  5  6  7  8   7  8  9 10 11 12 13   4  5  6  7  8  9 10  
 9 10 11 12 13 14 15  14 15 16 17 18 19 20  11 12 13 14 15 16 17  
16 17 18 19 20 21 22  21 22 23 24 25 26 27  18 19 20 21 22 23 24  
23 24 25 26 27 28 29  28 29 30 31           25 26 27 28 29 30 31  

Or you can show a whole year:

~ $ cal 2015
      January               February               March          
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  
             1  2  3   1  2  3  4  5  6  7   1  2  3  4  5  6  7  
 4  5  6  7  8  9 10   8  9 10 11 12 13 14   8  9 10 11 12 13 14  
11 12 13 14 15 16 17  15 16 17 18 19 20 21  15 16 17 18 19 20 21  
18 19 20 21 22 23 24  22 23 24 25 26 27 28  22 23 24 25 26 27 28  
25 26 27 28 29 30 31                        29 30 31              

       April                  May                   June          
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  
          1  2  3  4                  1  2      1  2  3  4  5  6  
 5  6  7  8  9 10 11   3  4  5  6  7  8  9   7  8  9 10 11 12 13  
12 13 14 15 16 17 18  10 11 12 13 14 15 16  14 15 16 17 18 19 20  
19 20 21 22 23 24 25  17 18 19 20 21 22 23  21 22 23 24 25 26 27  
26 27 28 29 30        24 25 26 27 28 29 30  28 29 30              

        July                 August              September        
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  
          1  2  3  4                     1         1  2  3  4  5  
 5  6  7  8  9 10 11   2  3  4  5  6  7  8   6  7  8  9 10 11 12  
12 13 14 15 16 17 18   9 10 11 12 13 14 15  13 14 15 16 17 18 19  
19 20 21 22 23 24 25  16 17 18 19 20 21 22  20 21 22 23 24 25 26  
26 27 28 29 30 31     23 24 25 26 27 28 29  27 28 29 30           
                      30 31                                       

      October               November              December        
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  
             1  2  3   1  2  3  4  5  6  7         1  2  3  4  5  
 4  5  6  7  8  9 10   8  9 10 11 12 13 14   6  7  8  9 10 11 12  
11 12 13 14 15 16 17  15 16 17 18 19 20 21  13 14 15 16 17 18 19  
18 19 20 21 22 23 24  22 23 24 25 26 27 28  20 21 22 23 24 25 26  
25 26 27 28 29 30 31  29 30                 27 28 29 30 31        


To print the date, just do:

~ $ date
Sat Dec 27 20:43:13 PST 2014

You can format the date however you like:

~ $ date +'%Y-%m-%d %H:%M:%S'
2014-12-27 20:45:07

Check out the manual page (man date) for more info about what options are available for date strings.


Sometimes it's handy to break long lines into shorter lines.

We can use the fold command to break some text at 30 characters:

~ $ head -n250 mobydick.txt | tail -n3 | fold -w 30
can see a whale, for the first
 discoverer has a ducat for hi
s pains....
I was told of a whale taken ne
ar Shetland, that had above a 
barrel of
herrings in his belly.... One 
of our harpooneers told me tha
t he caught

or to break on spaces instead, use -s:

~ $ head -n250 mobydick.txt | tail -n3 | fold -sw 30
can see a whale, for the 
first discoverer has a ducat 
for his pains....
I was told of a whale taken 
near Shetland, that had above 
a barrel of
herrings in his belly.... One 
of our harpooneers told me 
that he caught


curl is a handy little tool for making HTTP requests.

Here's a simple snippet to fetch my most recent RSA public key from github, wrapping the output to 75 character lines:

~ $ curl -s | tail -n1 | fold -w75


You can search for patterns in files or from stdin with the grep command.

The first argument is the pattern to search for as a regular expression.

Regular expressions are a language for pattern matching.

Here we can search for all lines matching "whaling" or "fishing":

~ $ grep -iE '(whal|fish)ing' mobydick.txt | tail -n5
Equatorial fishing-ground, and in the deep darkness that goes before the
the whaling season? See, Flask, only see how pale he looks--pale in the
preliminary cruise, Ahab,--all other whaling waters swept--seemed to
fixed upon her broad beams, called shears, which, in some whaling-ships,
years of continual whaling! forty years of privation, and peril, and

Check out the other workshop about regular expressions to learn more!


Sometimes it's useful to include the output of a program in the arguments list of another.

For example with the date command we can print the current year:

date +%Y

and we can use this value in a message with echo:

~ $ Greetings from the year `date +%Y`.
Greetings from the year 2014.


With $((...)) expressions, you can do simple arithmetic on the command line!

~ $ echo $((4*5+1))

I wouldn't go overboard with this feature, but it's handy sometimes.

environment variables

Environment variables are defined by the shell and shell scripts.

To list the current environment variables, type export:

~ $ export
declare -x DISPLAY=":0"
declare -x HOME="/home/substack"
declare -x HUSHLOGIN="FALSE"
declare -x LANG="en_US.UTF-8"
declare -x LD_LIBRARY_PATH="/home/substack/prefix/lib:/usr/local/lib:/usr/lib/x86_64-linux-gnu:/usr/lib:/lib64:/lib"
declare -x LIBGL_DRIVERS_PATH="/usr/lib/i386-linux-gnu/dri:/usr/lib/x86_64-linux-gnu/dri"
declare -x LOGNAME="substack"
declare -x MAIL="/var/mail/substack"
declare -x OLDPWD="/home/substack/projects/workshops"
declare -x PATH="/home/substack/prefix/bin:/usr/local/bin:/usr/bin:/bin:/sbin:/usr/sbin:/usr/local/games:/usr/games"
declare -x PREFIX="/home/substack/prefix"
declare -x PWD="/home/substack"
declare -x ROXTERM_ID="0x43962f0"
declare -x ROXTERM_NUM="15"
declare -x ROXTERM_PID="2521"
declare -x SHELL="/bin/bash"
declare -x SHLVL="3"
declare -x TERM="xterm"
declare -x USER="substack"
declare -x WINDOWID="8684328"
declare -x WINDOWPATH="7"
declare -x XAUTHORITY="/home/substack/.Xauthority"

You can use any environment variable by refering to its $NAME.

For example to print the value of $HOME do:

~ $ echo $HOME

You can use environment variables in any command:

~ $ ls /home/$USER
doc  media  notes.txt  projects

To define your own environment variable, just put its name followed by an equal sign (with no spaces) followed by its value:

~ $ ANIMAL=cats
~ $ echo $ANIMAL

Environment variables are almost always capitalized to distinguish them from variables in shell scripts but lower-case variables work too.


If you want to use characters like < or > in the arguments to a program, you will need to use quotes so that the shell doesn't try to interpret them.

For example, to echo the string <b>wow</b> we can use single quotes:

~ $ echo '<b>wow</b>'

Double quotes are similar but environment variables and backticks will be interpolated in-place (replaced with their value):

~ $ echo "There's no place like $HOME."
There's no place like /home/substack.
~ $ echo "So long `date +%Y`..."
So long 2014...
~ $ echo "So long `date +%Y`... next stop $((`date +%Y`+1))"'!'
So long 2014... next stop 2015!

You will also need to use quotes if one of the arguments you want to give has a whitespace character in it, because whitespace is otherwise used to split arguments.


Whenever you find yourself typing the same sequence of commands several times, consider making a script!

Just put the commands you would normally type into a file and add #!/bin/bash to the top of the file:

mkdir wow
cd wow
echo "yay" > zing.txt

Now make your script file executable:

~ $ chmod +x

And now you can do:

~ $ ./

to run the commands from your file!

script arguments

When you execute a script with arguments on the command-line, special environment variables $1, $2, $3... will be defined for each argument.

For example, if our script is:

echo first=$1
echo second=$2

Then we print out the first and second arguments:

~ $ ./ beep boop

There is a special variable $* that contains all the arguments separated by spaces. For a script of:

echo The arguments are: $*

And now we can get at all the arguments in one place:

~ $ ./ cats dogs ducks lizards
The arguments are: cats dogs ducks lizards


There is a special environment variable called $PATH:

~ $ echo $PATH

This variable contains a list of places separated by : that bash will look when you type a command.

If you put an executable file in one of the directories in your $PATH, you can make your own commands without needing to specify a relative or absolute path!

/usr/local/bin is the customary place to put system-specific scripts that are not managed by your system distribution. If you do:

~ $ sudo cp /usr/local/bin

Then you'll be able to type from any directory on the command-line!

You can rename that command that you type by renaming the file:

~ $ sudo mv /usr/local/bin/{,whatever}

and now the command is called whatever.


There is a special bash script called ~/.bashrc that runs whenever you start bash. You can edit this file to set up aliases, environment variables, and run commands when you start a new terminal session.

At the bottom of your ~/.bashrc file, try adding a command:

echo Greetings $USER. Nice to see you again.

Now open a new terminal and you should see a friendly new message!


Each file on a UNIX system belongs to a user and a group.

users are accounts on the system, like the one you log in with. groups are collections of users.

To see what groups you belong to, just type groups:

~ $ groups
substack cdrom floppy audio dip video plugdev

To inspect the permissions on a file, use ls -l:

~/doc $ ls -l b.txt
-rw-r--r-- 1 substack whatever 14 Dec 28 00:29 b.txt

Here we can see that the file b.txt is owned by the user substack and the group whatever. There's also this part on the left:


This string describes the permissions of the file.

The first character is reserved for some fancy uses, but after that there are 3 groups of 3 characters:


Each character describes a permission: (r)ead, (w)rite, and e(x)ecute. A - in place of one of those letters means the permission is not available.

If the e(x)ecute bit is enabled on a file for a user, it means the user can execute the file.

If the e(x)ecute bit is enabled on a directory for a user, it means the user can list the files in that directory.

  • The first rwx says what the owner can do.
  • The second rwx says what users in the group can do.
  • The third rwx says what everyone else can do.

These three categories are called user (u), group (g), and other (o).


To change the permissions on a file, first figure out which capabilities you want to grant or revoke (rwx) from which categories of users (ugo).

To allow the owner of a file to execute a script you can do:

~ $ chmod u+x

which is the same as:

~ $ chmod +x

because the u is implied if not specified.

You can also revoke permissions with a -. To make it so that other users can't write to a file:

~ $ chmod o-w wow.txt

You can grant and revoke permissions at the same time. Here we're adding read and execute permissions to the user while simultaneously revoking read and write from the group:

~ $ chmod u+rxg-rw

You can change the owner of a file with chown and the group with chgrp.

job control

Bash is built to handle multiple programs running in parallel.

time cat

Type time cat and then hit ctrl-c before one second, as close as possible without going over:

$ time cat

real    0m0.920s
user    0m0.004s
sys 0m0.000s


Terminate a process in the foreground.


Put a process in the background.

fg JOB

Move a process from the background to the foreground by its JOB.

~ $ cat
[1]+  Stopped                 cat
~ $ echo wow
~ $ fg %1

job syntax

When you background a process with ctrl-z, the shell prints a message with [N]. N is the job id. Use %N to refer to a particular job or:

  • %% - the most recent job


Another way to background a process is to use &:

$ ~ node &
[1] 29877

The job id of node is 1 and the process id is 29877. Job ids are local to a shell session, but process ids are global across the entire system.

~ $ perl &
[1] 29870
~ $ pgrep perl
~ $ kill %1
[1]+  Terminated              perl


Search for a process by its name.

kill ID

Kill a process by its process or job id.


You can use screen to run command-line programs and keep them running, even when you go away.

install screen

$ sudo apt-get install screen

create a new named screen

$ screen -S website

list screens

$ screen -list

connect to a screen

$ screen -x website

detach from a screen

From inside of a screen, press CTRL+A then d.

create a new window inside screen


go to the next window


go to the previous window


close a window

Just type exit to close a window.

irc from the command-line

Install irssi:

$ sudo apt-get install irssi

Then create a screen for irc:

$ screen -S irc

Then in a screen, you can run irssi to use irc from the command line.

  • /nick robowizard - set your nickname on the server
  • /connect - connect to
  • /join #cyberwizard - join the channel called cyberwizard
  • ESC+N or /win N - to jump to the window at number N

Once you're in a channel, type text like usual. CTRL+A d to detach and screen -x irc to resume.

run a web server

Make a web server.js:

var http = require('http');
var server = http.createServer(function (req, res) {
    res.end("YOU'RE A WIZARD.\n");

now run your server with node from inside a screen:

$ node server.js

then detach the screen with CTRL+A d.