Lesson 4: Shell Navigation

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The Shell

A shell is a text-based user-interface for a computer.
sea shell

Shells are the programs which allows you use a computer via text-based commands. Not to be confused with a terminal, which is the environment a shell runs in (GNOME terminal, tty1, etc). A shell is run in a terminal, a terminal is the widow you use the shell in.

Shell Examples

As with all things in Open Source, there are many options to choose from when deciding on a shell. Below is a short list of popular choices. Each shell has it’s positives and negatives. If you don’t have an opinion you should probably use what comes by default when you open your terminal.

sh Required by all POSIX Operating Systems.
bash Default on most GNU/Linux-based Operating Systems.
csh Default shell on most BSD (Unix) based Operating Systems
zsh The hip new shell on the block.
fish Yet another hip new shell on the block.

Basic Shell Commands

The most basic job of a shell is to enable the user to execute programs and navigate the OS. Below are a few of the commands you will use to do this.

# Prints the current working directory (where you are)
$ pwd
# Prints the contents of the current working directory
$ ls
# Navigates to a new directory.
$ cd <path/to/other/directory>
# Prints a string to the screen.
$ echo "some thing $AND_VARS"
# Prints the contents of a file(s) to the screen.
$ cat foo.txt bax.txt
# Searches `file.txt` for the string `foo`
$ grep foo file.txt
# Prints a file to the screen so you can arrow up/down.
$ less file.txt
# Prints environment variables to the screen.
$ env
# Prints out current user
$ whoami
# When in doubt, always type help.
$ help

Shell Scripts

Shell scripts are files which contain command run on your behalf by the shell.

Each shell is also a programming langauge, so you can write logic, loops, functions, and declare local variables in your scripts.

about_me.sh

#!/bin/sh
if [ $(whoami) == "root" ]; then
  echo "You're root!"
else
  echo "Your username is $(whoami)"
  echo "Your home-directory is $HOME"
  echo "Your current directory is $PWD"
  echo "Your computer's host-name is $HOSTNAME"
fi

Invoke with:

# Tell Linux that this can be run as a program
$ chmod +x about_me.sh
# Invoke the script.
$ ./about_me.sh
Your username is dobc
Your home-directory is /home/dobc
Your current directory is /home/dobc
Your computer's host-name is dobc

Useful Symbols

$ grep 'searchstring' files/* | less

$ true || echo 'never gets here'
$ false && echo 'never gets here'

$ echo 'this now an error message' 1>&2 | grep -v error
this is now an error message

!$ # last argument to last command
$ cat /dir
cat: /dir/: Is a directory
$ cd !$
cd /dir
$ pwd
/dir

More Useful Symbols

$ for x in 1 2 3; do echo $x; done # Use seq for longer sequences
1
2
3

$ var='this is a var'; echo ${var//this is } # Deletes 'this is '
a var

$ ls -l `which bash`
-rwxr-xr-x 1 root root 1029624 Nov 12 15:08 /bin/bash

Combining These Together

$ set -a blocks
$ blocks="10.0.0.0/24"
$ set -a ips
$ ips=`fping -g 10.0.0.0/24 2>&1 | grep unreachable | tr \\  \\n`
$ for ip in $ips; do
$   nmap -p 22 $ip && ips=`echo ${ips//$ip} \
    | tr -s \\n`
$ done
$ echo $ips

Function Definitions

name () {
# code goes here
}

Internal Variables

You should know the following:

Variable Meaning
$* All arguments passed
$? Return code of last command run
"$@" All arguments passed as a list
$CDPATH Colon-delimited list of places to look for dirs
$HOME Location of user homedir
$IFS Internal Field Seperator
$OLDPWD Previous PWD

Internal Variables

Variable Meaning
$PATH Colon-delimited list of places to find executables
$PWD Present Working Directory
$SHELL Path to running shell
$UID User ID
$USER Username

You should also read the EXPANSION section of the bash man page.

File Paths

File Paths describe the location of a file in the directory tree.

Files are an important part of using an OS through the shell. Being able to find, navigate to, and use files is important. The first part of doing that is understanding some basics about directories.

. The current directory
.. The parent directory
~ Alias for your home directory
/ Separates directories: one_dir/another_dir/last_dir Alone, or at the start of a path, it is the root directory. 
$ tree -F
.
├── bar/
│   ├── one/
│   └── two
├── baz/
└── foo/
    └── a/
        └── b
5 directories, 2 files

Special Characters

When using a shell (and in general) there are special characters. These serve a functional purpose (carry special meaning) in addition to being a character, so you can’t use them willy-nilly. You should know what those are and what they mean.

Wildcard (*)

Used as a stand-in for any character(s).

Example: cat *.log cats all files in the current working directory ending in .log.

End of line ($)
Used to specify the end of a regex. We’ll cover what regex is later.
Curl braces ({ })

Used to specify a set.

Example: ls {foo,bar,baz}ley-thing expands to ls fooley-thing barley-thing bazley-thing

Escape special characters (treat them as normal characters) with the escape character (\).

Type Less, Tab More

Pressing the tab key auto-completes a command, file-path, or argument in your shell.

Pressing tab multiple times completes the command to the best of the shells ability and then lists the possible completions (if there are any).

$ ls b    # <tab>
$ ls ba   # <tab>
bar_thing/ baz_thing/
$ ls bar  # <tab>
$ ls bar_thing

Text Editor: Nano

We are going to with a quick tangent by learning to use the terminal-based text editor Nano.

nano in action
  • User types like normal.
  • Arrow keys used to to navigate the cursor.
  • ^ + <key> Commands (control + key)

Nano is a great terminal text editor to start with. Later in your career you may start using emacs or vi/vim but to start with nano is familiar, easy to use, and gets the job done.

To use nano simply execute it like any other command in the terminal.

$ nano              # Open with empty file
$ nano <file_name>  # Edit a specific file

This editor is almost exactly like any word processor or plain-text editor except that you don’t have a mouse – only keyboard shortcuts. The instruction bar at the bottom of the screen is explains all of the key-bindings from saving, to exiting, to cut and pasting.

Bash Hello World

Using nano create a file called hello_world.sh and put the following in it:

#!/bin/bash
# declare STRING variable
STRING="Hello World"
# print variable on a screen
echo $STRING

Now make the script executable using chmod and run the script. What does it do?

$ chmod +x hello_world.sh
$ ./hello_world.sh
Hello World

Passing arguments to the bash script

When you pass arguments to a bash script, you can reference them inside of the script using $1, $2, etc. This means if you do something like ‘foo.sh bar‘, $1 will return bar. You can also use $@ to reference all arguments.

For example:

#!/bin/bash
echo $1   # prints argument #1 given to script
echo $@   # prints all arguments given to script

Reading User Input

You can also take input using the read command which then stores it in a variable. If you pass read ‘-a‘, it puts the input into a bash array.

For example:

#!/bin/bash
echo -e "Tell me a word: \c"
read word
echo "Your word is $word"

$ ./read.sh
Tell me a word: foo
Your word is foo

Simple Bash if/else statement

Bash conditionals use if, else, then and fi operators. You can compare strings, files and even command output. An example:

#!/bin/bash
if [ "$1" == "foo" ] ; then
  echo "You said $1"
else
  echo "You did not say foo"
fi

$ ./sayfoo.sh foo
You said foo
$ ./sayfoo.sh bar
You did not say foo

Exercises: Bash

  • Using the tar command, write a script named backup.sh which backs up the dobc user home directory into a file /tmp/dobc-backup.tar.gz . Hint: use the man page for tar or type ‘tar -h’.
  • Create a script called args.sh that takes three arguments and prints all of the args and then prints them in reverse using echo.
  • Create a script named input.sh which takes input for three args and then prints them in a sentence (of your choosing).
  • Create a script named ifelse.sh which takes two arguments. If both arguments match, print "Yay, they match!", if they don’t, then print "Boo, they don't match :(".

Exercise Answer Key

Simple Backup script

#!/bin/bash
# The flags for tar do the following:
# v - verbose
# c - compress
# z - use gzip
# f - output to file
tar -vczf /tmp/dobc-backup.tar.gz /home/dobc

$ ./backup.sh
tar: Removing leading `/' from member names
/home/dobc/
/home/dobc/backup.sh
/home/dobc/hello_world.sh
/home/dobc/.bash_profile
/home/dobc/.bashrc
/home/dobc/.bash_logout

Bonus: How could you list the contents of the file?

Passing arguments to the bash script

#!/bin/bash
echo $@
echo $3 $2 $1

$ chmod +x args.sh
$ ./args.sh DOBC is awesome
DOBC is awesome
awesome is DOBC

Bonus #1: What happens if you give the script nothing? Bonus #2: What happens if you give it the string “DOBC is awesome” with quotes?

Reading User Input

#!/bin/bash
echo -e "Tell me a noun: \c"
read noun
echo -e "Tell me a verb: \c"
read verb
echo -e "Tell me an adjective: \c"
read adj
echo "I plan to $verb a $adj $noun"

$ ./input.sh
Tell me a noun: apple
Tell me a verb: eat
Tell me an adjective: large
I plan to eat a large apple

Simple Bash if/else statement

#!/bin/bash
if [ "$1" == "$2" ] ; then
  echo "Yay, they match!"
else
  echo "Boo, they don't match :("
fi

$ ./ifelse.sh foo foo
Yay, they match!
$ ./ifelse.sh foo bar
Boo, they don't match :(

Further Reading

BASH Programming - Introduction HOW-TO
A free online resource of learning bash programming. Covers some concepts we’ll get to later in DOBC, but a good resource to have on hand.
Advanced Bash-Scripting Guide
An in-depth exploration of the art of shell scripting. Covers more advanced concepts with Bash.
Running rm -rf / on Linux
This video demonstrates what happens when you ‘delete your hard-drive’ on Linux. A fun watch!

Next: Lesson 5: Users, Groups, Permissions