Shell environment

Users, Groups, and Processes

Linux was created with the idea that multiple users can be using a computer at the same.

• The whoami command prints out the name of the currently logged in user.
• The users command prints out all the users that exist on the computer.
• The pinky command prints out more information about all the users.
• The groups command prints out all the groups you are a part of.
• The ps command prints out the processes you are currently running.
• The top command shows a constantly updating list of processes running and who is running them along with other helpful information such as CPU or memory usage, time running, etc.

Super Users are system administrators who have permissions to do essentially anything on a system including provisioning permissions for normal users. To run a command as a super user use the sudo (“super user do”) command which gives you administrator privileges on that command you type after it.

The /usr directory stands for Universal System Resources which contains the /bin or binary directory.

You can reference your last command in the shell by typing !! which means that you can type sudo !! when you want to give the command you ran earlier (that may have failed) super user permissions.

bashrc, bash_profile, and $PATH

.bash_profile is a shell script that runs on login to the shell. The bash command launches a new login shell. .bashrc file is similar to the .bash_profile file, but instead of only running on every login shell, it runs on every new shell, login or not. When making a change to either .bashrc or .bash_profile, instead of reloading the shell (logging in and out again), run source .bashrc which runs the commands in the file line-by-line.

$PATH is a special alias (variable name) that specifies directories for executable programs such as /bin. Programs that read the $PATH alias expect : delimiters. Make sure to leave no spaces on either side of the equal sign and quotes around the command.

Permissions

The permissions of each file and directory in the current directory appear in the first column of the output from the ls -l command in the format d rwx rwx rwx:

• The first character is either a d if the item is a directory, or nothing if it isn’t.
• The next three groups are in the order of owner (u), group (g), and others (o).
  • Read permissions are for operations such as the cat command, which only involves reading data.
  • Write permissions are for operations like vim or emacs for making changes to the file/directory.
  • Execute permissions are for running files that can be executed such as bash shell scripts.
• Each of these groups can have up to three different permissions: read (r), write (w), and execute (x).

The chmod command can change permissions.

• chmod u+x [filename] gives the owner execute permissions.
• chmod go-rw [filename] takes away the permission to read and write from the group and others.
• The other method is by using the NNN octal codes.
  • Each is a group(owner, group, others) and the number can be from 0-7.
  • +4 is read, +2 is write and +1 is for execution.
  • chmod 444 [filename] gives read permissions to every group.
  • chmod 600 [filename] gives read and write permissions to only the owner.

Directory permissions work differently.

• Read means whether or not they can run ls on the directory.
• Write determines if you can add, delete, or modify files in the directory.
• Execute is whether someone can cd into a particular directory.

The umask command sets the default permissions for newly created files. umask 0022 gives the owner full permissions and the group and others only read and execute permissions.

Our first script

A shell script is a series of shell commands run from top to bottom. The most basic one looks like this:

#!/bin/bash

echo "Hello World!"

The #!bin/bash is called a “shebang”. This tells your computer which “shell” should run the program; for this class, we’ll use bash.

Subsequent lines are run one after another. Note that if your lines cd to new directories or create new variables, that doesn’t carry over to the “parent shell”.

#!/bin/bash

mkdir bestfolder
cd bestfolder
touch bestfile

For example, the above shell script:

• will create a new folder called bestfolder
• will create a new file called bestfile
• will not move the shell that ran this script to bestfolder

Variables

To create variables, use the =:

color="red"

• don’t include spaces between the = symbol; color = "red" will not work
• by default, we use ". they are technically optional, but helpful when the input has spaces (e.g. color="sky blue" works while color=sky blue doesn’t)
• variables are case-sensitive (color is not the same as COLOR)

To reference a variable, use $:

color="red"
echo $color
echo "my fave color is $color"

• within double quotes, variables are “expanded” to their value
• within single quotes, strings are interpreted literally; echo 'My favorite color is $color' would print My favorite color is $color
• if a variable name is not found, bash does not throw an error; it just uses an empty string

Special Variables: Arguments

Bash “pre-fills” some variables for you that are related to the script’s arguments:

• $1 is the first argument, $2 is the second, …
• $0 is the filename of the script
• $# is the number of arguments provided (not counting the filename)
• $@ is all of the arguments at once

Exit Codes

The $? variable is also pre-filled by bash and tells you the exit code for the previous command.

Usually, a 0 indicates that the command was successful, while a non-zero code means there was a failure.

You can make a script exit with exit, e.g. exit 1.

For Loops

Bash has a special syntax for for loops, with the for, in, do, and done keywords. Here are some examples iterating over numbers, files, and arguments - all in one syntax style. Note how the first two use command substitution - the $() - to “capture” the output of a command.

#!/bin/bash

for i in $(seq 1 4); do
  echo $i
done

#!/bin/bash

for file in $(ls); do
  echo $file
done

#!/bin/bash

for arg in $@; do
  echo $arg
done

You can also put the do on a separate line:

#!/bin/bash

for arg in $@
do
  echo $arg
done

Arithmetic

To do arithmetic, we can either use let or $(()):

a=1
let b="$a + 2"
c=$(( $a + $b ))

If Statements

Bash has a very specific syntax for if statements - be very mindful of the spacing with the [ and ]!

Here are some examples:

if [ -n "$NAME"]; then
  echo 'Variable $NAME exists'
fi

if [ $a -lt 10 ] && [ $a -gt 5 ]; then
  echo "variable a is between 5 and 10"
fi

if [ $1 -lt $2 ]; then
  echo "arg1 is less than arg2"
elif [ $1 -eq $2 ]; then
  echo "arg1 equals arg2"
else
  echo "arg1 is greater than arg2"
fi

Arithmetic Operators

Operator	Description
-gt	greater than
-ge	greater than or equal to
-lt	less than
-le	less than or equal to
-eq	equal to
-ne	not equal to

Boolean Operators

Operator	Description
if [ expr1 ] && [ expr2 ]; then	boolean AND
if [ expr1 -a expr2 ]; then	boolean AND
if [ expr1 ] || [ expr2 ]; then	boolean OR
if [ expr1 -o expr2 ]; then	boolean OR
if [ ! expr1 ]; then	boolean NOT

String Operators

Operator	Description
=	equal to
!=	not equal to
-z	is empty
-n	is non-empty

File and Directory Operators

Operator	Description
-f	file exists
-d	directory exists
-r	file is readable
-w	file is writable
-x	file is executable