# Getting started with Bash
# Hello World
# Interactive Shell
The Bash shell is commonly used interactively: It lets you enter and edit commands, then executes them when you press the Return key. Many Unix-based and Unix-like operating systems use Bash as their default shell (notably Linux and macOS). The terminal automatically enters an interactive Bash shell process on startup.
Output Hello World by typing the following:
echo "Hello World"
#> Hello World # Output Example
# Notes
# Non-Interactive Shell
The Bash shell can also be run non-interactively from a script, making the shell require no human interaction. Interactive behavior and scripted behavior should be identical – an important design consideration of Unix V7 Bourne shell and transitively Bash. Therefore anything that can be done at the command line can be put in a script file for reuse.
Follow these steps to create a Hello World script:
touch hello-world.sh
#!/bin/bash
echo "Hello World"
Line 1: The first line of the script must start with the character sequence #!, referred to as shebang1. The shebang instructs the operating system to run /bin/bash, the Bash shell, passing it the script's path as an argument.
E.g. /bin/bash hello-world.sh Line 2: Uses the echo (opens new window) command to write Hello World to the standard output.
-
1. `./hello-world.sh` – most commonly used, and recommended
1. `/bin/bash hello-world.sh`
1. `bash hello-world.sh` – assuming `/bin` is in your `$PATH`
1. `sh hello-world.sh`
For real production use, you would omit the .sh extension (which is misleading anyway, since this is a Bash script, not a sh script) and perhaps move the file to a directory within your PATH so that it is available to you regardless of your current working directory, just like a system command such as cat or ls.
Common mistakes include:
1 Also referred to as sha-bang, hashbang, pound-bang, hash-pling.
# Hello World Using Variables
Create a new file called hello.sh with the following content and give it executable permissions with chmod +x hello.sh.
Execute/Run via: ./hello.sh
#!/usr/bin/env bash
# Note that spaces cannot be used around the `=` assignment operator
whom_variable="World"
# Use printf to safely output the data
printf "Hello, %s\n" "$whom_variable"
#> Hello, World
This will print Hello, World to standard output when executed.
To tell bash where the script is you need to be very specific, by pointing it to the containing directory, normally with ./ if it is your working directory, where . is an alias to the current directory. If you do not specify the directory, bash tries to locate the script in one of the directories contained in the $PATH environment variable.
The following code accepts an argument $1, which is the first command line argument, and outputs it in a formatted string, following Hello,.
Execute/Run via: ./hello.sh World
#!/usr/bin/env bash
printf "Hello, %s\n" "$1"
#> Hello, World
It is important to note that $1 has to be quoted in double quote, not single quote. "$1" expands to the first command line argument, as desired, while '$1' evaluates to literal string $1.
**Security Note:**
Read [**Security implications of forgetting to quote a variable in bash shells**](http://unix.stackexchange.com/q/171346/4667) to understand the importance of placing the variable text within double quotes.
# Hello World with User Input
The following will prompt a user for input, and then store that input as a string (text) in a variable. The variable is then used to give a message to the user.
#!/usr/bin/env bash
echo "Who are you?"
read name
echo "Hello, $name."
The command read here reads one line of data from standard input into the variable name. This is then referenced using $name and printed to standard out using echo.
Example output:
$ ./hello_world.sh
Who are you?
Matt
Hello, Matt.
Here the user entered the name "Matt", and this code was used to say Hello, Matt..
And if you want to append something to the variable value while printing it, use curly brackets around the variable name as shown in the following example:
#!/usr/bin/env bash
echo "What are you doing?"
read action
echo "You are ${action}ing."
Example output:
$ ./hello_world.sh
What are you doing?
Sleep
You are Sleeping.
Here when user enters an action, "ing" is appended to that action while printing.
# Importance of Quoting in Strings
Quoting is important for string expansion in bash. With these, you can control how the bash parses and expands your strings.
# There are two types of quoting:
- Weak: uses double quotes: "
- Strong: uses single quotes: '
If you want to bash to expand your argument, you can use Weak Quoting:
#!/usr/bin/env bash
world="World"
echo "Hello $world"
#> Hello World
If you don't want to bash to expand your argument, you can use Strong Quoting:
#!/usr/bin/env bash
world="World"
echo 'Hello $world'
#> Hello $world
You can also use escape to prevent expansion:
#!/usr/bin/env bash
world="World"
echo "Hello \$world"
#> Hello $world
For more detailed information other than beginner details, you can continue to read it here (opens new window).
# Viewing information for Bash built-ins
help <command>
This will display the Bash help (manual) page for the specified built-in.
For example, help unset will show:
unset: unset [-f] [-v] [-n] [name ...]
Unset values and attributes of shell variables and functions.
For each NAME, remove the corresponding variable or function.
Options:
-f treat each NAME as a shell function
-v treat each NAME as a shell variable
-n treat each NAME as a name reference and unset the variable itself
rather than the variable it references
Without options, unset first tries to unset a variable, and if that fails,
tries to unset a function.
Some variables cannot be unset; also see `readonly'.
Exit Status:
Returns success unless an invalid option is given or a NAME is read-only.
```
To see a list of all built-ins with a short description, use
```bash
help -d
```
## Hello World in "Debug" mode
```bash
$ cat hello.sh
#!/bin/bash
echo "Hello World"
$ bash -x hello.sh
+ echo Hello World
Hello World
```
The `-x` argument enables you to walk through each line in the script. One good example is here:
```bash
$ cat hello.sh
#!/bin/bash
echo "Hello World\n"
adding_string_to_number="s"
v=$(expr 5 + $adding_string_to_number)
$ ./hello.sh
Hello World
expr: non-integer argument
```
The above prompted error is not enough to trace the script; however, using the following way gives you a better sense where to look for the error in the script.
```bash
$ bash -x hello.sh
+ echo Hello World\n
Hello World
+ adding_string_to_number=s
+ expr 5 + s
expr: non-integer argument
+ v=
```
## Handling Named Arguments
```bash
#!/bin/bash
deploy=false
uglify=false
while (( $# > 1 )); do case $1 in
--deploy) deploy="$2";;
--uglify) uglify="$2";;
*) break;
esac; shift 2
done
$deploy && echo "will deploy... deploy = $deploy"
$uglify && echo "will uglify... uglify = $uglify"
# how to run
# chmod +x script.sh
# ./script.sh --deploy true --uglify false
```