Examples

• 55

  Hello World
  55

  Interactive Shell

  The Bash shell is commonly used interactively as a read-eval-print loop (REPL): It reads user commands, executes the commands, then prints the result. 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
  

  Pro Tip: You can change the shell by just typing the name of the shell in terminal. For example: sh, bash, etc...

  Note: echo is a Bash builtin command that writes the arguments it receives to the standard output. It appends a newline to the output, by default.

  ---

  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:

  1. Create a new file called hello-world.sh

     touch hello-world.sh
     

  2. Make the script executable by running chmod+x hello-world.sh

  3. Add this code:

     #!/usr/bin/env 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 command to write Hello World to the standard output.

  4. Execute the hello-world.sh script from the command line using one of the following:

     • ./hello-world.sh - most commonly used, and recommended
     • /bin/bash hello-world.sh
     • bash hello-world.sh - assuming /bin is in your $PATH
     • 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. Forgetting to apply execute permission on the file, i.e. chmod +x hello-world.sh, resulting in the output of ./hello-world.sh: Permission denied.

  2. Editing the script on Windows, which produces incorrect line ending characters that Bash cannot handle.

     A common symptom is : command not found where the carriage return has forced the cursor to the beginning of line, overwriting the text before the colon in the error message.

     The script can be fixed using the dos2unix program.

     An example use: dos2unix hello-world.sh

     dos2unix edits the file inline.

  3. Using sh ./hello-world.sh, not realizing that bash and sh are distinct shells with distinct features (though since Bash is backwards-compatible, the opposite mistake is harmless).

     Anyway, simply relying on the script's shebang line is vastly preferable to explicitly writing bash or sh (or python or perl or awk or ruby or...) before each script's file name.

  ---

  ¹ Also referred to as sha-bang, hashbang, pound-bang, hash-pling.

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• 18

  Hello World Using Variables
  18

  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 to understand the importance of placing the variable text within double quotes.

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• 6

  Hello World with User Input
  6

  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..

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