Shell

Shell vs. Bash

A shell is a command-line interface that interprets commands and executes them by accepting keyboard input and passing it to the OS to perform. It’s a programming environment (similar to Python or Ruby) that includes variables, conditionals, loops, and functions.

The Bourne Again Shell, or bash, is a specific type of UNIX shell and one of the most widely used. Another type of shell is Z Shell (zsh). While they may vary in the details, at their core they’re all roughly the same: they allow you to run programs, give them input, and inspect their output in a semi-structured way.

$PATH and Environment Variables

• If a command doesn’t match shell programming keywords, it checks the $PATH environment variable
• $PATH lists directories for the shell to search for programs
  • When executing a command like echo, the shell searches $PATH for the corresponding executable file
• Bypassing $PATH is possible by providing the full path to the executable file

Missing Semester: The shell

To oversimplify, every program has two “streams” → input and output. Bash also has built-in streams.

• By default, bash’s input is the keyboard, interacting through the terminal
• Following this, bash’s output (like when using echo) is also the terminal, where users can read text feedback

Shell gives you a way to manipulate where input and output are “fed” into, using special characters

• <: “rewire the input for this program to be the contents of this file”
• >: “rewire the output of the preceding program into this file”
• >>: appends to existing content
• |: “take the output of program to the left and make it the input of the program to the right”

$ cat < hello.txt
# shell opens hello.txt, takes its contents and uses it with cat

$ echo hello > hello.txt
# nothing gets printed, instead gets written to file hello.txt

Root User

On most UNIX-based systems, root is a special user with elevated privileges across the OS. This user is (almost) above all access restrictions, and can create, read, update, and delete any file in the system.

Although best practice is not to log in and operate from the root user, you can elevate privileges within the command line using the sudo command. Sudo (“sue-doe”) lets you “do” something “as su” (super user, or root).

Missing Semester: Shell Tools and Scripting

Argument	Description
$0	Name of the script
$1 to $9	Arguments to the script, $1 is the first function and so on
$@	All the arguments
$#	Number of arguments
$?	Return code of the previous command
$$	Process identification number (PID) for the current script
!!	The entire last command including arguments (common example is to run a script with elevated privileges, sudo !!)
$_	The last argument from the last command

Commands will return output using stdout and errors through stderr, as well as a Return Code to report errors in a more script-friendly manner. The return code (aka exit status) is the way scripts/commands have to communicate how the execution went – a value of 0 usually indicates everything went OK. Anything other than 0 indicates an error occurred. Exit codes can conditionally execute commands using && and || operators.

Manipulating streams and arguments

This is an example script showcasing a few of these features. It will iterate through the arguments provided and search for the string foobar using grep. The script redirects stdout and stderr so it won’t print anything to the terminal.

#!/bin/bash
 
echo "Start program at $(date)"
# Date will be substituted
echo "Running program $0 with $# arguments with pid $$"
 
for file in "$@"; do
    grep foobar "$file" > /dev/null 2> /dev/null
    # When pattern is not found, grep has exit status 1
    # We redirect STDOUT and STDERR to a null register since we do not care about them
    if [[ $? -ne 0 ]]; then
        echo "File $file does not have any foobar, adding one"
        echo "# foobar" >> "$file"
    fi
done

The conditional statement uses the test utility to compare if the grep command’s return code (#?) doesn’t equal (-ne) zero. Explore more evaluation options this utility provides using man test (online docs too).

Info

When performing comparisons in bash, try to use double brackets [[ ]] in favor of simple brackets [ ]

Shell Globbing

Globbing is a feature that allows you to use special characters called wildcards to match filenames and paths more flexibly when running commands. This makes it easier to work with groups of files or directories without listing them individually.

Character	Description
*	Matches any number of characters
?	Matches any single character
[]	Matches any single character within the specified range or set
{}	When you have a common substring, expands automatically
Scripting in bash is not the most intuitive, but other options like Python can still be executed from the terminal.

• The shebang line indicates which interpreter should execute a script
• Using the env command in shebang lines enhances script portability
• env resolves the command’s location via the PATH environment variable
• Example shebang line: #!/usr/bin/env python