Bash Scripting
Important Notice
- This tutorial is a work in progress, expect more topics to be covered later
- Everything in this tutorial works in Repl.it, unless stated otherwise
- This is important to mention because it can overcome a suprising number of Repl.it's limitiations
- This tutorial assumes that you have basic knowledge of programming, specifically variables and functions (also known as procedures, methods)
What is Bash
Bash (Bourne Again SHell) is a open-source shell for the GNU project (replacing the Bourne Shell). It is the default shell for most Linux distros, macOS aswell, and in Windows 10 via WSL.
The bash language is a shell scripting language, used for configuration, package management, automation of routine tasks and more. It is similar in purpose to powershell and (somewhat less so with) applescript.
Everything covered in this tutorial can be utilized in the Repl.it implemenation of Bash.
The (non comprehensive) list of known limitations (as of April 2019):
- unavailabilty sudo (for security reasons)
- aplay command (audio programming is not possible)
Basics of Bash
Hello World
The function echo
and printf
can be used to print to the terminal
> echo "hello world!" #echo automatically appends a newline hello world #'-n' flag is used to remove the newline (echo -n "str") > printf "hello world!\n" hello world
I recommend printf
if you need to format tabs or newline as it is more consistent across different terminals
Everything is a string
In bash everything is a string
and what they do is dependent on context.
Bash uses spaces as delimiters (seperators) so, hello world
is converted to the strings 'hello'
and 'world'
. Use single or double quotes to make it a single string, ie: "hello world"
or 'hello world'
.
The first string in a line is used as the command
> echo hello world #The strings are 'echo', 'hello', 'world' #The first string echo is the command #'hello' and 'world' are the first and second arguments
Note:
echo hello
, echo "hello"
, "echo" "hello"
are equivalent, but not "echo hello"
as the former are two strings and the latter is one string
Variables
It is important to be careful with spaces during assignment
> Name=Amjad #assign "Amjad" to Name > Name =Amjad #call Name with argument "=Amjad" > Name = Amjad #call Name with arguments '=' and "Amjad"
The $
in $Hello
tells Bash to replace the string Hello
with the variable Hello
.
> Name=Amjad #Mutable (non-constant) variables > echo $Name #The '$' is used Amjad
Reassignment & Constants
> Name="Haya" #Reassignment > echo "$Name" #"$NAME" is equal to $NAME Haya > echo '"$NAME"' #'"$NAME"' is not equal to $NAME $NAME readonly Change="Constant" #Constant variables > Change="Changing" #Causes error as variable is constant
Command Flags
Flags are used to toggle specific options
Single hypens '-' are used for multiple single letter flags
> rm -rf "dir" rm #remove command -r #option: remove folders recursively f #option: remove files "dir" #directory to delete
Double hypens '-' are used for one multi-letter flag
> git --help
Functions
Special Parameters
$1 #First parameter $2 #Second parameter $n #Nth parameter $# #Number of parameters $? #Return value of most recent function $_
#$* and [email protected] are identical if not quoted #Otherwise they are different "$*" = "$1 $2 $3 $4 ..." "[email protected]" = ""$1""$2""$3"..."
Function Declaration
Functions are declared as follows
fn_name(){ #arguments are not placed here #function body } fn_name args #function call
Bash as a Control Language
Bash is a fundamental domain specific language in linux, and serves as it's control interface, like Apple's applescript and Window's powershell
It can be used to automate many routine tasks such as file management
Linux File Commands
The basic file system of Linux is structured as follows (Only relevant files are shown)
The starting directory is called the root directory '/
', which is approximately equivalent to C:\
on windows
/bin /home /user /Desktop ... /usr /etc ...
File Command are
cd #change working (current) directory ls #list segments (folders) mkdir #make (new) directory (folder) rm #remove file or folder
Compiling with GCC
@Scoder12 had provided the general idea of using Bash to compile programs
GCC, Make and Git and installed in Repl.it
Makefiles
Makefiles can be used to reduce compilation time by only compiling the changed files
#Makefile .PHONY: build_c gcc -o output source.c ./output .PHONY: build_cpp g++ -o output source.cpp ./output
> make build_c > make build_cpp
Linking Libraries
> g++ -o main main.cpp -lNAME_OF_LIBRARY > g++ -o main main.cpp -lncurses #to link ncurses
Default Linux libraries (ncurses.h, unistd.h) are already included in Repl.it
Any unavailable libraries (SDL2, SFML, ...) can be downloaded from their respective sites and then uploaded to Repl.it to be used in your project or cloned and compiled via git
Assembly
GCC can compile C or C++ to assembly
//source.c //C source int main(){ return 0; }
> gcc -S -o assembly.s source.c #Compile to assembly
; assembly.s ; Assembly output .file "main.c" .text .globl main .type main, @function main: .LFB0: .cfi_startproc movl $0, %eax ret .cfi_endproc .LFE0: .size main, .-main .ident "GCC: (GNU) 8.1.0" .section .note.GNU-stack,"",@progbits
> gcc -c -o program.o assembly.s #Assemble > gcc -o executable program.o #Linking > ./executable #Execution
Git
It can also be used to access git
> git --help > git clone "git-repo-url"
Here is a good post on using git by @eankeen
Yeah, it does. I was how to get fiberglass out of skin surprised too. Even more surprised when I found out it can do links too. Like a markdown link with custom text. IT DOES THAT.
is it possible to use expect?
#!/usr/bin/expect doesn't work and neither does the expect command exist.
I assume we can’t have a way to do the equivalent of a “sudo apt-install g++”? In order to work with something like an assembly file with NASM/YASM.
g++ is installed by default, so you can compile assembly but you need to use AT&T syntax instead of NASM though.
install-pkg
which is basicallysudo apt-get install
. It is a makefile, not a bash script though.@SigmaPhi maybe you should put something like this in the tutorial
Makefiles are pretty relevant, so I will be adding more info. Also does Repl.it have a WYSIWYG markdown editor ? (That would be a cool feature!).