Intro to shell

Command Console and Scripting in Bash

Pipes and Redirections (‘|’ & ‘>’).

• Pipes: The Pipes, ‘

  ’, allow us to redirect the output of one process to the input of another.

• Redirections: The redirections, ‘>’, send the output of a process to the input of a specific file. For example, write a new file after reading another: “$> cat ‘existentFile’ > ‘newFile’”.
• Redirect standard error: “$> cat ‘nonExistentFile’ 2> /dev/null”, el 2> catch standard error and forward to “blackhole” /dev/null.

Processes that interact with “the external” do so from file descriptors, except in the case of daemons. In Linux, file descriptors are integers that can range from 0 to 1023, structures that specify some data input or output channel.

Stdin, Stdout, and Stderr

• Standard input: Stdin: 0, 0<
• Standard output: Stdout: 1, 1>
• Standard Error: Stderr: 2, 2>

Commands in a shell for basic file browsing:

• $> pwd It shows us in which file address we are located (PATH).
• $> ls We see the files in our current PATH
• $> ls -la We see the hidden files (those that are named starting with “.” and we also see information about groups, users and permissions)
• $> tree We see the files in our PATH in a tree format.
• $> tree -r We see PATH files and recursively their subdirectories.
• $> cd ‘argument’ We navigate to an absolute address, or relative to our PATH. With something like “$> cd ~” we mean the home folder of the user running the interpreter, something like “$> cd /” will take us to the root of the file system. To go back directories we use dots, we go back one level with “$> cd ..”, two levels with “$> cd ..”.
• $> find (arguments) for file search. We can use Pattern Matching, start from certain directories, and from other search criteria. Example: “$> find -not -path ‘~/NotHere’ / -iname ‘.deb’”, we look for .deb files in the name omitting the search for ~/NotHere.
• $> cat ‘filePath’ The cat command allows us to read a plain text file.

• *$> cat ‘filePath’

  less* Less is a plain text document viewer which, used in this way, will better display the file in FILEPATH.

• $> cat ‘filePath’ | more more is an evolution of less. Basic commands for creating files in bash.
• $> touch ‘new_nameFile’ Create file
• $> mkdir ‘new_nameFolder’ Create folder
• $> touch ‘new_nameFile new1_nameFile new2..’ Create multiple files
• $> mkdir ‘new_nameFolder new2_nameFolder ..’ Create multiple folders

Copy and move files in bash

• $> cp ‘pathFile’ ‘new_pathFile’
• $> mv ‘pathFile’ ‘destiny_path’

Modify filename in bash

• $> mv ‘./nameFile’ ‘./new_nameFile’ way to rename a file

Simple Basic Bash Script Creation: procedure to create a simple Script and give it execution permission:

1. $> touch basicScript.sh: Create a file called with the extension sh.
2. $> nano basicScript.sh: Open the nano editor.
   • Write “#!/bin/bash” en la primer linea.
   • Write ‘greet=”Hi, i’m the very basic bash script”’ in the second line.
   • Write “print $greet” in some next line.
   • Save the changes in the file with the key combination (Ctrl + O).
   • Exit nano with Ctrl + X.
3. Assign execute permissions to the file with “$> chmod u+x basicScript.sh”.
4. Execute the script with “./basicScript.sh”.