2 - Reading & Writing Files

Recall

• The most common *nix shell is BASH (Bourne Again SHell)

  • There are many (many) shells available for *nix operating systems. Most are derivatives of SH or BASH.

• Recently we learned how to...

• see the contents of a folder with ls

• move to a directory with cd (along with some shortcuts like ~ and ..)

• make new folders and empty files with mkdir and touch

• move or rename items with mv

• delete items with rm

• autocomplete with TAB

• see command history with 🠩 and 🠫

Let's continue!

The Lesson

Here's a few more interesting commands:

• pwd - not a password tool, this prints the current working directory

• ps - print the current process status (for this terminal)

• man - get the manual for a particular command

• top - a program to see computer resource usage and running programs

Inside most shells you can run programs after each other (asynchrounously) or at the same time (synchronously)

• • To run programs one after the other no matter what we use the semi-colon  ;

    • sudo apt update; sudo apt -y upgrade

  • But what if the first command fails?

    • cd my_directory; rm -fR *

  • To run programs one after the other only if the first was successful use two ampersands  &&

    • cd my_directory && rm -fR *

  • To run programs at the same time use a single ampersand  &

    • sudo apt update & vim my_script.sh

What was that vim command?

Reading & Writing Files

The basics of any OS is input and ouput (IO). As with anything, there are many tools designed to help with this in the shell.

• To write to a file there are several options - too many to list - but here's the basics:

  • touch <filename> - create a new, empty file

  • echo some text or information > filename.txt the chevron redirects the output to a new location (overwriting the file)

    • Note: one chevron overwrites the file but two chevrons concatonates onto the end of the file

  • vi <filename> - a very old file editing program that was improved with vim

  • vim <filename> - a complex and powerful text editor that takes lots of getting used to

  • emacs <filename> - another extremely popular and powerful text editor (overkill, really)

  • nano <filename> - a tiny text editor that gets the job done (Mr. Brash's typical choice)

    • Most distros have nano pre-installed but if not:  sudo apt install nano

• To list the contents of a file (read) there are a few tools:

  • more - older command that loads the entire file in memory first and displays it page-by-page

  • less - similar to more but moves line-by-line, reads the file sequentially, and has search tools

  • cat - used to concatonate files but if you don't give arugments, it lists the file contents

  • head - print the first 10 lines of a file (configurable)

  • tail - print the last 10 lines of a file (configurable)

  • echo "$(<filename)" - the actual BASH command to print a file's contents

Most people use less or cat