CIS241

System-Level Programming and Utilities

Linux Permissions

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

First, aliases!

Or, how to make your life easier in the Linux world

• Less keystrokes are good for everybody
• Short command for a longer series of commands/keys

Add in ~/.bashrc to make it persist!

• .bashrc runs when you login (source ~/.bashrc if you make changes and don't want to logout/login again)
• Example: alias la='ls -a'
• Depending on your distro, may go in ~/.profile or ~/.bash_profile
  • Lots of discussion: https://stackoverflow.com/questions/415403/whats-the-difference-between-bashrc-bash-profile-and-environment

Examples from the internet!

Sourced from https://www.reddit.com/r/linuxquestions/comments/16tgbh9/what_are_your_favorite_aliases_to_use/

Useful:

• alias up="sudo apt update && sudo apt upgrade"
• alias clearswap="sudo swapoff -a && sudo swapon -a"
• alias untar='tar -zxvf '
• alias eos=ssh username@eos18.cis.gvsu.edu

Oops:

• alias bim=vim

File permissions

Three aspects to permissions:

• Owner - file owner
• Group - members of group associated with file
• Everybody/thing else

Three types of access:

• Read
• Write
• Execute

Check permissions:

ls -l

• Char 0: filetype (directory (d) or file (-) or link (l))
• Char 1-9: file permissions
• ACL flag: # links, owner, group, size, mod date, filename

File permissions:

• rwx: Read, Write, eXecute
• 3 characters for each (owner, group, other)
  • Example: -rwxr-x---
    • File where owner has full permissions, group has read/execute, other has none

File permissions

Also can be written numerically (octal):

• Read: 4
• Write: 2
• Execute: 1

Sum to get the combination (min of 0, max of 7)

rwx : $4 + 2 + 1 = 7$
r-x : $4 + 0 + 1 = 5$

Modifying permissions

chmod

• + : add permission
• - : remove permission

Examples:

• chmod u+x filename
  • Give filename owner execute permission
• chmod o+rw filename
  • Give filename other (not owner/group) read/write permission
• chmod 755 filename
  • Give filename rwx r-x r-x permission

Environment variables

PATH: where shell looks for programs to execute

• You may have had to modify PATH in the past for things like, 'where is Java?' (or Python)

To modify:

• Open ~/.bashrc
• add export PATH="newpath:$PATH"
  • Adds your newpath onto the existing $PATH string

Searched in the order listed

How to check $PATH specifically?

Print all environment variables - printenv

Installing software from source

Why do this?

• Working through an example: sl

• First, let’s clone the repo:

  • git clone git@github.com:mtoyoda/sl.git

• If you don’t have ssh keys setup on EOS:

  • git clone https://github.com/mtoyoda/sl

• Build the application, if necessary

• Modify PATH to include this directory