In Unix everything is a file. Files are files, folders are files, disks are files, your keyboard is a file, your mouth is a file, the air is a file, you can't breathe, your file lungs fill with files and you try to scream but only files come out oh god Dennis how could you do thi
File Permissions
Permissions have 3 kinds of access:
- r - read access
- w - write/modify access
- x - execute permission (for dirs this means access subdirs or subfiles)
3 user sets:
- u - user, file owner
- g - group
- o - everyone else
These are presented like this in ls:
-rw- --- ---
Directories are the same but are preceeded with a d:
drw- --- ---
Setting file permissions can be done with chmod
- chmod <octal mode> <files>
- silly to use this 2023 except in shell scripts, but even then
- each group of 3 permission is views a 3 bits of an octal number
- 100 r perm
- 010 w perm
- 001 x perm
- chmod <symbolic mode> <files>
- chmod <user sets><+|-><perms> <files>
- chmod go+r file.txt
- may require multiple calls
Input/Output
Unix views all files as characters/bytes, leaving interpretation up to the programs.
- default input is stdin (file descriptor 0), which is mapped to the keyboard by default.
- default output is stdout (file descriptor 1), which is the screen
- default error output is stderr (filedescriptor 2), which also goes to the screen
- this allows errors to be seperately directed
Redirection
stdin/stdout/stderr can be redirected to other files
- < redirects stdin to file
- > redirects stdout to file
- >> redirects stdout to append to a file
- 2> redirects stderr to file
Piping
stdout/stderr can be redirected to other programs inputs, rather than files using piping.
date | wc
| only applies to stdout, but 2> errors | works equivalently for errors.
Scripts
Scripts are files that contains sets of commands on consecutive lives that can be repeatedly run.
The first line of a script says what interpreter should be used to execute the script.
#!/usr/bin/env bash
Script needs to be made executable
chmod u+x script.sh
From here Bash takes overs.