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Lecture 1: The Shell

What is the shell?​

  • textual interface
  • shell vs terminal
  • bash (Bourne Again Shell)

Using the shell​

Launch terminal to open shell prompt:

missing:~$
  • missing: machine
  • ~: short for home
  • $: you are not the root user

Echo:

missing:~$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
missing:~$ which echo
/bin/echo
missing:~$ /bin/echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
  • $PATH: lists which directories the shell should search for programs
  • Colon separated list
  • which: which file is executed for a given program name
missing:~$ pwd
/home/missing
missing:~$ cd /home
missing:/home$ pwd
/home
missing:/home$ cd ..
missing:/$ pwd
/
missing:/$ cd ./home
missing:/home$ pwd
/home
missing:/home$ cd missing
missing:~$ pwd
/home/missing
missing:~$ ../../bin/echo hello
hello
  • Linux & macOS: path separated by /; the path / is the root of the file system
  • Windows: path separated by \; one root for each disk partition (e.g. C:\)
  • cd ~: to home directory
  • cd -: to previous directory

ls command:

missing:~$ ls
missing:~$ cd ..
missing:/home$ ls
missing
missing:/home$ cd ..
missing:/$ ls
bin
boot
dev
etc
home
...
  • commands accept flags and options (flags with values)
  • exa: modern replacement for ls

ls --help:

  -l                         use a long listing format
missing:~$ ls -l /home
drwxr-xr-x 1 missing  users  4096 Jun 15  2019 missing
  • permissions: owner (missing) - group (users) - everyone else
  • rwx: read, write, execute
  • files: read; write; execute
  • directory: ls; rename/create/remove files in the directory; "search", to enter the a directory

Handy programs:

  • mv (rename/move file), cp (copy file), rm -r (remove recursively), rmdir (remove directory), mkdir (make directory)
  • ctrl+l to clear
  • man ls: show manual page of ls

Connecting programs​

  • Two primary "streams": input stream and output stream
  • redirect streams: < file and > file
  • append: >>

cat

  • tac: reverses the output
missing:~$ echo hello > hello.txt
missing:~$ cat hello.txt
hello
missing:~$ cat < hello.txt
hello
missing:~$ cat < hello.txt > hello2.txt
missing:~$ cat hello2.txt
hello

Pipes:

missing:~$ ls -l / | tail -n1
drwxr-xr-x 1 root  root  4096 Jun 20  2019 var
missing:~$ curl --head --silent google.com | grep --ignore-case content-length | cut --delimiter=' ' -f2
219
  • to "chain" programs such that the output of one is the input of another
  • head, tail

A versatile and powerful tool​

  • $: regular user ; #: root user
  • sudo: do as super user
  • sudo su: super user do; shell as super user, exit to return
  • find: search for file
  • tee: read from standard input, write to both standard output and file
  • xdg-open: open file with preferred application

e.g. change brightness:

$ sudo find -L /sys/class/backlight -maxdepth 2 -name '*brightness*'
/sys/class/backlight/thinkpad_screen/brightness
$ cd /sys/class/backlight/thinkpad_screen
$ sudo echo 3 > brightness
An error occurred while redirecting file 'brightness'
open: Permission denied
  • |, > and < are done by the shell

workaround:

echo 3 | sudo tee brightness

Exercises​

  • touch: create an empty file
  • single quotes (treated as literal string) vs double quote (allow variable expansion)
  • sh: Bourne shell
  • chmod: change file permissions
    • r=4, w=2, x=1
    • e.g. chmod 600: rw permission for owner, denies permission for group members and others
  • shebang: #! beginning of a script, specifies the intepreter to be used, e.g. #!/bin/bash