Shell

Navigating The Shell#

• cd -: Change directory back and forth between two

• ls -l: List files in long format

  • Presenting file permissions for the user, the user group and anyone else
  • For directory permissions:
    • Read: Whether it is allowed to "list" files in it.
    • Write: Whether it is allowed delete/rename/create files in it. If not, we can make them empty, but just cannot delete it.
    • Executable: Whether it is allowed to enter the directory.

• man ls: Show the mannual page of ls. man takes an argument of the program name to show its mannual page

• ctrl+L: Clean up the terminal and go back to the top

Connecting Programs#

• Two streams in the shell: input stream and output stream.

  • Input Stream: When the program tries to read input, it reads from the input stream. (Keyboard)
  • Output Stream: When the program prints something, it prints to the output stream. (Screen)

• Redirection

  • < fileA: input from fileA
  • > fileB: output to fileB
  • >> fileC: append to fileC instead of just overwrite

• Pipe: The output in the left is the input of the right

  • Print the last line of ls -l /'s output

    missing:~$ ls -l / | tail -n1
    drwxr-xr-x 1 root  root  4096 Jun 20  2019 var

  • Get the content-length of an http response. Select the second field of the space.

    missing:~$ curl --head --silent google.com | grep --ignore-case content-length | cut --delimiter=' ' -f2
    219

A Vertatile and Powerful Tool#

• The "root" user: above all access restrictions, and can create, read, update, and delete any file in the system.

  • To prevent accidental breakdown
  • sudo lets you do something "as superuser".

• Use sudo properly:

  $ 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

  The error occurs because |, <, > are done by the shell, not the program. The shell still does not have the permission. We can work around in this way:

  echo 3 | sudo tee brightness

  or

  sudo su
  echo 3 > brightness

  to get your shell as the superuser and be able to write to sysfs

• Could be used to write to the sysfs to configure some kernel parameter of your computer. Note that sysfs does not exist on Windows or MacOS.

  echo 1 | sudo tee /sys/class/leds/input6::scrolllock/brightness

Exercise#

• Write #!/bin/sh to a file named semester:

  echo '#!/bin/sh' > semester

• Use chmod to add execution permissions

  chmod +x semester

• Use commands to read the battery of the laptop

  cat /sys/class/power_supply/BAT1/capacity