Bash Basics Archive

Useful Stuff: Bash Basics

• Use the mkdir, touch, mv, cp, and ls commands to:
  • Create a file named test.
  • Move test to dir/test.txt, where dir is a new directory.
  • Copy dir/test.txt to dir/test_copy.txt.
  • List all the files contained in dir.

 mkdir lab1-0
 mkdir lab1-1
 ls
 >> lab1-0 lab1-1
 cd lab1-0
 touch test
 ls
 >> test
 mv ./test ../lab1-1/test.txt
 ls
 >>
 cd ../lab1-1; ls
 >> test.txt
 cp ./test.txt ./test_copy.txt
 ls
 >> test.txt test_copy.txt

• Use the grep command to:
  • List all the files from /etc containing the pattern 127.0.0.1.
  • Only print the lines containing your username and root in the file /etc/passwd (only one grep

 sudo grep -iR "127.0.0.1" /etc
 >>
   /etc/hosts:127.0.0.1    localhost
   /etc/default/docker:#export http_proxy="http://127.0.0.1:3128/"
   /etc/dhcp/dhclient.conf:#prepend domain-name-servers 127.0.0.1;
   /etc/dhcp/dhclient.conf:#  option domain-name-servers 127.0.0.1;
   /etc/security/access.conf:#+:root:127.0.0.1
   /etc/speech-dispatcher/modules/ivona.conf:#IvonaServerHost "127.0.0.1"
 
 sudo grep "parallels\|root" /etc/passwd
 >>
   root:x:0:0:root:/root:/bin/zsh
   parallels:x:1000:1000:parallels,,,:/home/parallels:/usr/bin/zsh

• Use the find command to:
  • List all the files from /etc that have been accessed less than 24 hours ago.
  • List all the files from /etc whose name contains the pattern “netw”.
  • List some of the useless information.

 sudo find /etc -atime -1
 >>
   /etc
   /etc/jupyter
   ... (there are too much, so omitted here)

 sudo find /etc -name "*netw*"
 >>
   /etc/modprobe.d/blacklist-rare-network.conf
   /etc/bluetooth/network.conf
   /etc/apparmor/init/network-interface-security
   /etc/default/networkd-dispatcher
   /etc/init.d/network-manager
   /etc/networkd-dispatcher
   /etc/systemd/networkd.conf
   /etc/systemd/network
   /etc/systemd/system/network-online.target.wants
   /etc/networks
   /etc/netplan/01-network-manager-all.yaml
   /etc/sysctl.d/10-network-security.conf
   /etc/network
sudo find ./ -name enc | cat -head -256
>>
	"ezcpb"

• In the bash man-page read the part related to redirections. Explain the following operators >, >>, <<<, >&1, and 2>&1 >. What is the use of the tee command.
  • >: Redirect output to specified file, rewrite this file.
  • >>: Redirect output to specified file, append to the end of it.
  • <<<: Redirect input using specified file as source, stop when a line containing a specified word is seen. The specification will be expanded using ~ expansion, parameter expansion and variable expansion.
  • >&1: Redirect the output to stdout.
  • 2>&1 >: Redirect the content of stderr to stdout.
  • tee: Reads from stdin and write to stdout and files. This is often used within a pipe, writing results to files.

• Explain the behaviour of the xargs command and of the | operator.
  • xargs: Constructs commands and execute. Used to execute commands (especially with those only taking input from stdin) on a list of arguments.
  • |: The pipe. Redirects the output of the command before it to the input of the command behind it.

• What are the head and tail commands? How to “live display” a file as new lines are appended?
  • head: Write the first 10 lines of the input file to stdout.
  • tail: Write the last 10 lines of the input file to stdout.
  • "Live Display": Use tail -f.

• How to monitor the system using ps, top, free, vmstat?
  • ps: Report a snapshot of processes running in system.
  • top: Display real-time view of processes and their information. (htop does this with better visualization)
  • free: Report a snapshot of memory/swap status of the system.
  • vmstat: Report virtual memory statistics such as processes, memory status, etc.

• What are the main differences between sh, bash, csh, and zsh?
  • sh: The original default shell in Unix systems.
  • csh: The C shell, with features such as aliases, command histor and so.
  • bash: Created by the GNU Project, the default shell in most Linux Distributions today.
  • zsh: The Z shell, contains features of bash, but also has additional features such as spell checking.

• What is the meaning of $0, $1,..., $?, $!?
  • They are positional arguments for shell scripting. ${N} stands for the N-th argument for a command, while ! in bash is expanded as the process ID of the most recently executed command and ? is executed as the exit status of the last executed command.

• What is the use of the PS3 variable? Provide a short code example.
  • The PS3 variable is used to select inside a script. Without using this variable the default #? fpr select command will be displayed.
  • Example:

 #!/bin/bash
 PS3="Select a Manuel's Course (1-4): "
 select i in ve472 ve475 ve477 ve482 exit
 do
   case $i in
     ve472) echo "VE472: Methods and Tools for Big Data";;
     ve475) echo "VE475: Introduction to Cryptography";;
     ve477) echo "VE477: Introduction to Algorithms";;
     ve482) echo "VE482: Introduction to Operating Systems"
     exit) exit;;
   esac
 done

• What is the purpose of the iconv command, and why is it useful?
  • iconv is used to convert some text from one encoding to another. It is useful when handling files with different encoding due to different localization.

• Given a variable $temp, what is the effect of ${#temp}, ${temp%%word}, ${temp/pattern/string}.
  • ${#temp}: Length of this variable.
  • ${temp%%word}: Content of $temp excluding the longest pattern matching word.
  • ${temp/pattern/string}: Content of $temp, replacing matches of pattern by string.

• Search online (not on the man pages), how files are organized on a Unix like system. In particular explain what are the following directories used for:
  • /: The root directory.
  • /lib: Libraries as dependency for the binary files in /bin.
  • /usr/lib: Libraries as dependency for the binary files in /usr/bin.
  • /srv: Data for services provided by the system.
  • /sbin: Executables for administrative purposes.
  • /bin: Binary executables, including essential tools such as ls, cp, ...
  • /mnt: Filesystem mount points for physical storages, remote/virtual filesystems, etc.
  • /usr/src: Source files that are system related.
  • /media: Default mount point for removable devices such as USB storage.
  • /dev: Mount points for peripheral devices as a file representation.
  • /boot: Files required for system booting process.
  • /usr/bin: Binary executables distributed with the system but not residing in /bin.
  • /proc: Virtual filesystem with runtime system information.
  • /opt: Reserved directory for softwares and other packages that are not of the default installation of the system distribution.
  • /vmlinuz: The compressed Linux kernel executable that is bootable.
  • /etc: Configuration files for the system.
  • /usr/share: Architecture independent data files of a given OS.
  • /sys: Interface to the kernel providing filesystem-like view of kernel information/configuration.
  • /var: A place for variable files (usually ) such as system logs.
  • /initrd.img: Link to the latest installed Linux initial RAM disk.
  • zp: Zero-padding, you know.

• Write a game where the computer selects a random number, prompts the user for a number, compares it to its number and displays “Larger” or “Smaller” to the user, until the player discovers the random number initially chosen by the computer.

 #!/bin/bash
 computer_number=$((($RANDOM)%100))
 user_number=101
 while [ $computer_number -ne $user_number ] ; do
   read -p "Guess a number: " user_number
   if [ $computer_number -eq $user_number ] ; then
     echo "Correct"
     break
   elif [ $computer_number -lt $user_number ] ; then
     echo "Smaller"
   else
     echo "Larger"
   fi
 done
 echo "The number is $computer_number"

Useless Stuff: 482 Jokes