System Tools
Lecture 2

cd ~/
pwd
ls
ls -l
ls -latrh
cd
cd /vagrant
cd ~/
cat
echo
echo "hello world" > newfile.txt
cat newfile.txt
echo "hello world" | cat
tac newfile.txt
less newfile.txt # q to quit

mkdir newdir
touch emptyfile.txt
cd newdir
pwd
ls
cd ../
cd ~/
rmdir newdir
rm newdir/empty
rmdir newdir
mkdir targetdir
cp emptyfile.txt targetdir
ls -R
ls targetdir/
touch filetomove.txt
mv filetomove.txt targetdir
ls -R
cd targetdir
mv filetomove.txt ../
cd ../
ls -R

ls <tab><tab>n<tab>
touch none.txt
ls <tab><tab>n<tab><tab>o<tab>

tree # to show hierarchy
ls /<tab><tab>v<tab><tab>
pwd
cd subdir
# absolute vs relative paths
cd ../ # parent dir
cd ./ # current dir
ls ./ # current dir
cat ./newfile.txt # same as cat newfile.txt
ls -la

ls > lsoutput.txt
gcc -o printer printer.c  # fprintf(stdout/stderr)
./printer > out  # ./printer 1> out
./printer 2> err
./printer > out 2> err
./print >out 2>&1

• Redirections stdio between processes (instead of just files)

cat file
cat file | grep "hello"
cat file | grep "hello" | cut c2-

ps
ps -aux
ps -aux | grep paul
ls
top
htop
echo $?

man cat
man atoi
man -a exec

tar -cvf packagename.tar file1 file2
tar -tvf packagename.tar
tar -xvf packagename.tar

• nano, emacs, vim

• Show differences between text files

• Show raw bytes of a file

cd project
git status
git add
git commit
git commit -p
git log

cpp -o file.i file.c

gcc -S -O0 hello.c
file hello.s

as -o hello.o hello.c
file hello.o

https://www.agix.com.au/creating-a-hello-world-program-in-assembly-language-in-5-minutes/

gcc -o hello.exe -v hello.o # shows linker
file hello.exe

https://systemoverlord.com/2017/03/19/got-and-plt-for-pwning.html

https://stackoverflow.com/questions/5469274/what-does-plt-mean-here

http://dustin.schultz.io/how-is-glibc-loaded-at-runtime.html

• Preprocessor: file.c -> file.i
• Compiler: file.i -> file.asm
• Assembler: file.asm -> file.o
• Linker: file.o, libraries -> file.exe

• (Diagram) https://thecustomizewindows.com/2012/07/kernel-of-operating-system/
• Mediates hardware access (processor, memory, I/O)
  • No (intentional) direct access to hardware permitted (hardware protection)
• Kernel provides a library of system calls (syscalls) to applications
• Kernel manages resources among running programs, i.e., processes
  • Illusion of simultaneous execution (time-sharing)

• In short we need to
  • Use an agreed-on binary file format, e.g., ELF for *nix
  • Provide the location of the first thing to run: _start by convention for C
  • Use the exec-family and exit syscalls to start and stop our program
• Lots of other details more appropriate for an OS course
  • Dynamic linking
  • Signals
  • I/O
  • Memory management
  • Process management

• Brings binary file into memory
• Begins execution
• Can take argc/argv, environment variables, pass along to process
• Sets up file, I/O

(exec ./hello.exe)

• Defines entrypoint _start
• crt sets up: signals, stdio, args (from loader), exit code (syscall)
• Calls main, passing argc/argv
• Takes main's return value passes it to the exit syscall
  • This is why main has a return value
  • The

https://wiki.osdev.org/How_kernel,_compiler,_and_C_library_work_together https://wiki.osdev.org/Creating_a_C_Library

• Compiler toolchain: file.c -> file.exe
• Loader: file.exe -> running process in RAM

• https://lwn.net/Articles/630727/
• https://lwn.net/Articles/631631/

• .o files symbol tables have missing entries
• Linker resolves missing entries

gcc -c caller.c
gcc -c callee.c
objdump -t caller.o
objdump -t callee.o
gcc -c main.c
objdump -t main.o
gcc -o main.exe main.o caller.o callee.o # link
objdump -t main.exe
readelf -a caller.o
readelf -a main.exe

Moving common declarations of external functions to a .h file.

• Conventions: all, clean (phony)
• built-in rules
• (Diagram) dependency graph

SRC := \
  caller.c \
  callee.c \
  main.c

OBJ := $(SRC:%.c=%.o)

.PHONY: all clean

all: main

main: $(OBJ)
  gcc -o $@ $^

%.o: %.c
  gcc -c $@ $<

clean:
  rm -f $(OBJ) main

• https://overthewire.org/wargames/
• https://ryanstutorials.net/linuxtutorial/commandline.php
• http://www.linuxcommand.org/lc3_learning_the_shell.php

• https://www3.ntu.edu.sg/home/ehchua/programming/cpp/gcc_make.html
• https://randu.org/tutorials/c/make.php
• https://stackoverflow.com/questions/6666805/what-does-each-column-of-objdumps-symbol-table-mean
• https://www.gnu.org/software/make/manual/html_node/Automatic-Variables.html

• https://guides.github.com/activities/hello-world/
• https://rogerdudler.github.io/git-guide/
• https://www.atlassian.com/git/tutorials/using-branches
• https://www.atlassian.com/git/tutorials/comparing-workflows