edX Linux Course Notes

Contents

• About
• Commands
• Descriptors
• Paths
• IP
• Variables
• Constructs
  • Conditionals
  • Looping constructs
  • Tests
    • File tests
    • Numerical tests
    • Arithmetic expressions
    • String tests
• Debugging
• Processes
• Terms

About

The free course is LFS101x: Introduction to Linux.

Some CLI utilities are not shipped with OS X, so I use Homebrew and Cakebrew to install them:

ruby -e "$(curl -fsSL https://raw.github.com/Homebrew/homebrew/go/install)"

and then open Cakebrew to check information of things I want to install before installation.

Commands

Descriptors

There are three file streams (or descriptors) in shell: standard in(put) (stdin), standard out(put) (stdout) and standard error (or stderr). Open files are represented by descriptors. stdin is 0, stdout 1, stderr 2, open file 1 descriptor 3… We can realise I/O Redirection using > (overwrite to outside command), >> (append to outside command), < (take in as part of command), << (take in the command by appending; I’m not so sure).

Paths

Initiating order:

1. ~/.bash_profile
2. ~/.bash_login
3. ~/.profile

IP

A–E class of IP: IP = Net ID + Host ID for A–C class; D class is used for multicast (broadcast information to multiple computers simultaneously); and E class is reserved for the future. IP is divided into four octets: octet1 is Net ID for A class (1.0.0.0–127.255.255.255), 1–2 B (128.0.0.0–191.255.255.255), and 1–3 C (192.0.0.0–223.255.255.255). IPs are requested from Internet Service Provider (ISP). IP can be static or dynamical which then will use Dynamic Host Configuration Protocol (DHCP) to assign IPs.

Variables

Functions are also called subroutines and $1 means the first argument. Beside, $0 means script filename, $* all arguments, $# number of arguments.

Constructs

Conditionals

if condition; then statement; fi

if condition
then
	statement
else
	statement
fi

if condition
then
	statement
elif condition
then
	statement
else
	statement
fi

case "$VAR" in
	"a"|"A") echo "Vowel" ;;
	"e"|"E") echo "Vowel" ;;
	"i"|"I") echo "Vowel" ;;
	"o"|"O") echo "Vowel" ;;
	"u"|"U") echo "Vowel" ;;
	*) echo "Consonant" ;;
esac

Looping constructs

$sum=0
for i in 1 2 3
do
	sum=$(($sum+$i))
done

for filename in $(ls)
	ext=${filename##*\.}
	case "$ext" in 
		c) echo "filename : C source file"
		...
		*) ...
	esac
done

while trueCondition
do
	statement
done

until falseCondition
do
	statement
done

Tests

File tests

• See see man 1 test
• -e file checks if exists.
• -d file checks if is a directory.
• -f file checks if is a regular file (i.e., not a symbolic link, device node, directory, etc.)
• -s file checks if is of non-zero size.
• -g file checks if has sgid (?) set.
• -u file checks if has suid (?) set.
• -r file checks if is readable.
• -w file checks if is writable.
• -x file checks if is executable.

Numerical tests

• exp1 -op exp2
• -eq means equal to
• -ne means not equal to
• -gt means greater than
• -lt means less than
• -ge means greater than or equal to
• -le means less than or equal to

Arithmetic expressions

Note the spaces: expr 8 + 8, echo $(expr 8 + 8), echo $((x+1)), var=$((...))

String tests

[ string1 > string2 ] compare the sorting order; [ string1 == string2 ] you know, compare; ! not.

Debugging

gdb and valgrind and I don’t know.

bash –x ./script.sh turns debug mode on. Or within script.sh, use set -x to turn on, set +x off.

Processes

There are interactive processes, batch processes, daemons, threads, and kernel threads; child process, parent process, zombie process. Process are tracked by process ID (PID) numbers, also Parent Process ID (PPID), Thread ID (TID), Real User ID (RUID), Effective User ID (EUID) of whom determines the access rights for the users, Real Group ID (RGID) about the group of users, and Effective Group ID (EGID). The lower the nice value ([-20, 19]), the higher the priority, the more time granted to use CPU, the less time to wait, aka, the less elapsed time. Besides, real-time priority is available.

Terms

tar means ‘tape archive’. tar archive file is called tarball.

gcc is C and C++ compiler.