Soma-notes - User contributions [en]

Talk:COMP 3000 2012 Midterm Material

2012-10-26T01:30:27Z

Windowpane: /* ls -l */

= Discussion =

= Only the facts =
I'll be putting up my notes of notes up here. Hopefully people can benefit from these. I'll be summing up the answers from the lab and adding a few more details here and there.

== Lab 1 ==
=== Types of shell commands ===
Built ins
Shell functions
binaries (scripts and binaries
alias

=== Shell builtins vs binaries ===
binaries exist elsewhere
faster, no forking needed
Kernel functions (IO redirection)

=== ls -l ===

1 2 3 4 5 6 7
-rwxr-xr-x 1 root root 104508 Mar 31 2012 /bin/ls

U G O
-rwxrwxrwx

To change user permission, use command: "chmod <UGO> <file/dir name>"
ex: chmod 777 file
The three digits represent User, Group or Others permissions. Represented as the sum of their permission values (read = 4, write = 2, execute = 1)

eg. 754 U = 7 = 4 + 2 + 1 = read, write and execute
G = 5 = 4 + 1 = read and execute
O = 4 = read only

eg. 623 U = 6 = 4 + 2 = read and write
G = 2 = write only
O = 3 = 2 + 1 = read and execute

Memnonic: 'You go hugo!'

- denotes a regular file
d denotes a directory
b denotes a block special file
c denotes a character special file
l denotes a symbolic link
p denotes a named pipe

2) 1 link to /bin/ls
3) user root
4) group root
5) size in bytes
6) date last modified
7) path/filename

=== I/O redirection ===
<< keyword HEREDOC. Input until program reads specified keyword
>> append
< read from path
> write to path, blow away anything there
| pipes

=== file descriptors ===
0 stdin
1 stdout
2 stderr
3-9 other

fd redirection
#> where # is a file descriptor
a>&b redirects file descriptor a to b.

COMMAND &>> file.txt
Appends BOTH STDERR and STDOUT to file.txt

=== For loops ===

//For all files in fold that start with l, echo the title
for i in l*; do
echo $i
done

=== Bg vs fg ===
Bash waits or doesn't for return
Both write to stdout as normal

=== Shell vars ===
shell vars only apply to shell. Live in shell
environment is more global, but typically a COPY is passed on to child processes
envp is a series of keyvalue strings passed in with exec*

== Lab2 ==

=== Processes ===

ps aux
ps -e
top

run pstree.
notice that cron, upstart-socket, and others are owned by init. These are daemons.

notice that gnome display manager is a child of lightdm. Killing lightdm kills
gnome. That's why we had to log into a virtual console

=== Upstart ===

init is upstart
started by kernel. It's the only think the kernel runs in userspace

shutdown -r now
switches to run level 6

shutdown -h now
switches to run level zero

symlinks in /etc/init.d point to /lib/init/upstart-job
successful sysV to upstart transition

Upstart's entire deal is revamp the ol' sysV boot seq

traditional init process is strictly synchronous, blocking future tasks until the
current one has completed.

upstart is async. and allows more parralleissimsimsmmmsm

=== Logs ===

/var/log

use lsof to check who's accessing a file.

Rule of thumb.
if logfile is generic, then written to by rsyslog (udev, auth, etc)
if logfile has program name, written to by that program (apache)

rsyslog
daemon that handles syslog protocol
handles generic logging for system

== Lab 3 ==

=== Tracing ===

ltrace, strace
Statically linking will make ltrace output nothing.

=== writing to files ===

File * f = fopen(path, "w")
fprintf(f,"blah")
fclose(f)

=== exec* ===

the exec family of functions replace the current process image with that of
another file/binary.
Does not return value if successful, -1 on failure
Process keeps same PID

execvp() inherits the current process's environment variables
execve() needs to have the environment variables passed in as an array of pointers
to null terminated strings.

=== Other ===

kill sends signals.

used as follows:
"kill <-command> <PID of program>"

Important kill signals:
-SIGINT or -2 -> (crtl-C) sends a kill signal to program, can be ignored. SIGINT is also the default signal used by kill. (ie. kill <PID> = kill -2 <PID>)
-SIGSTP -> (crtl-Z) sends suspend program signal, to resume it use fg <pid or jobnumber> or bg <pid or jobnumber> or send SIGCONT
-SIGSTOP -> same as SIGSTP, except no shortcut keys and it is unblockable.
-SIGCONT -> Continue signal, resumes a stopped program
-SIGFPE -> floating point exception (dividing by zero)
-SIGEGV -> Segmentation fault signal.
-SIGCHLD -> Signal sent to parent when child terminates.
-SIGKILL or -9 -> Unblockable kill signal

Talk:COMP 3000 2012 Midterm Material

2012-10-26T00:32:46Z

Windowpane: /* Other */

= Discussion =

= Only the facts =
I'll be putting up my notes of notes up here. Hopefully people can benefit from these. I'll be summing up the answers from the lab and adding a few more details here and there.

== Lab 1 ==
=== Types of shell commands ===
Built ins
Shell functions
binaries (scripts and binaries
alias

=== Shell builtins vs binaries ===
binaries exist elsewhere
faster, no forking needed
Kernel functions (IO redirection)

=== ls -l ===

1 2 3 4 5 6 7
-rwxr-xr-x 1 root root 104508 Mar 31 2012 /bin/ls

U G O
-rwxrwxrwx

Memnonic: 'You go hugo!'

- denotes a regular file
d denotes a directory
b denotes a block special file
c denotes a character special file
l denotes a symbolic link
p denotes a named pipe

2) 1 link to /bin/ls
3) user root
4) group root
5) size in bytes
6) date last modified
7) path/filename

=== I/O redirection ===
<< keyword HEREDOC. Input until program reads specified keyword
>> append
< read from path
> write to path, blow away anything there
| pipes

=== file descriptors ===
0 stdin
1 stdout
2 stderr
3-9 other

fd redirection
#> where # is a file descriptor
a>&b redirects file descriptor a to b.

COMMAND &>> file.txt
Appends BOTH STDERR and STDOUT to file.txt

=== For loops ===

//For all files in fold that start with l, echo the title
for i in l*; do
echo $i
done

=== Bg vs fg ===
Bash waits or doesn't for return
Both write to stdout as normal

=== Shell vars ===
shell vars only apply to shell. Live in shell
environment is more global, but typically a COPY is passed on to child processes
envp is a series of keyvalue strings passed in with exec*

== Lab2 ==

=== Processes ===

ps aux
ps -e
top

run pstree.
notice that cron, upstart-socket, and others are owned by init. These are daemons.

notice that gnome display manager is a child of lightdm. Killing lightdm kills
gnome. That's why we had to log into a virtual console

=== Upstart ===

init is upstart
started by kernel. It's the only think the kernel runs in userspace

shutdown -r now
switches to run level 6

shutdown -h now
switches to run level zero

symlinks in /etc/init.d point to /lib/init/upstart-job
successful sysV to upstart transition

Upstart's entire deal is revamp the ol' sysV boot seq

traditional init process is strictly synchronous, blocking future tasks until the
current one has completed.

upstart is async. and allows more parralleissimsimsmmmsm

=== Logs ===

/var/log

use lsof to check who's accessing a file.

Rule of thumb.
if logfile is generic, then written to by rsyslog (udev, auth, etc)
if logfile has program name, written to by that program (apache)

rsyslog
daemon that handles syslog protocol
handles generic logging for system

== Lab 3 ==

=== Tracing ===

ltrace, strace
Statically linking will make ltrace output nothing.

=== writing to files ===

File * f = fopen(path, "w")
fprintf(f,"blah")
fclose(f)

=== exec* ===

the exec family of functions replace the current process image with that of
another file/binary.
Does not return value if successful, -1 on failure
Process keeps same PID

execvp() inherits the current process's environment variables
execve() needs to have the environment variables passed in as an array of pointers
to null terminated strings.

=== Other ===

kill sends signals.

used as follows:
"kill <-command> <PID of program>"

Important kill signals:
-SIGINT or -2 -> (crtl-C) sends a kill signal to program, can be ignored. SIGINT is also the default signal used by kill. (ie. kill <PID> = kill -2 <PID>)
-SIGSTP -> (crtl-Z) sends suspend program signal, to resume it use fg <pid or jobnumber> or bg <pid or jobnumber> or send SIGCONT
-SIGSTOP -> same as SIGSTP, except no shortcut keys and it is unblockable.
-SIGCONT -> Continue signal, resumes a stopped program
-SIGFPE -> floating point exception (dividing by zero)
-SIGEGV -> Segmentation fault signal.
-SIGCHLD -> Signal sent to parent when child terminates.
-SIGKILL or -9 -> Unblockable kill signal

Talk:COMP 3000 2012 Midterm Material

2012-10-26T00:10:24Z

Windowpane: /* Other */

= Discussion =

= Only the facts =
I'll be putting up my notes of notes up here. Hopefully people can benefit from these. I'll be summing up the answers from the lab and adding a few more details here and there.

== Lab 1 ==
=== Types of shell commands ===
Built ins
Shell functions
binaries (scripts and binaries
alias

=== Shell builtins vs binaries ===
binaries exist elsewhere
faster, no forking needed
Kernel functions (IO redirection)

=== ls -l ===

1 2 3 4 5 6 7
-rwxr-xr-x 1 root root 104508 Mar 31 2012 /bin/ls

U G O
-rwxrwxrwx

Memnonic: 'You go hugo!'

- denotes a regular file
d denotes a directory
b denotes a block special file
c denotes a character special file
l denotes a symbolic link
p denotes a named pipe

2) 1 link to /bin/ls
3) user root
4) group root
5) size in bytes
6) date last modified
7) path/filename

=== I/O redirection ===
<< keyword HEREDOC. Input until program reads specified keyword
>> append
< read from path
> write to path, blow away anything there
| pipes

=== file descriptors ===
0 stdin
1 stdout
2 stderr
3-9 other

fd redirection
#> where # is a file descriptor
a>&b redirects file descriptor a to b.

COMMAND &>> file.txt
Appends BOTH STDERR and STDOUT to file.txt

=== For loops ===

//For all files in fold that start with l, echo the title
for i in l*; do
echo $i
done

=== Bg vs fg ===
Bash waits or doesn't for return
Both write to stdout as normal

=== Shell vars ===
shell vars only apply to shell. Live in shell
environment is more global, but typically a COPY is passed on to child processes
envp is a series of keyvalue strings passed in with exec*

== Lab2 ==

=== Processes ===

ps aux
ps -e
top

run pstree.
notice that cron, upstart-socket, and others are owned by init. These are daemons.

notice that gnome display manager is a child of lightdm. Killing lightdm kills
gnome. That's why we had to log into a virtual console

=== Upstart ===

init is upstart
started by kernel. It's the only think the kernel runs in userspace

shutdown -r now
switches to run level 6

shutdown -h now
switches to run level zero

symlinks in /etc/init.d point to /lib/init/upstart-job
successful sysV to upstart transition

Upstart's entire deal is revamp the ol' sysV boot seq

traditional init process is strictly synchronous, blocking future tasks until the
current one has completed.

upstart is async. and allows more parralleissimsimsmmmsm

=== Logs ===

/var/log

use lsof to check who's accessing a file.

Rule of thumb.
if logfile is generic, then written to by rsyslog (udev, auth, etc)
if logfile has program name, written to by that program (apache)

rsyslog
daemon that handles syslog protocol
handles generic logging for system

== Lab 3 ==

=== Tracing ===

ltrace, strace
Statically linking will make ltrace output nothing.

=== writing to files ===

File * f = fopen(path, "w")
fprintf(f,"blah")
fclose(f)

=== exec* ===

the exec family of functions replace the current process image with that of
another file/binary.
Does not return value if successful, -1 on failure
Process keeps same PID

execvp() inherits the current process's environment variables
execve() needs to have the environment variables passed in as an array of pointers
to null terminated strings.

=== Other ===

kill sends signals.

used as follows:
"kill <-command> <PID of program>"

Important kill signals:
-SIGINT or -2 -> (crtl-C) sends a kill signal to program, can be ignored.
-SIGSTP -> (crtl-Z) sends suspend program signal, to resume it use fg <pid or jobnumber> or bg <pid or jobnumber> or send SIGCONT
-SIGSTOP -> same as SIGSTP, except no shortcut keys and it is unblockable.
-SIGCONT -> Continue signal, resumes a stopped program
-SIGFPE -> floating point exception (dividing by zero)
-SIGEGV -> Segmentation fault signal.
-SIGCHLD -> Signal sent to parent when child terminates.
-SIGKILL or -9 -> Unblockable kill signal

Talk:COMP 3000 Lab 4 2012

2012-10-17T21:27:19Z

Windowpane: Created page with " Question aAbout padding from Part B - question 2: Does padding only occur at the end of the file? For example, if our file A is 256 bytes, the padding in file B will make i…"

Question aAbout padding from Part B - question 2:
Does padding only occur at the end of the file? For example, if our file A is 256 bytes, the padding in file B will make it 512 bytes?
Also, does a space character count as padding, or should we use null characters? (or are they the same)

Talk:COMP 3000 Lab 1 2012

2012-09-24T15:34:32Z

Windowpane:

Most useless piece of information I've ever received from a unix command:

spratt@lambda03:~$ type `which pwd`
/bin/pwd is /bin/pwd

- [[User:Sdp|sdp]]

Not true, it could be a symbolic link in some cases.

-windowpane