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utmp shows who is currently logged in user. wtmp shows all the history of logged in and logout both of them are binary so in order to see in human readilbe format we need to use last command dmesg = print or control the kernel ring buffer. kernel ring bufffer = rsyslogd = reliable and extended syslogd , is a system utility providing support for message logging. who writes these log file? rsylogd..

/sbin/init still exist for compatibility and both systemd ,init is soft link. systemctl == control the systemd system and service manager target == group units together , simliar to runlevel in order unix system one must not change /lib/systemd/system files if need it we can edit on /etc/systemd/system directory Runlevels are an obsolete way to start and stop gorups of services used in Sys init...

basically terminal is a file that program can read or write. major number = 8 identifies the driver associated with the device. for exmaple , /dev/null and /dev/zero are both managed by drvier 1, whereas virtual consoles and serial terminals are managed by driver 4. minor number = 0,1,2 used only by the drvier specified by the major number. the minor number provides a way for the driver to diffe..

in this file system there are 498784 inodes and so on.' tune2fs == allows the system adminstrator to adjust various tunable filesystem parameters on linux ext2, ext3, ext4 filesystems. i only typed less /etc/passwd but when you check out with lsof command with pid there are many files opened , because of dependency. 0u , 1u, 2u is the file descriptor stdin, stdout, stderr , and we have our less ..

$ su == substitute user // $su - unset Enviornment variable and working directory to use sudo 1. to use sudo your username should be inside /etc/sudoers to chage this file you should always use $visudo command 2. # usermod -G sudo k == make username member of this group 3. can check with $ less /etc/group command /etc/passwd (store user account information) 7 columns 1. username 2. password exis..

even though it has string even though directory's size is 0. in this case since it is "S" == sleeping in an interruptible wait file descriptor for stdin ,stdout ,stderr when i did the redirection of stdout "something" appeared in the othe terminal. every process has a thread and in this example 2852 is thread id all the numbers are the directories created on the fly for every process. other deal..

Task Scheduling 1. one time execution == (at,batch) is good to use 2. Recurring execution == want to execute every certain moment (cron, anacron) is good to use at == you need to give the time to execute this command batch == you don't need to specify time. executes command when system load is below certain level. when the load average drops below 1.5 or the value specified in the invocation of ..

signal == software interrupt deliverd by OS because 1. a process did something (wrong) accessed illegal memory read == SIGSEGV(11) , segment violation divide by zero == SIGFPE(8) try to execute illegal instructions == SIGILL(4) process try to write on a pipe that had been shutdown for writing or isn't connected anymore == SIGPIPE(13) 2. a process wants to tell something to another process to not..

$ wc 0file2 == $ cat >f2 op 2>err == f1 is passed as command line argument since file descriptor 1,2 is alreay redirected error message will go to err file for more explain == Bash One-Liners Explained, Part III: All about redirections (catonmat.net) Bash One-Liners Explain..

exporting user defined variable to sub shell preprocessor == include headerfile, expand macro , remove comments , generate modfied source program. gcc -E only preprocess compiler == generate assembly program. 1. check for syntax error 2. converts source to assembly of underlying processor(x86) Assembler == generate relocatable object program. assemble to machine depend code. Linker == combine al..