Linux in More Detail Shirley Moore CPS5401 August 29,

The Kernel The software layer that manages and allocates system resources (e.g., CPU, RAM, devices) Services provided by the kernel – Process scheduling preemptive multitasking – Memory management virtual address spaces – Provision of a file system – Creation and termination of processes – Access to devices – Networking – Provision of a system call application programming interface (API) 2

Linux Kernel Versions kernelnewbies.org/LinuxChanges/ kernelnewbies.org/LinuxVersions/ Most recent version is 3.10, released on 30 June 2013 perf_events for accessing hardware counters merged into the Linux kernel starting with version To access hardware counters from within a KVM virtual machine, you need Linux version 3.3 or later. 3

Kernel mode vs. user mode Areas of virtual memory are marked as being part of user space or kernel space – When running in user mode, CPU can only access memory in user space – i.e., user processes cannot access kernel instructions and data structures directly – When running in kernel mode, CPU can access both user and kernel memory space Some operations can be performed only while the processor is operating in kernel mode – accessing memory management hardware – initiating device I/O operations e.g., “A process can write data to a file” should really be “A process can request that the kernel write data to a file”. The shell runs as a user process. 4

File Permissions Linux systems are multi-user environments where multiple users run programs and share data. Files and directories have three levels of permissions: User, Group, and Others. The types of permissions a file can have are Readable, Writeable, and Executable User (owner)GroupOthers (everyone else) rw-r-- Read permissionWrite permissionExecute permission rwx 5

Changing Permissions chmod – command to change permissions on a file or directory chgrp – command to change group ownership to another group chown – change ownership of a file (can only be done by the superuser Above commands all support –R for recursion. 6

Environment List Each process has an environment list, which is a set of environment variables maintained in the process’s user space memory. – Each list element consists of a name and an associated value – Type the printenv command to see the environment list for your login shell 7

Setting Environment Variables bash export MYVAR=‘Hello world’ tcsh setenv MYVAR ‘Hello world’ The environment list can be inherited by a child process from the parent. 8

Environment Variables used by the Shell HOME – pathname of the user’s login directory PATH – list of directories that the shell should search for programs corresponding to commands entered by the user To append or prepend to the PATH variable – bash: export PATH=$PATH:/path/to/dir1:/path/to/dir2 export PATH=/path/to/dir1:$PATH:/path/to/dir2 – tcsh: set PATH = ($PATH /path/to/dir1 /path/to/dir2) set PATH = (/path/to/dir1 $PATH /path/to/dir2) Type the which command to see the complete path the shell is using for a program 9

I/O Redirection Standard output – When you issue a Unix command like date, the output of the command goes to what is called standard output. $ date Thu Aug 29 17:10:31 EDT 2013 – In Unix, you can redirect standard output to go to a file. $ date >date.txt $ cat date.txt Thu Aug 29 17:12:25 EDT

I/O Redirection (2) The >> symbol appends what would go to standard output to a file. $ date >>date.txt $ cat date.txt Thu Aug 29 17:12:25 EDT 2013 Thu Aug 29 17:15:10 EDT 2013 Redirection of standard input works similarly – eg., mail -s "unix-lab1” < date.txt (This example may not work on all Unix systems). 11

Pipes The symbol | is the Unix pipe symbol. It means is that the standard output of the command to the left of the pipe gets sent as standard input of the command to the right of the pipe. $ cat date.txt | wc Quiz question: How many processes are involved in executing the above pipeline? 12

Job Control Available in all shells except Bourne shell Allows a user to simultaneously execute and manipulate multiple commands or pipelines. All the processes in a pipeline are placed into a new process group, or job. A session is a collection of jobs. The session leader is the process that created the session. All the jobs created by a job-control shell belong to the same session as the shell, which is the session leader. 13

Job Control (2) Sessions usually have a controlling terminal. At any point in time, one job in a session is the foreground job, which can read input from the terminal and send output to it. If the user types the interrupt character (usually Control-C), or the suspend character (usually Control-Z) on the controlling terminal, the terminal driver sends a signal that kills, or suspends, the foreground job. A session can have any number of background jobs, which are created by ending a command with &. Useful commands: ps, bg, fg 14

Time Real time is measured from some standard point (calendar time) or from the start of the life of a process (elapsed or wallclock time) Process time, also called CPU time, is the toal amount of CPU time that a process has used since starting. – CPU time is further divided into system time (the time spent in kernel mode) and user time (the time spent in user mode) 15