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UNIX/LINUX Shells

UNIX/LINUX Shells. Glass & Ables ch. 5 A picture of the relationship between UNIX shells. T Shell. Korn Shell. C Shell. Bourne Shell. Common Core. Common Core. UNIX/LINUX Shells. At UMB LINUX, we use bash, the Borne Again shell Note "common core" for all shell families

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UNIX/LINUX Shells

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  1. UNIX/LINUX Shells • Glass & Ables ch. 5 • A picture of the relationship between UNIX shells T Shell Korn Shell C Shell Bourne Shell Common Core Common Core

  2. UNIX/LINUX Shells • At UMB LINUX, we use bash, the Borne Again shell • Note "common core" for all shell families • Figure shows division of core features • Many important features are there and we'll cover them first in Glass ch. 5 • More bash shell details in Glass ch. 6

  3. UNIX Processes • Basic to UNIX is the idea of a process • Each process contains a program in execution (It might be stopped, but it is in the system anyway). • Each process has own memory space with code, data, and program stack • Most programs we use daily are written in C and have "main (int argc, char *argv[])" through which they access the arguments on their command line • Even if not written in C, they have similar access

  4. UNIX Shells • Shells are just programs that provide a user with a command line interface • Each shell runs in its own process for you as a user and you interact with it via commands • Typically, have shell running in parent process handling command interface sometimes with a program running under it (e.g, a command) in its own child process

  5. Shell Functions Build-in Commands Scripts Redirection Wildcards Pipes Subshells Background Processing Command substitution Variables Sequences local Environment Conditional Unconditional Core Shell Functionality

  6. UNIX Shells • The shell is a program that is basically an initialization and then a loop processing user commands • Shell interprets a user command input, does what­ever that command requires, and waits for another command • Shell terminates when user types control-D at the beginning of a new line or enters the shell command to exit, typically “exit” or “logout”. • UMB standard .login files disable the control-D option • The "logout" command causes this shell and all other programs running in your UNIX session to “go away”

  7. UNIX Shells • Non-built-in shell commands are programs • “ls” or “lpr” or “vi” commands • “myprog” • How to see this – UNIX “which” command blade64(5)% which ls  find ls program path name /usr/ucb/ls  path name to the ls executable • These are all programs in system directories (or "myprog" which is in your own current dir) • UNIX shell simply runs the program in a child process, passing its arguments to it via argc/argv and waits for the child to exit before next prompt

  8. UNIX Shells • echo and cd are built-in shell commands--instead of running a program, the shell program detects these in the input line from the user and performs the right action itself blade64(2)% which cd  find cd program path name cd: shell built-in command. • Note that cd needs to be built-in to change the current dir in the shell process • Doing that action in a program run from the shell would only change the directory for the child process not for the parent shell process itself

  9. Alias • Alias defines a new command name or overrides an existing command name: blade64(36)% alias dir "pwd;ls -lg" blade64(37)% alias dir dir pwd;ls -lg blade64(38)% dir /home/cheungr total 6327 drwxr-s --- 2 cheungr others 512 Oct 26 20:57 bin . . . • To remove an alias: unalias dir User group Others

  10. Shell Variables • A shell variable or local variable is a name with a value in the current shell process space % set x=5 % set hwdir=~cheungr/cs240-1/hw4 • We access shell variable value via $name % echo cking variables: $x $hwdir cking variables: 5 /home/cheungr/cs240-1/hw4 % cd $hwdir • To delete the definition for a shell variable % unset x

  11. Display Shell Variables blade64(3)% set _ addsuffix argv () autologout 60 cwd /home/cheungr dirstack /home/cheungr echo_style bsd edit exec_prefix /tools/modules-2.2b1 filecomp gid 12 group others …

  12. Environment Variables • An environment variable is a name with a value that gets communicated from shell to programs running under the shell including other shells • To define an environment variable of your own using the C shell: % setenv y 10 % setenv printer lw_office

  13. Environment Variables • Values are accessed the same way as shell vars: % echo $y $printer 10 lw_office % lpr -P$printer *.c • To delete definition for an environment variable % unsetenv y • There are many preexisting environment variables. See Glass 92-95, particularly bottom of pg 93.

  14. Display Environment Variables blade64(4)% setenv USER=cheungr LOGNAME=cheungr HOME=/home/cheungr PATH=/tools/req/bin:/tools/backup/bin.sun4:/etc/operator/bin:/etc:/usr/etc:/sbin:/usr/sbin:/groups/ulab/bin:/groups/ulab/pcdev/bin:/tools/jdk-1.6.0_03/usr/jdk/jdk1.6.0_03/bin:/home/cheungr/bin:/usr/local/bin:/usr/local/hosts:/opt/SUNWns6:/usr/ucb:/usr/bin:/bin:/usr/local/gnu/bin:/usr/openwin/bin:/usr/dt/bin:/usr/ccs/bin:. MAIL=/var/mail//cheungr SHELL=/bin/tcsh …

  15. Forking a child process • Processes can give birth to other processes using the fork() system call (K&R, Chap 8) • Then, there are both a parent and a child process • Typically, the parent keeps track of the child but not vice versa • A common thing for a parent to do is just wait until the child finishes its work and exits by: • returning at level of main ( ) or • executing exit ( )

  16. Forking a child process • Which branch of the “if statement” executes? #include <stdio.h> #include <unistd.h> int main ( ) { int pid; pid = fork(); if (pid) printf("parent process %d forked child %d\n", getpid(), pid); else printf("child process %d from parent %d\n", getpid(), getppid()); return 0; }

  17. Forking a child process • Answer: Both!! blade64(5)% fork child process 29098 from parent 29097 parent process 29097 forked child 29098 blade64(6)% • One branch executes in the parent process and the other executes in the child process

  18. Forking a child shell process • A child shell inherits parent’s environment variables and gets a new clean copy of shell variables (Glass, Figure 3.5) Parent Shell Child Shell Environment Copied from parent Environment Local Local Clean, initialized

  19. Background Processing • Launch a command in a background (child shell) process using & at the end of the command line: blade64(54)% grep ju junk.c /* junk.c*/ blade64(55)% grep ju junk.c >junk.g & [1] 19913  PID for child process [1] Done grep ju junk.c > junk.g blade64(56)% cat junk.g /* junk.c*/ blade64(57)%

  20. Discussions on hw6 Memory Allocation Exercise 3 functions: void initalloc(void); char * alloc(int n); void freef(char *p); You are asked to write the freef() function The alloctest.c program is provided to test the functions

  21. initalloc function Initializes a large free buffer struct blockl{unsigned int tag:8;unsigned int size :24;struct blockl *nextp;struct blockl *prevp;}; F F Blockrp = allocbuf + ALLOCSIZE -TAGSIZE Cursorp = freep = allocbuf TAGSIZE allocbuf ALLOCSIZE Struct blockr{ unsigned int tag:8; unsigned int size:24;}; TAGSIZE = sizeof(blockr) MINFREESIZE= sizeof(blockl) + sizeof(blockr) USEDTAG = 0xaa FREETAG = 0x55

  22. Function alloc(n) Allocate a buffer of allocsize = n (in multiple of 4 ) + 2*TAGSIZE bytes from the free buffer pool The tags for both ends of the allocated buffer are of type struct blockr. They are not kept using a linked list (no need for the char * elements) Function returns a pointer = holdp + TAGSIZE pointer returned by alloc holdp U U n bytes in multiple of 4 TAGSIZE TAGSIZE

  23. Allocation Algorithm I) cursorp->size < allocsize=4*((n-1)/4 +1) +2*TAGSIZE return null if all free blocks are not big enough II) cursorp->size <allocsize + MINFREESIZE return pointer =holdp + TAGSIZE III) cursor->size > allocsize + MINFREESIZE return pointer =holdp + TAGSIZE blockrp = p-TAGSIZE holdp = p cursorp F U F U new free size allocsize p=cursorp=holdp blockrp = p +holdp->size -TAGSIZE U U allocsize blockrp =p + allocsize-TAGSIZE

  24. Enchaining Free Buffers Allocated buffers are not chained An example of chaining 4 free buffers nextp =3 prevp =1 Most recent freep cursorp Least recent BUF 4 BUF 3 BUF 2 BUF 1 nextp =2 nextp =1 nextp =4 prevp =4 prevp =3 prevp =2

  25. Unchaining free buffers nextp =3 nextp =2 prevp =1 prevp =1 Most recent Least recent BUF 4 BUF 3 BUF 2 BUF 1 nextp =2 nextp =1 nextp =4 prevp =4 prevp =3 prevp =2 freep cursorp BUF 4 BUF 2 BUF 1 nextp =1 nextp =4 prevp =4 prevp =2

  26. Multiple cases of freef F F U U U U Case 2: Left block is not free Case 1: Left block is free F F U U F F Case 3: Both left and right blocks are free

  27. Pseudocode for freef(char *p) p is the same pointer obtained from alloc() Change tag to FREETAG on blockl Change tag to FREETAG on blockr No need to update size on blockl and blockr because alloc() sets the sizes correctly Check if block on left is also free? case 1: yes, coalesce free block on left and p case 2: no, enchain buffer Check if block on right is also free? case 3: yes, unchain the block on right and coalesce p with block on right. return

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