제20강 :Shell, Script Revisited Shell, ScriptRevisited Newham & Rosenblatt, Learning the BASH shell, O'Reilly Gail Anderson & Paul Anderson, The UNIX C Shell, Field Guide.
Shell • UNIX command interpreter • UNIX Shell is just one of the user programs (a.out) • Bash Shell features • Read and interprete user’s command • Customize user’s system or environment (sh var) • Abbreviate long names, commands (alias) • Save old commands (history) • Provides job control (foreground/background) • High level programming using commands (script)
Bash Setup Files (p. 58) • .bash_profile • Bash executes .bash_profile when user logs in • Changes and settings necessary when user logs in • .bash_logout • bash executes .bash_logout when user logs out • .bashrc • executes at beginning of execution by each shell • every time it executes a script • defines special bash characteristics • Setting up terminal stty erase ^h stty kill ^x
Command Substitution • Using Backquote % d =`date` /* d is assigned the output of date*/ • back quote means “embedded command” • same as d = (Wed Jun 22 10:25:21 PST 1991) % echo $d Wed Jun 22 10:25:21 PST 1991 • Using $() % ls $(pwd) /* */ % vi $(ls *.c)
Command Group • (command1; command2 ; command3) % pwd /usr/bob % (cd down; gcc work.c) % pwd /usr/bob • Parent shell creates child shell (sub-shell) • when it encounters ( ) • child shell parses command group, which may change its own new environment • new prompt • when both commands • are completed • you never said “cd ..” • but you are back to original directory
struct user { int u_rsav[2]; /* save r5,r6 when exchanging stacks */ int u_fsav[25]; /* save fp registers */ char u_uid; /* effective user id */ int u_procp; /* pointer to proc structure */ char *u_base; /* base address for IO */ char *u_count; /* bytes remaining for IO */ char *u_offset[2]; /* offset in file for IO */ int *u_cdir; /* pointer to inode of current directory */ char u_dbuf[DIRSIZ]; /* current pathname component */ char *u_dirp; /* current pointer to inode */ struct { /* current directory entry */ int u_ino; char u_name[DIRSIZ]; } u_dent; int *u_pdir; /* inode of parent directory of dirp */ int u_ofile[NOFILE] /* pointers to file structures of open files */ int u_arg[5]; /* arguments to current system call */ int u_stime; /* this process system time */ u_utime; /* this process user time */ } u;
How sub-shell is created parent shell $ sh $ date $ exit $ ls sub-shell OR $ sh< script OR $ (ls; who) $ date
make - revisited Andrew Oram & Steve Talbott, Managing Projects with make, O’Reilly (p 59) a.o: a.c a.h cd down; gcc –o a.c a.h ar rcs libme.a a.o b.o Makefile: • Make treats eachcommand line as if it were executed in its own shell(\ is line continuation) cd down cd down \ cd down; gcc gcc (?) gcc (O) (O) Makefile same shell same shell different shells!
Built-in Command • Command which is internal part of shell program • sh executes directly (not fork/execexternal cmd) • cd must be built-in command, otherwwise % pwd /* Suppose cd is an external process. */ /here /* Then sh fork/exec cd process*/ % cd /there /* New proc changes its own directory & terminate */ % pwd /* Back to parent sh process, who is still in /here */ /here • All commands that change environment of a process must change environment in current process’s context eg = setenv history …..
I/O redirection • Diagnostic >& >>& % cc test.c |& more Pipes both std output & diagnostic % cat test >& save Redirect both standard output & diagnostic % (cat test> save1) >& save2 Separate std output and error file % cat test >>& save Append …
Protection against mistakes in I/O redirection • > , >> could destroy accidently • by redirecting to existing file or • appending to non-existing file % set noclobberoffers protection % unset noclobber remove protection % set noclobber <set redirection protection> % who > memo memo : File exists % who >! memo overrides % ls >> test test : No such file or directory % cat memo >>! test force the output append % unset noclobber
PID TTY STAT TIME COMMAND 62 001 R 0:01 du –s /etc > disk_storage 104 001 R 0:00 ls –l > directory.list 205 001 R 0:00 ps • Job control % du –s /etc > disk_storage & 6262 : process id % ls –l > directory.list & 104 % ps reports current process status % stop 62suspends process 62 % kill 62terminates process 62
Conditional command execution % grep koh test.c > save &&lpr save If grep succeeds, then execute lpr % grep koh test.c > save || lpr save If grep fails, …
In sum, sh is ... • sh is a language (very high level script program) • sh is interpreter (read & execute each line in script) • sh uses other components (which is a.out or other script) • sh glues them together • I/O redirection, • pipe, • IPC, • environment var, • control component execution: • conditional execution, • process control, ... date ls ONE=1 number=0 for filename in * do echo "$filename" | grep -q " " if [ $? -eq $ONE ] then let "number += 1" fi done
In sum, sh is … (cont’d) • Convenient language • C like syntax • variables are weakly defined (no type definition, no init) • can combine many kinds of components • command, script (sh, awk, perl, java, …), daemon, … • slow(many context switches) but powerful & easy • adequate for • quick prototyping • system programming (high level)
Many different kinds .. • Scripts on UNIX/Linux platform • Shell standard user command interface • Awk text processing • Tcl/tk X11 window programming • Scripts on Windows platform • Visual-Basic • Visual-C • Scripts common to Windows/UNIX(Linux) • Python • Perl text processing – CGI programming • Javascript Web page • Changing very rapidly • command + windows + web + …
Script & specialty area • glue various components together sh Tcl/tk Python Perl --- scripts (sh language) text file run by interpreter commands httpd DB Win --- components (C, Java) (sort, date,..) binary run by hardware
Why Script? • Easy to learn • days, weeks • Productivity • Glue various components (C, Java, command, DB, web ..) • Software reuse • Productivity: (60 times than C) (30% than OO) • Slow • 10 – 20 times (less problem as hardware improves) • Shell & C/Java comparison • Script: integration, quick prototyping • C, Java: component coding (library, process, …) for complex/performance-sensitive problems
