U NIX C OMP -145 L ECTURE 4: T HE S HELL C OMMAND S TRUCTURE S OURCE : S. D AS, “Y OUR U NIX : T...

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UNIX COMP-145

LECTURE 4: THE SHELL COMMAND STRUCTURE

SOURCE: S. DAS, “YOUR UNIX: THE ULTIMATE GUIDE”, 2ND EDITION, MCGRAW HILL, 2006

BROOKDALE COMMUNITY COLLEGE7/17/2009 rwj

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OBJECTIVES

• REVIEW OF FILE ACCESS PERMISSIONS /chmod• UNDERSTAND THE SHELL AS A COMMAND

PROCESSOR• LEARN ABOUT WILD CARDS ( *, ?, [] ).• UNDERSTAND HOW THE UNIX PIPE FACILITY

WORKS.• LEARN ABOUT I/O REDIRECTION


BROOKDALE COMMUNITY COLLEGE 3

WHAT FILE PERMISSIONS RESULT FROM THE FOLLOWING COMMANDS?$ chmod a=rwx <FILE>

• SET READ, WRITE & EXECUTE ACCESS FOR ALL

$ chmod a=rw <FILE>• SET READ, & WRITE ACCESS FOR ALL

$ chmod o+x <FILE>• ADD EXECUTABLE ACCESS FOR OTHERS

$ chmod u-rwx <FILE>• REMOVE ALL ACCESS FOR OWNER

$ chmod 777 <FILE>• SET READ WRITE & EXECUTE ACCESS FOR ALL

$ chmod 750 <FILE>• SET READ, WRITE & EXECUTE FOR USER, READ EXECUTE FOR GROUP, NO

ACCESS OTHERS

REVIEW OF FILE PERMISSIONS

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BROOKDALE COMMUNITY COLLEGE 47/17/2009 rwj

• STEPS:1. ISSUES PROMPT AND WAITS FOR USER INPUT2. SCANS FOR META-CHARACTERS IN COMMAND LINE

AND EXPANDS ABBREVIATIONS3. PASSES COMMAND LINE TO KERNEL FOR PROCESSING4. SHELL WAITS FOR COMMAND TO COMPLETE,

NORMALLY LOCKS TERMINAL5. RETURNS TO STEP 1

SHELL AS A COMMAND PROCESSOR


DETERMINING THE COMMAND TO RUN

• WHEN A COMMAND IS INVOKED WITH A FULLY QUALIFIED PATHNAME (E.G., /bin/echo), THE SHELL RUNS PROGRAM (E.G., ECHO) FOUND IN THE SPECIFIED LOCATION.

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PATTERN MATCHING: WILD CARDS

• SUBSET OF META-CHARACTERS USED IN AN EXPRESSION TO MATCH MULTIPLE BUT SIMILAR FILENAMES (SEE TABLE 7.1 OF TEXT)

• SHELL CREATES LIST OF FILENAMES BEFORE ALLOWING COMMAND TO RUN.

• EXPANSION CAN BE PREVENTED BY QUOTING AND ESCAPING.

• FEATURE ALSO FOUND IN FIND AND SHELL’S FOR AND CASE CONSTRUCTS.

• FILENAMES MUST NOT CONTAIN WILD-CARD CHARACTERS.

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* ANY CHARACTER OR SEQUENCE OF CHARACTERS? ANY SINGLE CHARACTER[ ] ANY OF THE CHARACTERS SPECIFIED IN THE

BRACKETS! NOT— IDENTIFIES A RANGE OF CHARACTERS OF NUMBERS

E.G., ls *[ab]ls *[5-7]

WILD-CARDS SET FOR ls/mv/cp/rm, ETC.


* ANY NUMBER OF CHARACTERS INCLUDING NONE

ls *.lst LISTS ALL FILES WITH EXTENSION .LST.

? A SINGLE CHARACTER

rm ??* REMOVES ALL FILES COMPRISING AT LEAST 2 CHARACTERS.

[ch] A SINGLE CHARACTER THAT IS EITHER A c OR h

cp *.[ch] cprogs COPIES ALL FILES WITh .c OR .h EXTENSION.

[!ch] A SINGLE CHARACTER THAT IS NOT A c OR h

rm *[!a-zA-Z]* REMOVES FILES NOT CONTAINING AT LEAST ONE LETTER.

ls .??* LISTS ALL FILENAMES BEGINNING WITH A DOT AND

COMPRISING AT LEAST TWO MORE CHARACTERS.

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WILD-CARDS SET (CONT’D)


THE SHELL META-CHARACTERS

• WILD-CARD CHARACTERS LIKE THE *, ?, ETC.

• REDIRECTION CHARACTERS LIKE >, >>, <, ETC.

• THE PIPE CHARACTER, |

• COMMAND SUBSTITUTION CHARACTERS (` `)$ echo The date today is `date`$ Echo “There are `ls | wc –l` files in the current directory”

• THE $ AS A VARIABLE PREFIX, E.G., $PATH


THE SHELL METACHARACTERS (CONT’D)

cat chap* SHELL EXPANDS * TO MATCH ALL FILENAMES IN THE CURRENT DIRECTORY THAT BEGIN WITH CHAP.

date > foo SHELL SEES THE > FIRST, OPENS THE FILE foo AS A NEW FILE

AND CONNECTS THE DATE OUTPUT TO IT.

date >> foo SHELL SEES THE >> FIRST, OPENS THE FILE FOO IN THE

CURRENT DIRECTORY AND APPENDS THE DATE OUTPUT TO IT.

who | sort SHELL UNDERSTANDS THE STRINGS ON EITHER SIDE OF THE | AS TWO SEPARATE PROGRAMS AND CONNECTS THEM.

ls `cat foo` SHELL FIRST RUNS cat AND SUPPLIES THE OUTPUT AS ARGUMENTS TO ls.

echo $HOME EVALUATES $HOME AS A VARIABLE BEFORE RUNNING echo.


• THE PIPE CHARACTER = SPECIAL OPERATOR REDIRECTS THE OUTPUT OF ONE COMMAND INTO INPUT FOR ANOTHER COMMAND.

$ ls | wc –l

• THE “>” CHARACTER = REDIRECTS COMMAND OUTPUT FROM STANDARD E.G., THE SCREEN, TO A FILE

$ ls | wc –l > wordCountOfFiles

Redirection Characters


• THE SHELL INPUT REDIRECTION:– STANDARD INPUT (THE KEYBOARD) - DEFAULT– USE < TO CHANGE THE DEFAULT INPUT TO BE A FILE– USE | TO CHANGE THE DEFAULT INPUT TO BE A

COMMAND – A VARIABLE, E.G., $PRINTER

• SHELL REDIRECTION OF OUTPUT:– USE > TO CREATE IT OR >> TO APPEND TO EXISTING

FILE– USE | TO SEND OUTPUT TO ANOTHER COMMAND

I/O REDIRECTION SUMMARY


ESCAPING (USING A \ BEFORE A CHARACTER)

• REVERSES USUAL MEANING OF METACHARACTER FOLLOWING IT. (rm \* removes a file named *.)

• CAN ALSO PROTECT ITSELF. (echo \\ prints a \.)

• PROTECTS SPACE AND [ENTER]. (cd My\ Documents will work.)

• INCONVENIENT TO USE WHEN COMMAND LINE CONTAINS TOO MANY METACHARACTERS THAT NEED TO BE ESCAPED.• PRINCIPLE ALSO USED BY COMMANDS IN THEIR EXPRESSIONS. (grep “\.” foo looks for a dot in foo.)


QUOTING

• Protects most metacharacters from interpretation by the shell, i.e., indicates to be treated literally.

$ echo “*” prints an asterisk (*)• More convenient than escaping when protecting a

group of metacharacters.• Quoted string understood as a single argument by

shell and C programs. (a.out foo “My Documents” has 2 arguments and not 3.)

• Double quotes and single quotes are not equivalent. (echo “$SHELL” not the same as echo ‘$SHELL’)

• Quoting doesn’t protect the backward slash (\),escaping is also required.


SINGLE QUOTES OR DOUBLE QUOTES?

• SINGLE QUOTES PROTECT ALL CHARACTERS EXCEPT \ $ echo ‘\’ won’t work

• DOUBLE QUOTES PROTECT ALL CHARACTERS EXCEPT THE \, $ AND `

$ echo “$” doesn’t print a $

• SINGLE QUOTES PROTECT THE “• DOUBLE QUOTES PROTECT THE ‘•DOUBLE QUOTES PERMIT VARIABLE EVALUATION AND

COMMAND SUBSTITUTION.

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THE USER MASK

• REASSIGNS DEFAULT FILE AND DIRECTORY PERMISSIONS.

• DEFAULT PERMISSIONS BEFORE APPLYING MASK ARE COMPLETELY INSECURE:

• 666 FOR FILES• 777 FOR DIRECTORIES

• SYSTEM-WIDE DEFAULT CHANGED BY umask (A SHELL BUILT-IN).

• umask STATEMENT PLACED IN A STARTUP SCRIPT (TYPICALLY, /etc/profile).



FILE SYSTEMS

• SYSTEM OF ORGANIZING FILES INTO MULTIPLE MANAGEABLE UNITS.

• EACH FILE SYSTEM HAS A SEPARATE DIRECTORY STRUCTURE WITH AT TOP.

• FOR A FILE TO BE VISIBLE, ITS FILE SYSTEM MUST BE ATTACHED TO THE MAIN FILE SYSTEM.

• TWO FILES IN TWO FILE SYSTEMS MAY HAVE THE SAME INODE NUMBER.

• NOT EASY TO UNDERSTAND WHETHER A DIRECTORY STRUCTURE COMPRISES MULTIPLE FILE SYSTEMS.

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DETERMINING THE COMMAND TO RUN (CONT’D)

• WHEN A COMMAND IS INVOKED WITHOUT A PATHNAME, THE SHELL FIRST CHECKS WHETHER IT IS AN ALIAS OR BUILT-IN:• IF ALIAS OR BUILT-IN, THE SHELL RUNS IT

WITHOUTLOOKING IN DISK.TYPE, WHATIS AND WHICH = EXAMPLES OF BUILT-IN COMMANDS

• IF NOT, THE SHELL LOOKS AT THE PATH VARIABLE FOR DIRECTORIES WHERE THE COMMAND MAY RESIDE.$ ECHO $PATH WHERE ECHO=SHELL, $PATH = INPUT

VARIABLE7/17/2009 rwj

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FILE ATTRIBUTES STORED IN INODE

• TYPE: WHETHER ORDINARY, DIRECTORY, DEVICE, ETC.• PERMISSIONS: DETERMINES WHO CAN READ, WRITE OR EXECUTE A FILE.• LINKS: NUMBER OF NAMES A FILE CAN HAVE. A PROGRAM CAN BE DESIGNED TO BEHAVE DIFFERENTLY DEPENDING ON THE NAME BY WHICH IT IS INVOKED.• OWNER: A FILE IS OWNED BY A USER, BY DEFAULT ITS CREATOR. THE OWNER CAN CHANGE MANY FILE ATTRIBUTES AND SET THE PERMISSIONS.• GROUP OWNER: THE GROUP WHICH OWNS THE FILE. THE OWNER BY DEFAULT BELONGS TO THIS GROUP.• FILE SIZE: NUMBER OF BYTES OF DATA CONTAINED.• FILE TIME STAMPS:

• DATE AND TIME OF LAST MODIFICATION• DATE AND TIME OF LAST ACCESS


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DIRECTORY PERMISSIONS

• READ PERMISSION: WHETHER FILENAMES IN DIRECTORY CAN BE LISTED BY A PROGRAM (LIKE ls).• WRITE PERMISSION: WHETHER FILES AND DIRECTORIES CAN BE CREATED IN THE DIRECTORY.• EXECUTE OR SEARCH PERMISSION: WHETHER ONE CAN PASS THROUGH DIRECTORY TO SEARCH FOR FILENAMES.• DESIRABLE PERMISSION SETTING: 755


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AN OWNERSHIP-PERMISSIONS PROBLEM

ASSUMPTION: ROMEO AND JULIET BELONG TO THE USERS GROUP.OWNERSHIP AND PERMISSIONS OF FILE FOO AND ITS DIRECTORY

$ who am iromeo$ ls -l foo-r-x-w-r-x 1 juliet users 7017 2004-11-14 13:53 foo$ ls -ld .drwxr-xr-x 21 romeo users 8192 2004-11-28 11:40 .

NOTE: foo IS OWNED BY JULIET BUT DIRECTORY IS OWNED BY ROMEO.JULIET:

• CAN’T EDIT foo WITHOUT CHANGING THE PERMISSIONS.• CAN CHANGE PERMISSIONS (AS OWNER) AND THEN EDIT foo.• CAN’T DELETE foo (DIRECTORY WRITE-PROTECTED FOR GROUP).

ROMEO:• CAN EDIT OR DELETE foo.• CAN’T CHANGE PERMISSIONS OF foo.• CAN’T DISPLAY OR COPY foo.BROOKDALE COMMUNITY COLLEGE7/17/2009 rwj


THE INODE REVISITED

• SYSTEM OF ORGANIZING FILE ATTRIBUTES SEPARATELY FROM CONTENT.

• IDENTIFIED BY INODE NUMBER BUT INODE DOESN’T CONTAIN THIS NUMBER.

• INODE NUMBER DISPLAYED BY ls -i• BOTH INODE AND DIRECTORY ENTRIES ARE

LOOKED UP BY INODE NUMBER.• POSSIBLE TO CONSUME ALL INODES EVEN WHEN

THERE IS ADEQUATE DISK SPACE.7/17/2009 rwj


MAKING A PROGRAM BEHAVE DIFFERENTLY

• USING ARGUMENTS AND OPTIONS.• MANIPULATING A CONFIGURATION FILE READ BY

A PROGRAM ON STARTUP.1. MODIFIABLE TEXT FILES BY HAND OR VIA

SHELL SCRIPTS • USING DIFFERENT NAMES FOR THE SAME FILE.

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LINKS OR HARD LINKS

• MECHANISM BY WHICH A FILE IS ALLOWED TO HAVE MULTIPLE NAMES.

• LINKED FILENAMES SHARE INODE BUT HAVE SEPARATE DIRECTORY ENTRIES.

• EACH LINK INCREMENTS LINK COUNT IN INODE BY 1 AND ADDS AN ENTRY TO THE DIRECTORY.

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• FILE CONSIDERED TO BE DELETED AND INODE FREED ONLY WHEN LINK COUNT DROPS TO 0.

• LINKED FILENAMES EQUIVALENT IN ALL RESPECTS.

• THE ln COMMAND CREATES LINKS AND rm REMOVES THEM1. INODE # REVEALS WHETHER FILENAMES ARE LINKS

OR COPIES. 2. TWO FILENAMES IN THE SAME DIRECTORY AND

HAVING THE SAME INODE NUMBER MUST BE LINKED TO EACH OTHER.

LINKS OR HARD LINKS (CONT’D)


LINKS OR HARD LINKS (CONT’D)

QUESTION:IF TWO FILES IN TWO DIFFERENT DIRECTORIES HAVE THE SAME INODE NUMBER, HAVE A LINK COUNT OF 2, AND ALSO HAVE IDENTICAL CONTENT, ARE THEY LINKED FILENAMES?

ANSWER:

NOT NECESSARILY; THEY CAN BE TWO SEPARATE FILES ON TWO FILE SYSTEMS HAVING THE SAME INODE NUMBER BY COINCIDENCE. THEY MAY ALSO HAVE THEIR OWN SEPARATE HARD LINKS. IT’S DIFFICULT TO CREATE THIS SITUATION, BUT IT’S NOT IMPOSSIBLE.BUT LINKS CAN BE IN DIFFERENT DIRECTORIES, SO HOW DOES ONE LOCATE THEM? USE THE -INUM OPTION OF THE FIND COMMAND THAT CAN TRACK DOWN A FILE BY ITS INODE NUMBER.

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ADVANTAGES OF HARD LINKS

• BACKUP: PREVENTION FROM ACCIDENTAL DELETION.

• ALLOWS THE SAME FILE TO BE EXECUTED AS TWO SIMILAR BUT SEPARATE PROGRAMS.

• TAKES CARE OF OLD PROGRAMS THAT ACCESSES A FILE WHOSE NAME OR LOCATION HAS CHANGED.


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LIMITATIONS OF HARD LINKS

• CAN’T LINK DIRECTORIES.

• CAN’T LINK ACROSS FILE SYSTEMS.


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SYMBOLIC LINKS OR SYMLINKS

• SEPARATE FILE TYPE AND HAVING ITS OWN INODE.

• CONTAINS THE PATHNAME OF ANOTHER FILE OR DIRECTORY.

• CAN LINK ACROSS FILE SYSTEMS.

• LINK AND FILE POINTED TO ARE NOT EQUIVALENT.

• PATHNAME MAY BE STORED EITHER IN INODE OR IN A SEPARATE FILE.


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UNDERSTANDING OWNERSHIP AND GROUP OWNERSHIP

ENTRY FOR romeo in /etc/passwd: romeo:x:1003:101::/export/home/romeo:/usr/bin/ksh

ENTRY FOR romeo’s GROUP (101) in /etc/group: users::101:

• A FILE HAS A UID (NAME AND NUMBER).• BOTH UID PARAMETERS MAINTAINED IN /ETC/PASSWD (1ST AND

3RD FIELD).• A FILE HAS A GID (NAME AND NUMBER).• BOTH GID PARAMETERS MAINTAINED IN /ETC/GROUP (1ST AND 3RD

FIELD).• THE NUMERIC GID ALSO MAINTAINED IN /ETC/PASSWD.


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OWNERSHIP IN SVR4 AND BSD

ATTRIBUTE SVR4 BSD

OWNERSHIP CAN BE TRANSFERRED ONLY BY SUPERUSER BUT CAN BE REMOVED.

GROUP OWNERSHIP CAN BE TRANSFERRED ONLY TO ANOTHER GROUP TO REMOVED

BUT CAN BE MODIFIED.



Backup Slides

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FILE ACCESS PERMISSIONS

-rwxrwxrwx

- rwx rwx rwx

File Type Owner Group World

- = filed = directory

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OWNER, GROUP, THE WORLD

• OWNER– THE CREATOR OF THE FILE

• GROUP– A SET OF USERS GROUPED TOGETHER

• THE WORLD– EVERY OTHER ACCOUNT NOT IN THE GROUP



READ, WRITE, EXECUTE

• READ– FILE CAN BE READ, BUT NOT MODIFIED

• WRITE– PERMISSION IS GRANTED TO MODIFY THE FILE

• EXECUTE– RUN DIRECTLY AS IF THE FILE IS A PROGRAM– ALL PROGRAMS SHOULD BE EXECUTABLE (/BIN)

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WHAT PERMISSIONS MEAN ON A DIRECTORY

• READ– USERS CAN GET A LISTING OF THAT DIRECTORY

• WRITE– USERS CAN CREATE AND REMOVE FILES IN THAT

DIRECTORY

• EXECUTE– USERS CAN EXAMINE FILES IN THAT DIRECTORY

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CHANGING PERMISSIONS WITH CHMOD

chmod <SETTINGS> <FILE>

chmod a-x testFile

u = user

g = group

o = other

a = all

+ (add)

- (remove)

= (set)

r = read

w = write

x = execute

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MORE EXAMPLES

• SET READ AND WRITE ACCESS FOR ALL?– chmod a=rw <FILE>

• ADD EXECUTABLE ACCESS FOR OTHERS?– chmod o+x <FILE>

• REMOVE ALL ACCESS FOR OWNER?– chmod u-rwx <FILE>

• SET READ, WRITE & EXECUTE ACCESS FOR ALL?– chmod a=rwx <FILE>

• 777 PERMISSIONSo OCTAL REPRESENTATION OF -rwxrwxrwxo THREE BINARY DIGITS OR BITS CORRESPONDS TO ONE OCTAL DIGIT:

Read = 4, Write = 2 Execute =1 i.e., rwx = 4+2+1 = 7 chmod 750 <file> same as chmod u=rwx,g=rx,o= <file>

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ADVANCED CHMOD USAGE

• MOST UNIX HACKERS DON’T USE THIS FORM• THEY PREFER THE MORE DIRECT APPROACH

– SET PERMISSIONS FOR OWNER, GROUP, AND OTHERS ALL WITH ONE NUMBER

• UNFORTUNATELY, THIS APPROACH REQUIRES A LITTLE BIT OF INFORMATION


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