9/19/2015course material created by d. woit 1 cps 393 introduction to unix and c start of week 4...

04/19/23 Course material created by D. Woit 1

CPS 393Introduction to Unix and C

START OF WEEK 4 (UNIX)



Environment Variables • -always available• -shell provides values• -e.g., HOME -full path name of users home dir• PATH -command search path• PS1 -primary prompt string• PS2 -secondary prompt string (happens if it is

waiting for the rest of a cmd, but you hit enter)• PWD -current dir• PRINTER -what your default printer is• CC -default C compiler (usually set to gcc) • (can set in makefile: CC=gcc )



Environment Variables cont.

• env command lists all your env vars and what they're set to– check the output of env

• We can change env variables interactively e.g:– PS1='${PWD}> '



HMWK

• 1. write a shell program called myenv which takes one argument. The argument should be the name of an

environment variable, such as PATH HOME etc.

• myenv should print out the value of the variable given as the argument. If no argument is given, or the argument

• is invalid, your program could do unpredictable things!• e.g., myenv PRINTER

– should print a line such as: PRINTER=kc3500

• e.g., myenv HOME PRINTER– should print a line such as: HOME=/home/dwoit



Local variables

-need no declaration

• just assign value like in this example: • myname=Jelena #no spaces around = or Unix thinks it is shell cmd• myname='Dr. Misic' #quote if spaces or special chars • to access:

– echo "Professor is ${myname}" # {} optional but prevents ambiguities• to forget:• unset myname #as if it never existed • length operator #

• myname=Jelena

– echo ${#myname} #prints 6 (length of Jelena)

• to forget:

– unset myname #as if it never existed



Local variables• -Arrays:• Starkkids[0]=Robb

• Starkkids[1]=Sansa

• Starkkids[2]=Arya – echo ${Starkkids[1]} # Sansa– echo ${Starkkids[*]} # Robb Sansa Arya

– echo ${#Starkkids[*]} #3 • or can set arrays in one step• Starkkids=(Robb Sansa Arya Bran Rickon) #here, Starkkids[0]=“Robb" • Typeset command below declares variable X as integer and gives it value:• typeset -i X=0• echo ${Starkkids[X]}

man bash says that typeset exists for compatibility with Korn shell and is deprecated in favor of declare



Quoting

• We need to protect white-space characters in strings which we want to submit to command because:– shell scans cmd line & divides into words by white space– evaluates and substitutes for special chars– evaluates as cmd -options filelist

• e.g., grep Jon Snow got.txt # is wrong– What happened here is that :– String Jon is looked up in files Snow and cast_got.txt

• grep “Jon Snow” got.txt # is correct



Single versus double quotes• single quotes are most restrictive:

– do not evaluate anything within– ignore metacharacters, special chars, whitespace, etc

• double quotes:– as above, but evaluate $ (variable or command substitution),

– ` (back-quote chars) and \ (backslash char)

– can use to output a single quote: echo "That's all"

• quotes stop filename substitution: * (like in echo files are “*” )

• E.G lets us evaluate variables• e.g., echo "dir is ${PWD}" # dir is /home/jmisic/cps393• echo 'dir is ${PWD}' # dir is ${PWD}• e.g., grep "${Starkkids[2]}” got.txt # looks for Arya for example• grep '${PWD}' * # looks for ${PWD}



Role of backslash character

• as we saw before, backslash protects following char

• e.g., to print: $amount is what ??• echo \$amount is what \?\?• backslash char \ also allows splitting of long cmd lines, for

example:• /home/jmisic> cat got.txt | grep \• > “Lannister"• e.g., /home/dwoit> grep cat #looks in stdin for “cat"• /home/dwoit> grep Ghost \ • > direwolves.txt #looks in file direwolves.txt for “Ghost"• note: ">" is my current secondary prompt PS2, to see yours do either:

• env | grep PS2 or echo ${PS2}



Meaning and use of backquotes• Shell treats string in backquotes (accent graves) as a

command and evaluates it (in sub-shell) and substitutes cmd result in its place

• e.g.,• TODAY=`date`• echo $TODAY

– prints: Sat Jan 18 17:12:06 EST 2014

• However, without the quotes string is not evaluated e.g.:• TODAY=date• echo $TODAY

– prints: date

• grep `pwd` * – Will print all lines which contain string which is output of pwd command.



conditional test is heart of control structure• check if file is: readable, writable, executable, a file, a dir• check if 2 strings or intigers are >, < or =• can do AND OR or NOT logic, etc.• shell variable $? is assigned with result of test:

– 0 = true 1 = false (yes, backwards from C!)

• e.g., test -x direwolves.txt

• echo $? – 0 printed if file fname is executable; 1 otherways

• Additional switches for test fname:

– -r for readable; -w for writable

– -f for regular file; -d for directory– -G if group of fname is same as users current group



Test –conditional test, cont.

• NAME=“Arya"• test $NAME = “Arya“ # put a space around =• echo $?

– 0

• • test n1 -eq n2 ## $? true if number n1 equal to n2• also -ne, -lt, -gt, -ge, -o -a• e.g., test -x fname -a -d bin

– test true (0) if fname is executable AND bin is a dir

• - man test gives all the options



EXPR

• evaluates arguments and *returns* true or false (1 or 0 like C!!) to stdout (not in $? )– e.g., a=1– b=2– expr $a = $b # put a space around =– 0

• STR="MYNAME"• expr "$STR" = "MYNAME“

– 1

• Can return a numerical value e.g:– expr $a + $b– 3



Expr cont.

• operators: *,/,%,+,-,=,!=,<,>,<=,>=• \_______/\_____________/• int true/false

• Following examples are equivalent (but no spaces between d and expr please)

• d=`expr $a \* 5` or

• d=`expr $a '*' 5` or• d=$(expr $a \* 5)

– echo $d



Arithmetic Evaluation • bash can also do arithmetic without expr construct:• typeset -i x=1

• x=x+1• echo $x

– 2 # is printed on stdout• Notice the difference if you do:• y=1• y=y+1• echo $y

– y+1 #is printed on stdout

• $(( )) does arithmetic evaluation• e.g.,• unset a ; b=5• a=$(($b * 5)) ; echo $a # no spaces around =• 25 is printed on stdout



Control Structures • ; separates commands e.g.: typeset -i x=1; x=x+1; echo $x• () execute in sub-shell - has all variables from calling shell

– any variables it sets are gone when done– anything that alters shell environment is gone when done

• {} execute in current shell - has all variables from calling shell– any variables it sets are still set in calling shell when done– some things that alter shell environment are still set when done (man

bash search subshell)• e.g., (but space after { please )• /home/dwoit: x=4;y=2; ( x=9;y=9; echo $x $y; ); echo $x $y

– 9 9

– 4 2

• /home/dwoit: x=4;y=2; { x=9;y=9; echo $x $y; }; echo $x $y• 9 9• 9 9



Control Structures cont.• Note: if you put a shell program in {}, and that shell program

changes the environment (sets a variable for example), it is NOT changed in calling shell of {} (because SHELL PGMS cannot change calling environment).

• this is because separate process is created to execute script• E.g. if shell program xy contained statements :

– x=9;y=9; echo $x $y;

• then all these would give same output: "9 9 \n 4 2"• x=4;y=2; ./xy; echo $x $y• x=4;y=2; ( ./xy; ); echo $x $y• x=4;y=2; { ./xy; }; echo $x $y # please note a space in { ./xy



Control Structures cont.• To get a shell program to run in current environment, do • . ./shell.pgm # (which is dot space dot backslash shell.pgm)• e.g.,• /home/dwoit: x=4;y=2; . ./xy; echo $x $y

– 9 9

– 9 9

• both () and {} group stdout of multiple cmds into 1 stream • Eg. (cat got.txt; cat direwolves.txt) | grep -c Stark

– remember -c counts matching lines and returns a number– prints num of lines in f1 and f2 containing “Stark"

• without the ( ) what happens? Only direwolves.txt goes to grep and got.txt is catt-ed to stdout 1st

• Note: don't need spaces around ( ) but do need spaces around { ; } and semi-colon



Control Structures cont.• && test for true and execute

– cmd1 && cmd2 #-- execute cmd2 if cmd1 returns true (true=0)– test -r direwolves.txt && cat direwolves.txt

• || test for false and execute– cmd1 || cmd2 #-execute cmd2 if cmd1 returns false (false=1) – E.g: test –e mydir || mkdir mydir

• e.g., cmd1 || ( cmd2; cmd3 ) – is it the same as cmd1 || cmd2; cmd3 ?? NO!

• ! expr returns false if expr true; true if expr false• [ cmd ] evaluates cmd and returns status of 0 if cmd true, 1 if false it is a

short form for test cmd (man test)

• Note: [[ cmd ]] same but more functionality (for example,

• it can do > < for strings, where as [ ] cannot (man bash) )



Conditional construct - if• if test-conditions• then• cmd-list1• elif test-conditions #optional• then #optional• cmd-list2 #optional• else #optional• cmd-list3 #optional• fi• test-conditions is evaluated. If it returns a 0 exit status• the if-part is done, Otherwise the next part is done.



If cont.• e.g., you can type lines below into command line• if test -f fn1 # or if [ -f fn1 ]• then• cp fn1 fn2 #only done if fn1 is a file• fi• E.g

• if [ -f fn1 -a -x fn1 ] # -a means and

• then• echo fn1 is an executable file• else• echo fn1 is not executable and/or not a file• fi



If cont.

• typeset -i a=0

• if [ $a -eq 1 ] #both -eq and = OK• then• echo YES #not echoed• fi• #another example

• xvar=dog #a string

• if [ $xvar = "dog" ]• then• echo YES #echoed• fi



If cont.

• Note: -eq, -lt etc, are used for numbers and• = != for strings• str is true if str is not the null string “”• eg:• xvar=dog #a string• if [ $xvar ]• then• echo YES #echoed• fi• Eg.• If [ “” ]; then echo YES; else echo NULL; fi

– Output NULL



Example program ff string• #!/bin/bash• #source ff• if [ "`grep $1 direwolves.txt`" ]• then• echo string $1 is contained in file direwolves.txt• else• echo string $1 is not contained in file direwolves.txt• fi• After chmod +x ff, we execute ./ff Ghost• #works because grep prints null to stdout if not found and• #some string if it is found• #what happens if run this with $1 not set? Remember fix it with “$1”

like in ff1• #what happens if $1 is contained on many different lines of f1?



Test operators

• For test operators (things that can be used in test or [ ])• see man test• Some interesting ones:• -n string #true, if length of string is non-zero.• -z string #true, if length of string is zero.• -e file #true, if file exists.• file1 -nt file2 #true, if file1 is newer than file2.

• As mentioned above, man bash gives what can be used in [[ ]], which is slightly more extensive than what can be used in [ ]



HMWK1• Write a shell program called sg that takes 2 command line arguments.• The first argument is a textual string; the second a file name.• If other than 2 args are supplied it prints: • sg: Usage: sg str file

• (the sg is printed so that if you change the name of the program,

• the "Usage line" changes automatically.)• If the second arg is not a readable file, print: • sg: file xxx invalid or not readable• where "xxx" is arg2• Your program will return 0 if string is contained somewhere in the• file, and 1 if something wrong with args, and 2 if string not in

• file (but args OK). (use grep in your program).



HMWK 2

• Re-write shell program ff from above slides so that it does something reasonable when the user invokes it without a command argument

(or with a null argument as in: ff "" )



HMWK 3• Write a shell program that will tell you how long another user's

session has been idle.• If the user has more than one session, just look at the first one.• To see idle time, use the "w" command or the who command

with an option.• The program will take a userid as an argument• and will do one of (a), (b), or (c) below:• (a) print "user userid is not logged in" and exit 1• (b) print "user userid has been idle at least 1 minute and exit

0• (c) print "user userid has been idle less than 1 minute and

exit 0



HMWK 4

• 1.re-write your homework program called nw (from unix3.txt) so that if no argument is passed, all of your entries in the current dir are printed (instead of only 10 as before). Change it so the argument is a simple integer, instead of -integer as before.

• 2. write a shell program called x that makes your most recently created file executable.



case• #!/bin/bash• #Source: datecase• mnth=`date +%m` # formats it as

mm (month )• case ${mnth} in• 01)• echo "January"• ;;• 02)• echo "February"• ;;• 03)• echo "March"• ;;

• 11)• echo "November"• ;;• 12)• echo "December"• ;;• *)• echo "some other month"• ;;• esac• exit 0



Case cont.• case ${user-date} in• 01|Jan|January) <-- multiple matches• echo "first month"• ;;• 02|Feb|February)• .• . .• esac• Can use glob constructs (explains why default is *)• If wanted to match 01 or anything starting with Jan: 01|Jan*) • If wanted to match anything with exactly 3 chars: ???)• Match exactly 3 digits: [0-9][0-9][0-9])



Case cont.

• # in example below two levels of are needed due to• # arithmetic evaluation• case $(($count < 100)) in # outcome is 1 if true• 1) echo $count is less than 100;;• 0) echo $count is more than 99;;• esac



For

• for variable in value1 value2 value3 ... valuen

• do• ...• done # loops n

times, • # 1st. pass: $variable has

value1• # 2nd. pass: $variable has

value2 etc.

• Example

• #!/bin/bash• #Source tryfor• echo "not much" > f1• cp f1 f2; cp f1 f3; cp f1 f4• for file in f1 f2 f3 f4• do• chmod +x $file• done• ls -l f?• rm -i f1 f2 f3 f4• exit 0



For examples• #!/bin/bash• #source cla• #program to print out all

command line args one at a time• for i • do• echo $i• done• exit 0• Note: for i in "$@" also loops over

command line args, like in cla1• for i in $@• do• echo $i• done• echo “all params are $@”• exit 0

• #!/bin/bash• #Source filedir#tells if each item in current dir is file or dir for file in * do• if [ -f $file ] # or if test -f $file• then• echo "$file is file"• else• if [ -d $file ] # or if test -d

$file• then• echo " $file is dir. "• fi• fidoneexit 0



For examples• #!/bin/bash• #source renamePgm• #renames all files whose name contains "sample" so that the• #"sample" part of the name becomes "Ch1.sample"• for i in *sample*• do• #in following line destination file name for copy is obtained by sed• cp $i `echo $i | sed -e 's/sample/Ch1.sample/'` 2>/dev/null• if [ $? = "0" ] # variable $? Contains status of prev. operation• then• echo copied $i• else• echo could not copy $i• fi• done



Hmwk1.write a shell pgm called lst which acts like ls -F, using a shell for loop

(Note ls -F puts a '/' after dir names and a '*' after any files that are user executable and a '@' before any that are symbolic links. Look at 'man bash' or 'man test' to find the test for symbolic links)

2. write a shell program called num that prints the number of entries in the current directory. USE A LOOP and COUNTER. Use no pipes.

• 3. modify your program nw from previous homework so that if the argument is a number larger than the number of entries IN the current directory, the following message is printed: there are only $num entries in this directory The new nw should use your program num from above to assign the number of files/dirs to a variable.



Example scripts for, if etc• How would you copy all files in the current dir into a backup directory

called /home/userid/bak (where userid is replaced by your userid),– assuming that if bak exists, it is used; if not, it is created first?

• #!/bin/bash• #Source: backup• test -d /home/jmisic/cp393/Unix4/bak || mkdir

/home/jmisic/cps393/Unix4/bak• for f in * # * means all files in current directory• do• if [ -f $f ] #or if test -f $f• then• cp ${f} /home/jmisic/cps393/Unix4/bak/${f} # works also without braces• fi• done• exit 0



Read command• read line < fn• echo $line #prints line 1 of file fn

– in general reads 1 line of stdin

– #returns non zero status value (in $?) if EOF (end-of-file)

• read arg1 arg2 rest – #reads "words" on one line (strings separated by whitespace)– reads word1 into arg1, word2 into arg2, and the rest of the line into

rest.– #linux bug: won't work if stdin from a pipe (unless within a "while" statement.

Or if () after |, as in echo "arg1 arg2" | (read x y; echo $x $y).

– However, #$x and $y only have value INSIDE () • read -n1 line # reads one character at a time ( so -nx reads x at a

time)• Read utility is good for asking user for additional information



Example for use of read• In script below we remind user to enter arguments.

• #!/bin/bash

• #Source: backup1• if [ $# -eq 0 ] # variable $# is number of arguments • then• echo "Enter filename"• read fn• else• fn=$1• fi• cp $fn ${fn}.bak• exit 0



while

• while [ ... ] # test command• do• ...• done

• Executes commands between do and done as long as exit status of test command is 0.

• #!/bin/bash• #Source: list10• #Note: command seq 1 10 • #does the same thing• x=1• while [ $x -le 10 ] • #or while test $x -le 10• do • echo $x • x=$((x+1))• done• exit 0• #if we had used typeset -i x=1

then could have just done x=x+1• See list10a



While cont.

• while [ 1 ] infinite-loop

• Note: nothing special about the "1", any non-null string will result in infinite loop like ones below:

• while [ 0 ] , while [ mouse ] , while [ "mouse" ]• However while [ "" ] and while [ ] are both false (null string)• Example: • y=`grep blah fn1` #all lines of fn1 containing blah put in var y• if [ "$y" ] test is true if blah found at least once in fn1

– otherways $y is null "" and if-condition is false



Example with infinite loop• #!/bin/bash• #Source: countod• #read in a number of strings from user (until user enters END or end)• # and output the total number of strings that are files & the total• #number that are directories.• #note: if the user enters more than one string on a line,• # will get if [ -f str1 str2 etc ] bash syntax error

• typeset -i ordfile=0• typeset -i dirfile=0• while [ true ]• do• echo Enter a file name or END to quit• read filenam



Example with infinite loop cont.• if [ "$filenam" = "END" -o "$filenam" = "end" ]• then• break #breaks out of the while loop, like in C• #fyi. continue statement also available• fi• if [ -f $filenam ]• then• ordfile=ordfile+1• elif [ -d $filenam ]• then• dirfile=dirfile+1• fi• done• echo -e "The number of ordinary files:\t $ordfile“ # -e enables \t “• echo -e "The number of directories:\t $dirfile"• exit 0



While examples cont.

• #Source: profgone• # does nothing until prof logs off• # should run this in the background with &• while [ "`who | grep jmisic`" ] #backquotes to evaluate • # outcome of previous statement false when null string i.e. when jmisic logs off

• do• sleep 60 #1 minute• done• echo "she's finally gone" • exit 0



While examples cont.• #!/bin/bash• # Source: add:• # adds : to front of each line of several files & concatenates result into #

big-file• for file in fn? #<-- fn1, fn2, fn3 etc• do• cat $file | \ # remember \ extends the command to multiline• while read line #<-- reads line 1 of stdin into var line• do• echo ":$line"• done | cat >> big-file #just done >> big-file works too• done #need >> or last file will clobber• exit 0



Other control constructs • man bash gives lots of other control constructs, e.g.,

• for (( expr1 ; expr2 ; expr3 )) ; do commands ; done– Expr1 is evaluated first – Expr2 is evaluated repeatedly until it evaluates to 0– Each time expr2 evaluates to non-zero value, commands are executed

and expr3 is evaluated– This is similar to C which we will study soon.

• select name [ in word ] ; do list ; done– The list of words following in is expanded generating list of items.– The set of expanded words is printed on stderr each preceeded by a

number.• Select fname in *

• do– Echo you picked $fname $$REPLY$– break;

• done04/19/23 Course material created by D. Woit 46


HMWK1. If the numbers from 1 to 4321 were written out, how many times would

the digit '5' appear? Write a shell program to figure this out.

2. write a shell program called numit which reads lines from stdin and outputs the lines with line numbers.

• e.g., if stdin was• this is a line• this is now another line• and here is line • • Then numit would print• 1. this is a line• 2. this is now another line• 3. and here is line



HMWK cont• And if file dog contained• Abcde• • klm• n• Then numit <dog would print• 1. Abcde• 2. • 3. klm• 4. n



HMWK cont.• write a program like numit above, except that IF a command line arg is

given, it will check to see if the arg is a file, and if so, will output to stdout that file with line numbers as above. If it's not a file, or not readable, it will print an error message. Call this program num2

• Modify the program num2 above so that it will take any number of files

as command line args, and number them consecutively on stdout. Call this

program num3 . For example:

• if file f1 was:– abc def

– gh ij

– k

• and file f2 was:– a b c

– deflsjk

– Ds

– fkl04/19/23 Course material created by D. Woit 49


HMWK cont.

• Then num3 f1 f2

• would print• 1. abc def• 2. gh ij• 3. k• 4. a b c• 5. deflsjk• 6. ds• 7. fkl



HMWK cont.• Write a program called biggest that takes any number of arguments.• For all the arguments that are files, it finds• the file with the most words in it, and prints a line such as:

• File whatever has largest number of words (37)

• assuming the file called "whatever" has 37 words, which is more (or the• same) as any other files in the current directory.• If no arguments were valid files, then the following• line should be printed to stderr:• biggest: no valid filenames were specified• You may find the wc command useful, especially wc -w



HMWK cont.

• Use arrays to write a shell program called revarg that will print out its command line args in reverse order.



HMWK cont.• Write a shell program that tells you how many windows a user has

open. Note that each open window is assigned a unique port number, which you can see with the w command. e.g.,

• > w• dwoit pts/0 141.117.18.31 ....• eharley pts/3 141.117.18.31 ....• dwoit pts/2 moon.scs.ryer ....

• This shows dwoit has 2 windows open (pts/0 and pts/2)

• The user is given as a command line argument to your program ($1).• B.• Extend your solution above to work for any number of userids• on the command line, e.g.,• > numWindows dwoit eharley• dwoit: 2• eharley: 1


9/19/2015course material created by d. woit 1 cps 393 introduction to unix and c start of week 4...

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