- SHELL (bash, ksh, bourne, c shell itp)
- C/C++
- PERL
- SHELL / AWK / SED
Zastosowanie tych jezyków jest następujące :
SHELL
Jest to bardzo prosty język programowania pozbawiony wielu zaawansowanych struktur danych takich jak chociażby : tablica wielowymiarowa, zmienna liczbowa zmiennoprzecinkowa, brak zdefiniowanych funkcji, problemy z parsowaniem textu.
Z uwagi na swoją prostotę i domyslną obecność we wszystkich odmianach unix-a/linux-a tuż po instalacji systemu, jest on wykorzystywany przez administratorów w celu automatyzacji czynności lub wyciagania informacji z systemu.
C/C++
Jest to obiektowy C++ i nieobiektowy C jezyk programowania. Jeden z najcięższych w jakich programuje człowiek.
Jeżeli chodzi o ich zastosowanie to w językach tych :
- pisze się sterowniki
- tworzy systemy operacyjne
- tworzy aplikacje użytkowe pod platforme np. unix/linux
- mechanizmy zarządzania pamięcia w unix/linux np. semafory, kolejki, pamięc współdzielona
PERL
Język, który jest obency w każdej implementacji systemu unix/linux. Posiada rozbudowaną semantykę znaczników do wspołpracy z textem. Ma zdefiniowane wiele funkcji z których można skorzystać bez konieczności manualnego ich oprogramowywania.
Idealny jezyk do parsowania textu.
SHELL / AWK / SED
Połączenie tych 3 jezyków programowania pozwala administratorowi na zastąpienie umiejetności parsowania textu przez PERl-a.
Celem tego tutorial-u jest omówienie podstawowych zagadnień związanych z programowaniem w języku SHELL i jego odmianą o nazwie Korn Shell (ksh) wykorzystywaną np. w Solaris10.
Można go również używać pod innymi platformami unix/linux ale może wymagać doinstalowania, w solaris10 jest natywnie obency po instalacji oesa.
W systemie Solaris10 mamy do dyspozycji 3 podstawowe shell-e :
- Bourne Shell
- C Shell
- Korn Shell
Bourne Shell
Oryginalny UNIX shell i jest domyślny dla
root użytkownika. Domyślny znak zachęty dla użytkownika zwykłego to $ natomiast dla root znak ten to #C Shell
Posiada szereg funkcji, których Bourne Shell nie posiada : historia komend, aliasy kontrola job-ów. Domyślny znak zachęty dla użytkownika zwykłego to
hostname% natomiast dla root znak ten to hostname#Korn Shell
Łączy w sobie funkcjonalność Bourne + C SHell. Domyślny znak zachęty dla użytkownika zwykłego to
$ natomiast dla root znak ten to #Solaris 10 posiada trzy dodatkowe alternatywne SHell-e :
- Bash
- Z Shell
- TC Shell
Poniżej znajduje się omówienie semantyki programowania w języku Korn SHell :
- Arrays
- Basic Math
- Case Statement 1
- Case Statement 2
- Debugging 1
- Debugging 2
- Else If Statement 1
- Else If Statement 2
- If Statement with Math Operations 1
- If Statement with Math Operations 2
- If Statement with String Operations
- Loop Break & Continue Statement
- Loop For
- Loop Until
- Loop While
- More Pattern Matching
- Pattern Matching 1
- Pattern Matching 2
- Pattern Matching 3
- Pattern Matching 4
- reading Input
- Tests On Files
Arrays
#!/bin/ksh
#Arrays.ksh
# Introducing arrays: a group of similar variables.
# A god use ofr an array would be an array to hold
# a list of groceries of the months of a year.
#
# - Jim
# To create an array:
# ArrayName[ElementNumber]="Value"
# Defining a counter:
index=7
# Example
Child[0]="Tom"
Child[1]="Kate"
Child[7]="Colleen"
# Example of not using index 0.
Month[1]="January"; Month[2]="February"
Month[3]="March"; Month[4]="April"
Month[5]="May"; Month[6]="June"
Month[7]="July"; Month[8]="August"
Month[9]="September"; Month[10]="October"
Month[11]="November"; Month[12]="December"
# Printing a variable:
print "index is $index"
# Can not access an array element by $Month[7]. Ksh auto interprets everything that goes after $sign as variable
# To access the value in an array element.
print "The fifth month is: ${Month[5]}"
# To print all values in an array:
print "The whole array is: ${Month[*]}"
#
#
exit
Basic Math
#!/bin/ksh
# Korn shell allows basic math: +, -, *, /,
# and % (remainder of the division).
# Korn shell only performs amth on integers.
# It will truncate 7.9 to 7 and 2.1 to 2.
# - Norbert 666-666-6666
#Define some variables:
x=5; print "x is: $x" #multiple cmds in one line by using ; sign
y=11; print "y is: $y"
z=18; print "z is: $z"
# To perform math, use double parentheses.
# However, no need to use $ in front of the variable.
(( Result = y + x ))
print "y + x is: $Result"
(( Result = z - y ))
print "z - y is: $Result"
(( Result = y / x ))
print "y / x is (Remember returns whole number): $Result"
(( Result = y % x ))
print "y % x is: (This is the remainder operation): $Result"
# Korn shell truncates numbers to make the integers.
(( Result = 1.9 + 1.9 + 1.9 ))
print "In Korn Shell, 1.9 + 1.9 + 1.9 is: $Result" # there are version of korn shell with decimal points
# Adjusting position within an array/list:
Position=1
Color[$Position]="Red"
(( Position = Position + 1 ))
Color[$Position]="Orange"
print "The contents of the array \"Color\" are: ${Color[*]}" # add second "" inside of ""
# To print just a -5 use -R.
print -R "-5" #if not used with -R, it will tell shell to treat "-5" as -5 flag
exit
Case Statement 1
#!/bin/ksh
# NonCase.ksh
# Reviews if-elif-else construct.
# - jakubn
# If you want to test 1
# variable for several
# conditions, you can use
# if-elif-else construct
# if (( test1 ));then
# command group 1
# elif (( test2 )); then
# command group 2
# elif (( test3 )); then
# command group 3
# else
# command group 4
# fi
# Remember the FIRST test that
# is true gets executed. After,
# the program automatically
# breaks out of the if-elif
# structure, and does not perform
# any more tests.
print "Please enter the type of "
print "backup you want done:[ABC]."
print "A: Daily"
print "B: Weekly"
print "C: Quarterly:
print "D: Special"
print -n "Response: "
read Response
if [[ $Response = [Aa] ]]
then
# Do a Daily backup
print "Starting daily backup."
# /#oracle/backup.daily.ksh
elif [[ $Response = [Bb] ]]
then
# Do a Weekly backup
print "Starting weekly bakup."
# /oracle/backup.weekly.ksh
elif [[ $Response = [Cc] ]]
# Do a Quartlerly backup
print "Starting quarterly backup."
# /oracle/backup.quarterly.ksh
elif [[ $Response = [Dd] ]]
then
# Do a Special backup
print "Starting daily backup."
# /oracle/backup.daily.ksh
print "Please take out daily tapes."
print "Please put in weekly tapes,"
print -n "and hit 'enter' to continue." #gives you time to get tapes
read Waiting
print "Starting weekly backup."
# /oracle/backup.weekly.ksh
else
# Person entered in bad data.
# Tell them to enter a right
# choice
print "'$Response' is not a valid choice."
fi
#
#
exit
Case Statement 2
#!/bin/ksh
# Case.ksh
# Introduces the case statement
# which allows you to test 1
# variable against several
# different values. The 1st test
# that is true will result in
# the code assigned to it to be
# executed until a ;; is found
# After, it breaks out of the case statement
# without performing any more tests
# You need to end code with
# ;; before the next pattern
# matching test. No fall through
# allowed.
# Format of case statement:
# case $Var in
# Pat1)
# Code 1 ;;
# Pat2)
# Code 2 ;;
# Pat3)
# Code 3 ;;
# *)
# Default Code ;;
# esac
print -n "Enter a value: "
read Var
# To read in to a var., don't
# use $ sign. To get info out,
# use $ sign.
case $Var in
# comment
1) print "value is 1";;
2) print "value is 2";;
3|71) print "value is 3 or 71";;
*) print "Please enter 1, 2, 3, or 71";;
esac
print "Please enter the type of "
print "backup you want done:[ABCD]."
print "A: Daily"
print "B: Weekly"
print "C: Quarterly"
print "D: Special"
print -n "Response: "
read Response
case $Response in
[Aa])
# Do a Daily backup
print "Starting daily backup."
# /oracle/backup.daily.ksh
;;
@(B|b)) #requirement of 1 of whatever it is in ()
# Do a Weekly backup
print "Starting weekly backup."
# /oracle/backup.weekly.ksh
;;
[Cc])
# Do a quarterly backup
print "Starting quarterly backup."
# /oracle/backup.quarterly.ksh
;;
[Dd])
# Do a special backup
print "Starting daily backup."
# /oracle/backup.daily.ksh
print "Please take out daily tapes."
print "Please put in weekly tapes,"
print -n "and hit 'enter' to continue." #gives you time to get tapes
read Waiting
print "Starting weekly backup."
# /oracle/backup.weekly.ksh
;;
*)
# Person entered in bad data.
# Tell them to enter a right
# choice
print "'$Response' is not a valid choice."
;;
esac
Debugging 1
#!/bin/ksh
# Debugging1.ksh
# This script introduces debugging
# using "#!/bin/ksh -x" or
# set -x (and set +x).
# Also, it reminds you how useful
# print statements can be.
# - jakubn 666-666-6666
# Turn on debugging. # you can put multiple set -x and set +x statements in your script
set -x
y="Monday"
print "Save the cheerleader, save the world."
# Turn off debugging.
set +x
#print statement for debugging
print "After cheerleader statemetn, the value of y is: $y"
#The following print statement won't work
# because it has an extra "t" in the word print.
printt "This is an invalid print statement."
Debugging 2
#!/bin/ksh -x
# Debugging2.ksh ---> puts whole code in debugging mode ksh -x
# This script introduces debugging
# using "#!/bin/ksh -x" or
# set -x (and set +x).
# Also, it reminds you how useful
# print statements can be.
# - jakubn 666-666-6666
y="Monday"
print "Save the cheerleader, save the world."
#print statement for debugging
print "After cheerleader statemetn, the value of y is: $y"
#The following print statement won't work
# because it has an extra "t" in the word print.
printt "This is an invalid print statement."
exit
Else If Statement 1
Przetwarzanie bloku kodu "if else" wygląda następująco.
#!/bin/ksh
# elif.ksh
# This program demonstrates the use
# of elif, the ability to make a
# 2nd test if the 1st if false.
# - Jim
# Assign a default grade.
Grade="F"
#Clear the screen.
clear #This is unix command.
print -n "Enter number grade: "
read NumberGrade
if (( NumberGrade > 89 )); then
#Person gets an "A".
Grade="A"
elif (( NumberGrade > 79 )); then
#Person
Grade="B"
elif (( NumberGrade > 69 )); then
#Person
Grade="C"
elif (( NumberGrade > 59 )); then
#Person
Grade="D"
fi
print "Letter grade is: $Grade"
exit
Else If Statement 2
#!/bin/ksh
# elif2.ksh
# This program demonstrates the use of else at the end of an elif statement
# - Jim
# Let's buy a bus ticket.
# The price may change
# depending on the age of the apssenger.
print -n "Enter your age: "
read Age
if (( Age < 5 )); then #The Cost if free TicketCost="Free" elif (( Age > 54 )); then
#Senior citizens get
#1/2 price.
TicketCost="\$1" #it means 1 dollar, so it string 1$ dollar sign
else
#Normal price.
TicketCost="\$2"
fi
print "Cost is: $TicketCost"
exit
If Statement with Math Operations 1
#!/bin/ksh
# IfMath1.ksh
# This program introduces the "if" statement.
# - Jim
# prompt the user to enter a number.
print -n "Enter your age: "
read Age
# Create an if statement.
if (( Age >= 18 )) #don't put comments between if and then keywords
then #double (( means math, we can also do math comparisons apart of +,- etc
print "Vote!" #you can't create (( 18 >= Age, number must be on right side
fi
# Math tests: #you can do var to var or var to nr not nr to var
#
# (( Num1 == Num2 )) Note: == not =
# (( Num1 != Num2 ))
# (( Num1 < Num2 )) # (( Num1 > Num2 ))
# (( Num1 >= Num2 ))
# (( Num1 <= Num2 )) exit If Statement with Math Operations 2
#!/bin/ksh
# IfMath2.ksh
# This program introduces multiple tests
# ( AND => "&&" and OR => "||" )
# inside of an "if" statement.
# - Jim
# prompt the user to enter a number.
print -n "Enter your age: "
read Age
print -n "Enter the number of children you have: "
read NumberOfChildren
if ( (( Age < 30 )) && ((NumberOfChildren == 0 )) ); then #you don't need () single paratheses here # Note the use of == and not = print "Single and loving it!" fi if ( (( Age >= 30 )) || (( NumberOfChildren > 0 )) ); then
print "Bummer!"
print "Things could be worse. You could be a Cube fan."
fi
exit
If Statement with String Operations
#!/bin/ksh
#IfStrings.ksh
#Goes over testing strings
#instead of match. Also, it
#introduces pattern matching.
# - Norbert
# Here are math tests and the
# corresponding string tests:
# (Notice the square brackets
# and the double quotes.)
# Also, you need white space
# between [[ and var name.
#
# Equals:
# (( num1 == num2 )) #we didn't use $ operator cause Korn Shell knows that everything that is in (( is a variable
# [[ "$s1" = "$s2" ]] #if want test string vs string or string vs value; "$s1" --> variable s1
#
# To assign:
# s1=$s2 #s2="New York City" must use "" cause white space between words ksh interpr as 3 different vars
# #you use double quotes "" in case the value in var has spaces in it!
#
# Not equals:
# (( num1 != num2 ))
# [[ "$s1" != "$s2" ]]
#
#
# Less then:
# Before:
# (( num1 < num2 )) # [[ "$s1" < "$s2" ]] # # Note: This is based off of the # ASCII character set. # Therefore "Z" is before "a". # Also, in ASCII 19 is before 2. #cause it compares 1 against 2 then 19 against 2 # # # Greater than: # After: # (( num1 > num2 ))
# [[ "$s1" > "$s2" ]]
#
#
# Less than or equal to:
# (( num1 <= num2 )) # ( [[ "$s1" < "$s2" ]] || [[ "$s1" = "$s2" ]] ) # # Greater then or equal to: # (( num1 >= num2 ))
# ( [[ "$s1" > "$s2" ]] || [[ "$s1" = "$s2" ]] )
#
#
# Zero length:
# [[ -z "$s1" ]]
#
# Nonzero length:
# [[ -n "$s1" ]]
#
#
#
exit
Loop Break & Continue Statements
#!/bin/ksh
#
# break -> statement means to stop the loop
# if you are in the middle of do and done
# it will bring to the end of the loop write where after done is
#
#
# continue -> if you are in the middle of the cmd's
# and you decide that you don't want to execute all
# of the cmd's after it, it will move you back to the top of your # loop
# and start executing from there, it bypasses that are after it
#
#
# it is best to not use break or continue statements
# example1
for Var in 1 2 3 5 6 7 8
do
if (( Var == 3 ))
then
continue
fi
print "\$Var is $Var"
if (( Var == 6 ))
then
print "At 6; breaking!"
break
fi
done
print "Outside of for loop"
# example2 - multilevel breaking
# while test; do
# while test; do
# continue 2 #continue from outer loop(2lev loop)
# done
# done
# while test; do
# while test; do
# break 2 #break to outer loop level 2
# done
# done
Loop For
# In a list, when Korn shell sees
# a white space (space, a tab, or a
#
# boundary for a list entry.
# for Flower in rose Water Lily sunflower # got four entries
# for Flower in rose "Water Lily" sunflower # got three entries
# n=0
# while (( n < 6 )) # do # cube n # print cube # increment n by 1 # done # the same as in while above by using for loop for n in 0 1 2 3 4 5 do (( Cube = n * n * n )) print "$n cubed is $Cube." done print print -n "Waiting. Hit
read Waiting
print "The shells you can use are: "
for Shell in $(grep / /etc/shells) # $() -> whatever is inside do #() is unix cmd
print "\t$Shell" # \ -> means add to or remove from
done # meaning of the next character
# \t -> means indent tab
Loop Until
# while (test) # this test was true, do cmd's below
# do
# commands
# done
# until (test) # if test is false, then do cmd's below
# do
# commands
# done
# basically, the until loop
# is the opposite of the
# while loop.
# while loop runs a
# test. if the test is
# true, then the code
# inside of do to done
# is repeatedly executed
# until the test is false.
# "while (test)" is the same
# as "until ! (test)". And,
# "while ! (test)" is the same
# as "until (test)".
# "while not true" is the same as "until true".
# And "while true" is the same as "until not true"
#
# it means basically this :
# run code between do and done until (test) is false
# while (true)
# until (false)
# Make sure that you have the
# space between the ! and the test.
# Structure:
# until test
# do
# cmd's
# done
print "until with !."
n=0
# while (( n < 6 )); do until ! (( n < 6 )); do (( Cube = n * n * n )) print "$n cubed is $Cube." (( n = n + 1 )) done print print -n "Waiting. Hit
read Waiting
print "until without !."
n=0
until (( n>=6 )); do
(( Cube = n * n * n ))
print "$n cubed is $Cube."
(( n = n + 1 ))
done
Loop While
#!/bin/ksh
# while loop runs a
# test. if the test is
# true, then the code
# inside of do to done
# is repeatedly executed
# until the test is false
# if test
# then
# commands
# fi
# structure:
# while test
# do
# commands
# done
n=0
while (( n < 6 )); do (( Cube = n * n * n )) print "$n cubed is $Cube." # n do szescianu (( n = n+1 )) done while [[ -a /tmp/a.txt ]] #a -> any type of file, does this file exist
do
sleep 2 #unix cmd, means pause for 2 seconds
done
print "/tmp/a.txt is gone."
# we might e.g. check with df cmd whether ceratain fs is
# less then 85 %, as logn at it is we can wait e.g. for 10 seconds
# and perform another the same test and so on.
# that's how we use while with sleep cmd
More Pattern Matching
#!/bin/ksh
print -n "Enter yes or no: "
read Answear
#
#
#
if [[ "$Answear" = [yY]* ]]
then
# Matches "y*" or "Y*".
# Also works for more than 2
# characters. For example:
# [aeiouAEIOU]
print "Matched y* or Y*"
fi
#
print -n "Enter 1st word: "
read Answear
#
#
#
if [[ "$Answear" = [0-9]* ]]
then
# A range [0-9]
print "This is not a word."
fi
#
#
if [[ "$Answear" = [2-4]* ]]
then
# Smaller range.
print "Starts a 2, 3, or 4."
fi
#
#
if [[ "$Answear" = [13579]* ]]
then
# Odd number answear
print "Starts w/ an odd number."
fi
print -n "enter 2nd word: "
read Word
if [[ "$Word" = [a-zA-Z][a-zA-Z][a-zA-Z]* ]]
then
# Is a word of at least 3 characters.
print "Is a word of at least 3 chars."
fi
print -n "Enter 3rd word: "
read Answear
if [[ "#Answear" = [!a-z]* ]] # ! ---> means opposite is true e.g. Upper Case Letters and Numbers and other signs
then
print "Sth"
fi
exit
Pattern Matching 1
#!/bin/ksh
print -n "Enter a username: "
read UserName
if [[ "$UserName" = "bob" ]]
then
# match "bob".
# does not match "b0b", nor "Bob".
print "Matched \"bob\""
fi
if [[ "$FileName" = oracle??? ]] # without "" cause if with this, shell will interprete this as oracle???
then
# matches oracle followed
# by any 3 characters.
print "Matched \"oracle???\""
fi
if [[ "$FileName" = a* ]]; then
# matches: "a" followed by
# anything or nothing.
print "Matched \"a\" followed by"
print "anything or nothing."
fi
if [[ "$FileName" = "a*" ]]; then
# Matches "a*" only.
print "Matched \"a*\" only."
fi
exit
Pattern Matching 2
#!/bin/ksh
print -n "Enter a name: "
read Name
# ?() matches 0 or 1 of what
# is in the ().
# ? (pat)
# ? (pat1|pat2|pat3)
if [[ $Name = tom?(my) ]]
then
# tom, tommy
print "Hi Tom"
fi
if [[ $Name = [jJ]ohn?(ny|boy|nie) ]]
then
# [jJ]ohn, [jJ]ohnny,
# [jJ]ohnboy, [jJ]ohnnie
print "Hi John"
fi
if [[ $Name = ann?([ae]) ]]
then
# ann, anna, anne
print "Welcome Ann"
fi
exit
Pattern Matching 3
#!/bin/ksh
# ?(optional patterns) String
# String? (optional patterns)
# String? (optional patterns) String2
# String? (pat1|pat2|pat3)
#
# @() matches exactly 1 of what
# is in the ().
# @ (pat1|pat2|pat3)
# Useful for looking for a list
# of things
if [[ $Name = T@(homas|om|ommy) ]]
then
# Tom, Thomas, Tommy
print "Hi Tom"
fi
if [[ $Name = [jJ]ohn@(|ny|boy|nie) ]] # notice (|ny....) means that john is ok
then
# [jJ]ohn, [jJ]ohnny,
# [jJ]ohnboy, [jJ]ohnnie
print "Hi John"
fi
print -n "Enter a color: "
read Color
if [[ $Color = @(white|lemon|sky blue) ]]
then
# Nice colors.
print "relaxing color"
fi
if [[ $Name = ch@(|ap?(t)|apter)01.txt ]]
then
# ch01.txt, chap01.txt
# chapt01.txt, chapter01.txt
print "Found chapter01"
fi
exit
Pattern Matching 4
#!/bin/ksh
#
# @(req. patterns)String
# String@(req. patterns)
# String@(req. patterns)String2
# String@(pat1|pat2|pat3)
#
# *() matches 0 or more of what is
# in the () in any combination.
# *(pat1|pat2|pat3)
#
if [[ $Name = T*(homas|om|ommy) ]]
then
# T
# Tom, Thomas, Tommy
# Tomommy, Tommyhomas
# Tomommyommyom
print "Hi Tom"
fi
print -n "Enter an answear: "
read Answer
if [[ Answer = [Aa]*([a-z]) ]]
then
# Won't match "don't".
# Starts with a letter a or A.
print "Starts with a or A."
fi
# +(req pats) => matches one
# or more of the req. patterns.
# +([a-z]) => chars only
# + ([0-9]) => numbers only
# above matches 0008, 91, 0.
if [[ $Answer = +([a-z]\'+([a-z]) # if sbd type word don't
then
# could be a contracted would.
print "contraction."
fi
# !(excluded patterns) =>
# matches anything but what
# is in ()
# Think of it as * - excluded
# patterns.
# Examples:
read User
if [[ $User = !(root) ]]
then
# Anybody but root.
print "Anybody but root."
fi
read File
if [[ $File = Ora_!(jan|feb).txt ]]
then
# matches Ora_mar.txt, Ora_.txt
# doesn't march:
# Ora_feb.txt, Ora_jan.txt
# Found file
print "Found File!"
fi
exit
Reading Input
#!/bin/ksh
#ReadingInput.ksh
#
# This script shows how to read input
# from the user. Moreover, it warns you
# about how whitespace is used to
# input more then one variable.
# jakubn 666-666-6666
#Printing with the
print "Please enter a city name: "
#print a blank line
#Printing without the
print -n "Please enter a city name: " #cursor didn't go to the next line,
#Read in value: #read statement auto assumes when you type space within string as a end of variable
read City
print "The name of the city is: $City" #assign info to variable doesn't need $ sign, when getting info from var you need $ sign
#####
# Problems with white spaces:
print -n "Please enter the names of 2 cities: " #if you type New York Trenton, read will set New as City1 and York
Trenton as City2
#You can read in multiple values at once. #it is best to read one var at a time
read City1 City2
print "The name of the FIRST city is: $City1"
print "The name of the SECOND city is: $City2"
exit
Tests On Files
# Does the file exist?
if [[ -a /etc/passwd ]] #[] string test
then
print "passwd file exists."
fi
#
# Think of "a" as any
# type of file.
# ===
# Is it a regular file?
# Is it a file that has
# regular data?
if [[ -f /bin/ksh ]] #-f means file
then
# has text or is
# executable
print "ksh is Regular file"
fi
if [[ -d /etc ]] #-d means directory
then
print "Good. /etc exists."
fi
# Is it a shortcut?
# Is it a symbolic link?
if [[ -L /etc/aliases ]] #-L means symbolic link
then
ls -l /etc/aliases
fi
# Does the file have a size
# to it?
# if [[ -s file ]] #does a file has size or data in it, if file exist but is empty, this test is faulse
# -s for size
# Empty file or zero size:
if [[ ! (-s /tmp/a.txt) ]] #() is optional
then
print "/tmp/a.txt is empty."
fi
# Can YOU read the file?
# [[ -r file ]]
# if dir, then you can
# do an ls on dir
# but not ls -l
# Can YOU write to the file?
# [[ -w file ]]
# if dir, then you can
# write to that dir if
# you can also execute.
# can YOU execute the file?
# [[ -x file ]]
# if dir, then you can
# execute files in the dir
# and execute (rm, cd, ls -l)
# for that dir
# Are you the owner of the file?
# [[ -O file ]]
# Is your primary group the
# owner of the file?
# [[ -G file ]]
# [[ f1 -nt f2 ]]
# f1 is newer than f2
# [[ f1 -ot f2 ]]
# f1 is older than f2.
