11.8. Handling Time Zones, Daylight Saving Time, and Leap Years
by Cameron Newham, JP Vossen, Carl Albing
bash Cookbook
- bash Cookbook
- A Note Regarding Supplemental Files
- Preface
- 1. Beginning bash
- 0. Why bash?
- 0. The bash Shell
- 1.1. Decoding the Prompt
- 1.2. Showing Where You Are
- 1.3. Finding and Running Commands
- 1.4. Getting Information About Files
- 1.5. Showing All Hidden (dot) Files in the Current Directory
- 1.6. Using Shell Quoting
- 1.7. Using or Replacing Built-ins and External Commands
- 1.8. Determining If You Are Running Interactively
- 1.9. Setting bash As Your Default Shell
- 1.10. Getting bash for Linux
- 1.11. Getting bash for xBSD
- 1.12. Getting bash for Mac OS X
- 1.13. Getting bash for Unix
- 1.14. Getting bash for Windows
- 1.15. Getting bash Without Getting bash
- 1.16. Learning More About bash Documentation
- 2. Standard Output
- 2.1. Writing Output to the Terminal/Window
- 2.2. Writing Output but Preserving Spacing
- 2.3. Writing Output with More Formatting Control
- 2.4. Writing Output Without the Newline
- 2.5. Saving Output from a Command
- 2.6. Saving Output to Other Files
- 2.7. Saving Output from the ls Command
- 2.8. Sending Both Output and Error Messages to Different Files
- 2.9. Sending Both Output and Error Messages to the Same File
- 2.10. Appending Rather Than Clobbering Output
- 2.11. Using Just the Beginning or End of a File
- 2.12. Skipping a Header in a File
- 2.13. Throwing Output Away
- 2.14. Saving or Grouping Output from Several Commands
- 2.15. Connecting Two Programs by Using Output As Input
- 2.16. Saving a Copy of Output Even While Using It As Input
- 2.17. Connecting Two Programs by Using Output As Arguments
- 2.18. Using Multiple Redirects on One Line
- 2.19. Saving Output When Redirect Doesn’t Seem to Work
- 2.20. Swapping STDERR and STDOUT
- 2.21. Keeping Files Safe from Accidental Overwriting
- 2.22. Clobbering a File on Purpose
- 3. Standard Input
- 4. Executing Commands
- 4.1. Running Any Executable
- 4.2. Telling If a Command Succeeded or Not
- 4.3. Running Several Commands in Sequence
- 4.4. Running Several Commands All at Once
- 4.5. Deciding Whether a Command Succeeds
- 4.6. Using Fewer if Statements
- 4.7. Running Long Jobs Unattended
- 4.8. Displaying Error Messages When Failures Occur
- 4.9. Running Commands from a Variable
- 4.10. Running All Scripts in a Directory
- 5. Basic Scripting: Shell Variables
- 5.1. Documenting Your Script
- 5.2. Embedding Documentation in Shell Scripts
- 5.3. Promoting Script Readability
- 5.4. Separating Variable Names from Surrounding Text
- 5.5. Exporting Variables
- 5.6. Seeing All Variable Values
- 5.7. Using Parameters in a Shell Script
- 5.8. Looping Over Arguments Passed to a Script
- 5.9. Handling Parameters with Blanks
- 5.10. Handling Lists of Parameters with Blanks
- 5.11. Counting Arguments
- 5.12. Consuming Arguments
- 5.13. Getting Default Values
- 5.14. Setting Default Values
- 5.15. Using null As a Valid Default Value
- 5.16. Using More Than Just a Constant String for Default
- 5.17. Giving an Error Message for Unset Parameters
- 5.18. Changing Pieces of a String
- 5.19. Using Array Variables
- 6. Shell Logic and Arithmetic
- 6.1. Doing Arithmetic in Your Shell Script
- 6.2. Branching on Conditions
- 6.3. Testing for File Characteristics
- 6.4. Testing for More Than One Thing
- 6.5. Testing for String Characteristics
- 6.6. Testing for Equal
- 6.7. Testing with Pattern Matches
- 6.8. Testing with Regular Expressions
- 6.9. Changing Behavior with Redirections
- 6.10. Looping for a While
- 6.11. Looping with a read
- 6.12. Looping with a Count
- 6.13. Looping with Floating-Point Values
- 6.14. Branching Many Ways
- 6.15. Parsing Command-Line Arguments
- 6.16. Creating Simple Menus
- 6.17. Changing the Prompt on Simple Menus
- 6.18. Creating a Simple RPN Calculator
- 6.19. Creating a Command-Line Calculator
- 7. Intermediate Shell Tools I
- 7.1. Sifting Through Files for a String
- 7.2. Getting Just the Filename from a Search
- 7.3. Getting a Simple True/False from a Search
- 7.4. Searching for Text While Ignoring Case
- 7.5. Doing a Search in a Pipeline
- 7.6. Paring Down What the Search Finds
- 7.7. Searching with More Complex Patterns
- 7.8. Searching for an SSN
- 7.9. Grepping Compressed Files
- 7.10. Keeping Some Output, Discarding the Rest
- 7.11. Keeping Only a Portion of a Line of Output
- 7.12. Reversing the Words on Each Line
- 7.13. Summing a List of Numbers
- 7.14. Counting String Values
- 7.15. Showing Data As a Quick and Easy Histogram
- 7.16. Showing a Paragraph of Text After a Found Phrase
- 8. Intermediate Shell Tools II
- 8.1. Sorting Your Output
- 8.2. Sorting Numbers
- 8.3. Sorting IP Addresses
- 8.4. Cutting Out Parts of Your Output
- 8.5. Removing Duplicate Lines
- 8.6. Compressing Files
- 8.7. Uncompressing Files
- 8.8. Checking a tar Archive for Unique Directories
- 8.9. Translating Characters
- 8.10. Converting Uppercase to Lowercase
- 8.11. Converting DOS Files to Linux Format
- 8.12. Removing Smart Quotes
- 8.13. Counting Lines, Words, or Characters in a File
- 8.14. Rewrapping Paragraphs
- 8.15. Doing More with less
- 9. Finding Files: find, locate, slocate
- 9.1. Finding All Your MP3 Files
- 9.2. Handling Filenames Containing Odd Characters
- 9.3. Speeding Up Operations on Found Files
- 9.4. Finding Files Across Symbolic Links
- 9.5. Finding Files Irrespective of Case
- 9.6. Finding Files by Date
- 9.7. Finding Files by Type
- 9.8. Finding Files by Size
- 9.9. Finding Files by Content
- 9.10. Finding Existing Files and Content Fast
- 9.11. Finding a File Using a List of Possible Locations
- 10. Additional Features for Scripting
- 11. Working with Dates and Times
- 11.1. Formatting Dates for Display
- 11.2. Supplying a Default Date
- 11.3. Automating Date Ranges
- 11.4. Converting Dates and Times to Epoch Seconds
- 11.5. Converting Epoch Seconds to Dates and Times
- 11.6. Getting Yesterday or Tomorrow with Perl
- 11.7. Figuring Out Date and Time Arithmetic
- 11.8. Handling Time Zones, Daylight Saving Time, and Leap Years
- 11.9. Using date and cron to Run a Script on the Nth Day
- 12. End-User Tasks As Shell Scripts
- 13. Parsing and Similar Tasks
- 13.1. Parsing Arguments for Your Shell Script
- 13.2. Parsing Arguments with Your Own Error Messages
- 13.3. Parsing Some HTML
- 13.4. Parsing Output into an Array
- 13.5. Parsing Output with a Function Call
- 13.6. Parsing Text with a read Statement
- 13.7. Parsing with read into an Array
- 13.8. Getting Your Plurals Right
- 13.9. Taking It One Character at a Time
- 13.10. Cleaning Up an SVN Source Tree
- 13.11. Setting Up a Database with MySQL
- 13.12. Isolating Specific Fields in Data
- 13.13. Updating Specific Fields in Data Files
- 13.14. Trimming Whitespace
- 13.15. Compressing Whitespace
- 13.16. Processing Fixed-Length Records
- 13.17. Processing Files with No Line Breaks
- 13.18. Converting a Data File to CSV
- 13.19. Parsing a CSV Data File
- 14. Writing Secure Shell Scripts
- 14.1. Avoiding Common Security Problems
- 14.2. Avoiding Interpreter Spoofing
- 14.3. Setting a Secure $PATH
- 14.4. Clearing All Aliases
- 14.5. Clearing the Command Hash
- 14.6. Preventing Core Dumps
- 14.7. Setting a Secure $IFS
- 14.8. Setting a Secure umask
- 14.9. Finding World-Writable Directories in Your $PATH
- 14.10. Adding the Current Directory to the $PATH
- 14.11. Using Secure Temporary Files
- 14.12. Validating Input
- 14.13. Setting Permissions
- 14.14. Leaking Passwords into the Process List
- 14.15. Writing setuid or setgid Scripts
- 14.16. Restricting Guest Users
- 14.17. Using chroot Jails
- 14.18. Running As a Non-root User
- 14.19. Using sudo More Securely
- 14.20. Using Passwords in Scripts
- 14.21. Using SSH Without a Password
- 14.22. Restricting SSH Commands
- 14.23. Disconnecting Inactive Sessions
- 15. Advanced Scripting
- 15.1. Finding bash Portably for #!
- 15.2. Setting a POSIX $PATH
- 15.3. Developing Portable Shell Scripts
- 15.4. Testing Scripts in VMware
- 15.5. Using for Loops Portably
- 15.6. Using echo Portably
- 15.7. Splitting Output Only When Necessary
- 15.8. Viewing Output in Hex
- 15.9. Using bash Net-Redirection
- 15.10. Finding My IP Address
- 15.11. Getting Input from Another Machine
- 15.12. Redirecting Output for the Life of a Script
- 15.13. Working Around “argument list too long” Errors
- 15.14. Logging to syslog from Your Script
- 15.15. Sending Email from Your Script
- 15.16. Automating a Process Using Phases
- 16. Configuring and Customizing bash
- 16.1. bash Startup Options
- 16.2. Customizing Your Prompt
- 16.3. Change Your $PATH Permanently
- 16.4. Change Your $PATH Temporarily
- 16.5. Setting Your $CDPATH
- 16.6. Shortening or Changing Command Names
- 16.7. Adjusting Shell Behavior and Environment
- 16.8. Adjusting readline Behavior Using .inputrc
- 16.9. Keeping a Private Stash of Utilities by Adding ~/bin
- 16.10. Using Secondary Prompts: $PS2, $PS3, $PS4
- 16.11. Synchronizing Shell History Between Sessions
- 16.12. Setting Shell History Options
- 13. Creating a Better cd Command
- 16.14. Creating and Changing into a New Directory in One Step
- 16.15. Getting to the Bottom of Things
- 16.16. Adding New Features to bash Using Loadable Built-ins
- 16.17. Improving Programmable Completion
- 16.18. Using Initialization Files Correctly
- 16.19. Creating Self-Contained, Portable RC Files
- 16.20. Getting Started with a Custom Configuration
- 17. Housekeeping and Administrative Tasks
- 17.1. Renaming Many Files
- 17.2. Using GNU Texinfo and Info on Linux
- 17.3. Unzipping Many ZIP Files
- 17.4. Recovering Disconnected Sessions Using screen
- 17.5. Sharing a Single bash Session
- 17.6. Logging an Entire Session or Batch Job
- 17.7. Clearing the Screen When You Log Out
- 17.8. Capturing File Metadata for Recovery
- 17.9. Creating an Index of Many Files
- 17.10. Using diff and patch
- 17.11. Counting Differences in Files
- 17.12. Removing or Renaming Files Named with Special Characters
- 17.13. Prepending Data to a File
- 17.14. Editing a File in Place
- 17.15. Using sudo on a Group of Commands
- 17.16. Finding Lines in One File But Not in the Other
- 17.17. Keeping the Most Recent N Objects
- 17.18. Grepping ps Output Without Also Getting the grep Process Itself
- 17.19. Finding Out Whether a Process Is Running
- 17.20. Adding a Prefix or Suffix to Output
- 17.21. Numbering Lines
- 17.22. Writing Sequences
- 17.23. Emulating the DOS Pause Command
- 17.24. Commifying Numbers
- 18. Working Faster by Typing Less
- 19. Tips and Traps: Common Goofs for Novices
- 19.1. Forgetting to Set Execute Permissions
- 19.2. Fixing “No such file or directory” Errors
- 19.3. Forgetting That the Current Directory Is Not in the $PATH
- 19.4. Naming Your Script Test
- 19.5. Expecting to Change Exported Variables
- 19.6. Forgetting Quotes Leads to “command not found” on Assignments
- 19.7. Forgetting That Pattern Matching Alphabetizes
- 19.8. Forgetting That Pipelines Make Subshells
- 19.9. Making Your Terminal Sane Again
- 19.10. Deleting Files Using an Empty Variable
- 19.11. Seeing Odd Behavior from printf
- 19.12. Testing bash Script Syntax
- 19.13. Debugging Scripts
- 19.14. Avoiding “command not found” When Using Functions
- 19.15. Confusing Shell Wildcards and Regular Expressions
- A. Reference Lists
- A.1. bash Invocation
- A.2. Prompt String Customizations
- A.3. ANSI Color Escape Sequences
- A.4. Built-in Commands and Reserved Words
- A.5. Built-in Shell Variables
- A.6. set Options
- A.7. shopt options
- A.8. Adjusting Shell Behavior Using set, shopt, and Environment Variables
- A.9. Test Operators
- A.10. I/O Redirection
- A.11. echo Options and Escape Sequences
- A.12. printf
- A.13. Date and Time String Formatting with strftime
- A.14. Pattern-Matching Characters
- A.15. extglob Extended Pattern-Matching Operators
- A.16. tr Escape Sequences
- A.17. Readline Init File Syntax
- A.18. emacs Mode Commands
- A.19. vi Control Mode Commands
- A.20. Table of ASCII Values
- B. Examples Included with bash
- C. Command-Line Processing
- D. Revision Control
- E. Building bash from Source
- Index
- About the Authors
- Colophon
- Copyright
Don’t. This is a lot trickier than it sounds. Leave it to code that’s already been in use and debugged for years, and just use a tool that can handle your needs. Odds are high that one of the other recipes in this chapter has covered what you need, probably using GNU date. If not, there is almost certainly another tool out there that can do the job. For example, there are a number of excellent Perl modules that deal with dates and times.
Really, we aren’t kidding. This is a real nightmare to get right. Save yourself a lot of agony and just use a tool.
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