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Only a few of Perl special variable have direct counterpart in Python. Another half dozen can be easily emulated (for example $.).The rest represent a real problem in translation.
Adapted from PERL TO PYTHON QUICK REFERENCE - Perl To Python Migration [Book] by Martin C. Brown
| Perl | Python equivalent | Reference |
|---|---|---|
| $!, $ERRNO, $OS_ERROR | Errors and messages are handled through the exception system. If you need to translate error numbers to strings use the os.strerror() function. To compare error numbers use the errno module | 96 |
| $", $LIST_SEPARATOR | Use string.join() to manually bond lists together | 189 |
| $#, $OFMT | Use the format % tuple operator to format a number | 68 |
| $$, $PID, $PROCESS_ID | os.getpid() | 176 |
| $%, $FORMAT_PAGE_NUMBER | N/A | |
| $&, $MATCH | You'll need to use a Match object returned by the functions in the re module to obtain the matches for individual groups | 201 |
| $(, $GID, $REAL_GROUP_ID | os.getgid() | 176 |
| $), $EFFECTIVE_GROUP_ID, $EGID | os.getegid() | 176 |
| $*, $MULTILINE_MATCHING | Use the M flag to the functions in the re module to perform multiline matches and substitutions | 199 |
| $-, $FORMAT_LINES_LEFT | N/A | |
| $', $POSTMATCH | You'll need to use a Match object returned by the functions in the re module to obtain the matches for individual groups | 201 |
| $,, $OFS, $OUTPUT_FIELD_SEPARATOR | You need to explicitly define any output record separator during data output | |
| $. $NR, $INPUT_LINE_NUMBER | There is no direct counterpart but you can emulate it easlity by using a counter within a loop to get this information | |
| $/, $RS, $INPUT_RECORD_SEPARATOR | You'll need to manually read the information and then use string.split() on a specific character to emulate this functionality | 189 |
| $:, $FORMAT_LINE_BREAK_CHARACTERS | N/A | |
| $;, $SUBSCRIPT _SEPARATOR, $SUBSEP | N/A | |
| $?, $CHILD_ERROR | You'll need to use the return value from the wait() call to determine the child's exit status | 191 |
| $@, $EVAL_ERROR | Errors through Python's exec statement and eval() function are handled through the exception mechanism | 147 |
| $[ | N/A | |
| $\, $ORS, $OUTPUT_RECORD_SEPARATOR | The print statement automatically adds the correct line separator for the current platform. To determine the correct value, access the os.linesep object. Note that changing the value of this object does not affect the print statement | 176 |
| $], $OLD_PERL_VERSION | sys.version[0] | 170 |
| $^, $FORMAT_TOP_NAME | N/A | |
| $^A, $ACCUMULATOR | N/A | |
| $^C, $COMPILING | N/A | |
| $^D, $DEBUGGING | N/A | |
| $^E, $EXTENDED_OS_ERROR | N/A | |
| $^F, $SYSTEM_FD_MAX | N/A | |
| $^H | N/A | |
| $^I, $INPLACE_EDIT | N/A | |
| $^L, $FORMAT_FORMFEED | N/A | |
| $^M | N/A | |
| $^O, $OSNAME | sys.platform() or os.uname(). The latter return 'posix" for all Unixes so it is useful only for distinguishing Unix from Windows | 173, 177 |
| $^P, $PERLDB | N/A | |
| $^R, $LAST_REGEXP_CODE_RESULT | N/A | |
| $^S, $EXCEPTIONS_BEING_CAUGHT | N/A | |
| $^T, $BASETIME | N/A | |
| $^V, $PERL_VERSION | sys.version actually returns a list, elements zero and one are the major and minor version numbers for the interpreter | 170 |
| $^W, $WARNING | N/A | |
| $^X, $EXECUTABLE_NAME | sys.executable() | |
| $_ $ARG | Python has no "scratchpad" variable ( $_ in Perl) – if a function is expecting an argument and you don't supply one the interpreter will raise an exception. | |
| $`, $PREMATCH | You'll need to use a Match object returned by the functions in the re module to obtain the matches for individual groups | 201 |
| ${ ^WARNING_BITS} | N/A | |
| ${ ^WIDE_SYSTEM_CALLS} | N/A | |
| $|, $AUTOFLUSH, $OUTPUT _AUTOFLUSH | Use f.flush() to manually flush output to a file | 215 |
| $~, $FORMAT_NAME | N/A | |
| $+, $LAST_PAREN_MATCH | You'll need to use a Match object returned by the functions in the re module to obtain the matches for individual groups | 201 |
| $<, $REAL_USER_ID, $UID | os.getuid() | |
| $=, $FORMAT_LINES_PER_PAGE | N/A | |
| $>, $EFFECTIVE_USER_ID, $EUID | os.geteuid() | |
| $0, $PROGRAM_NAME | sys.argv[0] | |
| $0..x | You'll need to use a Match object returned by the functions in the re module to obtain the matches for individual groups. To include a group within a regular expression replacement string use the \# format | 201 |
| $ARGV | N/A | |
| %^H | N/A | |
| %ENV | os.environ/os.putenv() the os.environ object is a mapping object, so changes to the elements within the object are reflected in the environment variables for the current process. This is identical to the %ENV hash in Perl | 175 |
| %INC | sys.modules the sys.modules list includes information about the module names, where they were loaded from, and also their aliaswithin the symbol table | 171 |
| %SIG | signal.getsignal(), signal.signal() you must use the signal module to install or release signal handlers | 183 |
| @- | Use the Match object returned by the functions in the re module to get individual matches | 201 |
| @_, @ARG | Arguments are extracted from a function call by name in the function definition | |
| @+ | Use the Match object returned by the functions in the re module to get individual matches | 201 |
| @ARGV | sys.argv[1:] | 67 |
| @INC | sys.path | 127, 172 |
| @ISA | Class inheritance is handled on a class by class basis. During class definition you can access the base classes for a class using the __bases__ attribute for a given class | 135 |
| _ (stat/lstat filehandle cache) | N/A | |
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Mar 05, 2020 | www.networkworld.com
One quick way to determine whether the command you are using is a bash built-in or not is to use the command "command". Yes, the command is called "command". Try it with a -V (capital V) option like this:
$ command -V command command is a shell builtin $ command -V echo echo is a shell builtin $ command -V date date is hashed (/bin/date)When you see a "command is hashed" message like the one above, that means that the command has been put into a hash table for quicker lookup.
... ... ... How to tell what shell you're currently usingIf you switch shells you can't depend on $SHELL to tell you what shell you're currently using because $SHELL is just an environment variable that is set when you log in and doesn't necessarily reflect your current shell. Try ps -p $$ instead as shown in these examples:
$ ps -p $$ PID TTY TIME CMD 18340 pts/0 00:00:00 bash <== $ /bin/dash $ ps -p $$ PID TTY TIME CMD 19517 pts/0 00:00:00 dash <==Built-ins are extremely useful and give each shell a lot of its character. If you use some particular shell all of the time, it's easy to lose track of which commands are part of your shell and which are not.
Differentiating a shell built-in from a Linux executable requires only a little extra effort.
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PERL TO PYTHON QUICK REFERENCE - Perl To Python Migration [Book]
Perl To Python Migration [Book]
Perl to Python Migration Martin C. Brown 9780201734881 Amazon.com Books
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