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Perl special variable mapping to Python

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] How to tell if you're using a bash builtin in Linux

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 using

If 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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