just want to know what are the main differences among them? and the power of each language (where it’s better to use it).

Edit: it’s not “vs.” like topic, just information.

In order of appearance, the languages are sed, awk, perl, python.

The sed program is a stream editor and is designed to apply the actions from a script to each line (or, more generally, to specified ranges of lines) of the input file or files. Its language is based on ed, the Unix editor, and although it has conditionals and so on, it is hard to work with for complex tasks. You can work minor miracles with it – but at a cost to the hair on your head. However, it is probably the fastest of the programs when attempting tasks within its remit. (It has the least powerful regular expressions of the programs discussed – adequate for many purposes, but certainly not PCRE – Perl-Compatible Regular Expressions)

The awk program (name from the initials of its authors – Aho, Weinberger, and Kernighan) is a tool initially for formatting reports. It can be used as a souped-up sed; in its more recent versions, it is computationally complete. It uses an interesting idea – the program is based on ‘patterns matched’ and ‘actions taken when the pattern matches’. The patterns are fairly powerful (Extended Regular Expressions). The language for the actions is similar to C. One of the key features of awk is that it splits the input automatically into records and each record into fields.

Perl was written in part as an awk-killer and sed-killer. Two of the programs provided with it are a2p and s2p for converting awk scripts and sed scripts into Perl. Perl is one of the earliest of the next generation of scripting languages (Tcl/Tk can probably claim primacy). It has powerful integrated regular expression handling with a vastly more powerful language. It provides access to almost all system calls and has the extensibility of the CPAN modules. (Neither awk nor sed is extensible.) One of Perl’s mottos is “TMTOWTDI – There’s more than one way to do it” (pronounced “tim-toady”). Perl has ‘objects’, but it is more of an add-on than a fundamental part of the language.

Python was written last, and probably in part as a reaction to Perl. It has some interesting syntactic ideas (indenting to indicate levels – no braces or equivalents). It is more fundamentally object-oriented than Perl; it is just as extensible as Perl.

OK – when to use each?

• Sed – when you need to do simple text transforms on files.
• Awk – when you only need simple formatting and summarisation or transformation of data.
• Perl – for almost any task, but especially when the task needs complex regular expressions.
• Python – for the same tasks that you could use Perl for.

I’m not aware of anything that Perl can do that Python can’t, nor vice versa. The choice between the two would depend on other factors. I learned Perl before there was a Python, so I tend to use it. Python has less accreted syntax and is generally somewhat simpler to learn. Perl 6, when it becomes available, will be a fascinating development.

(Note that the ‘overviews’ of Perl and Python, in particular, are woefully incomplete; whole books could be written on the topic.)

After mastering a few dozen languages, you get tired of people like S. Lott (see his controversial answer to this question, nearly half as many down-votes as up (+45/-22) six years after answering).

Sed is the best tool for extremely simple command-line pipelines. In the hands of a sed master, it’s suitable for one-offs of arbitrary complexity, but it should not be used in production code except in very simple substitution pipelines. Stuff like ‘s/this/that/.’

Gawk (the GNU awk) is by far the best choice for complex data reformatting when there is only a single input source and a single output (or, multiple outputs sequentially written). Since a great deal of real-world work conforms to this description, and a good programmer can learn gawk in two hours, it is the best choice. On this planet, simpler and faster is better!

Perl or Python are far better than any version of awk or sed when you have very complex input/output scenarios. The more complex the problem is, the better off you are using python, from a maintenance and readability standpoint. Note, however, that a good programmer can write readable code in any language, and a bad programmer can write unmaintainable crap in any useful language, so the choice of perl or python can safely be left to the preferences of the programmer if said programmer is skilled and clever.

I wouldn’t call sed a fully-fledged programming language, it is a stream editor with language constructs aimed at editing text files programmatically.

Awk is a little more of a general purpose language but it is still best suited for text processing.

Perl and Python are fully fledged, general purpose programming languages. Perl has its roots in text processing and has a number of awk-like constructs (there is even an awk-to-perl script floating around on the net). There are many differences between Perl and Python, your best bet is probably to read the summaries of both languages on something like Wikipedia to get a good grasp on what they are.

First, there are two unrelated things in the list “Perl, Python awk and sed”.

Thing 1 – simplistic text manipulation tools.

• sed. It has a fixed, relatively simple scope of work defined by the idea of reading and examining each line of a file. sed is not designed to be particularly readable. It is designed to be very small and very efficient on very tiny unix servers.

• awk. It has a slightly less fixed, less simple scope of work. However, the main loop of an awk program is defined by the implicit reading of lines of a source file.

These are not “complete” programming languages. While you can — with some work — write fairly sophisticated programs in awk, it rapidly gets complicated and difficult to read.

Thing 2 – general-purposes programming languages. These have a rich variety of statement types, numerous built-in data structures, and no wired-in assumptions or shortcuts to speak of.

• Perl.

• Python.

When to use them.

• sed. Never. It really doesn’t have any value in the modern era of computers with more than 32K of memory. Perl or Python do the same things more clearly.

• awk. Never. Like sed, it reflects an earlier era of computing. Rather than maintain this language (in addition to all the other required for a successful system), it’s more pleasant to simply do everything in one pleasant language.

• Perl. Any programming problem of any kind. If you like free-thinking syntax, where there are many, many ways to do the same thing, perl is fun.

• Python. Any programming problem of any kind. If you like fairly limited syntax, where there are fewer choices, less subtlety, and (perhaps) more clarity. Python’s object-oriented nature makes it more suitable for large, complex problems.

Background — I’m not bashing sed and awk out of ignorance. I learned awk over 20 years ago. Did many things with it; used to teach it as a core unix skill. I learned Perl about 15 years ago. Did many sophisticated things with it. I’ve left both behind because I can do the same things in Python — and it is simpler and more clear.

There are two serious problems with sed and awk, neither of which are their age.

1. The incompleteness of their implementation. Everything sed and awk do can be done in Python or Perl, often more simply and sometimes faster, too. A shell pipeline has some performance advantages because of its multi-processing. Python offers a subprocess module to allow me to recover those advantages.

2. The need to learn yet another language. By doing things in Python (or Perl) your implementation depends on fewer languages, with a resulting increase in clarity.

When to use them: awk – never – S. Lott.

I think S. Lott slightly missed the mark with this recommendation. The fact is, on Linux and the other UNIX environments, awk is a useful tool to be used with bash, sh, and ksh for quick text processings. The idea of scripting itself is you solve your problem by gluing together this tool, that tool. Hence in admin scripts, it is common to has ls, grep, |, awk, time, ps, etc. Each is a tool that the scripter combines like a builder brick by brick to finish the building (to solve the problem at hand).

For instance I am a team member of the team managing paintball gear supplies dotcom. This e-commerce site is based on the LAMP stack. For automated processing and normalizing data feeds from various suppliers into the back end database, we employ and maintain a diversified mix of scripts, including bash, perl, php, and even expect. Each has its strengths based on the available modules and API. In the bash scripts we do quick patterns match and appropriate actions on the patterns as needed using awk without the need to switch to PERL. One thing I would also like to point out, which has not been emphasized in the thread, is that a fair number of these scripts were purchased, or gotten from the open source. If the script came as Perl, we maintain it as Perl; if the script came as Php, we maintain it as Php; if it came as bash, we maintain it as bash; we do not re-write it in another language just because we think it is less efficient in the original language.