This question already has answers here:
What is the difference between $(command) and `command` in shell programming?
(6 answers)
Closed 8 years ago.
What's the preferred way to do command substitution in bash?
I've always done it like this:
echo "Hello, `whoami`."
But recently, I've often seen it written like this:
echo "Hello, $(whoami)."
What's the preferred syntax, and why? Or are they pretty much interchangeable?
I tend to favor the first, simply because my text editor seems to know what it is, and does syntax highlighting appropriately.
I read here that escaped characters act a bit differently in each case, but it's not clear to me which behavior is preferable, or if it just depends on the situation.
Side question: Is it bad practice to use both forms in one script, for example when nesting command substitutions?
There are several questions/issues here, so I'll repeat each section of the poster's text, block-quoted, and followed by my response.
What's the preferred syntax, and why? Or are they pretty much interchangeable?
I would say that the $(some_command) form is preferred over the `some_command` form. The second form, using a pair of backquotes (the "`" character, also called a backtick and a grave accent), is the historical way of doing it. The first form, using dollar sign and parentheses, is a newer POSIX form, which means it's probably a more standard way of doing it. In turn, I'd think that that means it's more likely to work correctly with different shells and with different *nix implementations.
Another reason given for preferring the first (POSIX) form is that it's easier to read, especially when command substitutions are nested. Plus, with the backtick form, the backtick characters have to be backslash-escaped in the nested (inner) command substitutions.
With the POSIX form, you don't need to do that.
As far as whether they're interchangeable, well, I'd say that, in general, they are interchangeable, apart from the exceptions you mentioned for escaped characters. However, I don't know and cannot say whether all modern shells and all modern *nixes support both forms. I doubt that they do, especially older shells/older *nixes. If I were you, I wouldn't depend on interchangeability without first running a couple of quick, simple tests of each form on any shell/*nix implementations that you plan to run your finished scripts on.
I tend to favor the first, simply because my text editor seems to know what it is, and does syntax highlighting appropriately.
It's unfortunate that your editor doesn't seem to support the POSIX form; maybe you should check to see if there's an update to your editor that supports the POSIX way of doing it. Long shot maybe, but who knows? Or, maybe you should even consider trying a different editor.
GGG, what text editor are you using???
I read here that escaped characters act a bit differently in each case, but it's not clear to me which behavior is preferable, or if it just depends on the situation.
I'd say that it depends on what you're trying to accomplish; in other words, whether you're using escaped characters along with command substitution or not.
Side question: Is it bad practice to use both forms in one script, for example when nesting command substitutions?
Well, it might make the script slightly easier to READ (typographically speaking), but harder to UNDERSTAND! Someone reading your script (or YOU, reading it six months later!) would likely wonder why you didn't just stick to one form or the other--unless you put some sort of note about why you did this in the comments. Plus, mixing both forms in one script would make that script less likely to be portable: In order for the script to work properly, the shell that's executing it has to support BOTH forms, not just one form or the other.
For making a shell script understandable, I'd personally prefer sticking to one form or the other throughout any one script, unless there's a good technical reason to do otherwise. Moreover, I'd prefer the POSIX form over the older form; again, unless there's a good technical reason to do otherwise.
For more on the topic of command substitution, and the two different forms for doing it, I suggest you refer to the section on command substitution in the O'Reilly book "Classic Shell Scripting," second edition, by Robbins and Beebe. In that section, the authors state that the POSIX form for command substitution "is recommended for all new development." I have no financial interest in this book; it's just one I have (and love) on shell scripting, though it's more for intermediate or advanced shell scripting, and not really for beginning shell scripting.
-B.
You can read the differences from bash manual. At most case, they are interchangeable.
One thing to mention is that you should escape backquote to nest commands:
$ echo $(echo hello $(echo word))
hello word
$ echo `echo hello \`echo word\``
hello word
The backticks are compatible with ancient shells, and so scripts that need to be portable (such as GNU autoconf snippets) should prefer them.
The $() form is a little easier on the eyes, esp. after a few levels of escaping.
Related
I find makefiles very useful, and the header of each recipe
<target> : [dependencies]
is helpful. Within a recipe, the prefixes # and - are useful, as well as the automatically-defined variables like $# and $?. However, besides that, I find the way of coding the actual recipe to be strange and unhelpful. There are so many questions on StackOverflow along the lines of "how to do this in a makefile" for something that's simple (or at least more familiar) to do in bash.
Is there a reason why the recipe contents are not just interpreted as a regular shell script? Reading the manual pages, there seems to be many tools with equivalent functionality to a shell script but with different syntax. I end up specifying .ONESHELL and escaping $ with $$, or sometimes just call a script from the recipe when I can't figure out how to make it work in a makefile. My question is whether this is just unfortunate design, or are there are important features of makefiles that force them to be designed this way?
I don't really know how to answer your question. Probably that means it's not really appropriate for StackOverflow.
The requirement for using $$ instead of $ is obvious. The reasoning for using a separate shell for each logical line of a makefile instead of passing the entire recipe to a single shell, is less clear. It could have worked either way, and this is the way it was chosen.
There is one advantage to the way it works now, although maybe most people don't care about it: you only have to indent the first recipe line with TAB, if you use backslash newline to continue each line. If you don't use backslash newline, then every line has to be indented with TAB else you don't know where the recipe ends.
If your question is, could Stuart Feldman have made very different syntax decisions that would have made it easier to write long/complex recipes in makefiles, then sure. Choosing a more obscure character than $ as a variable introducer would reduce the amount of escaping (although, shell scripting uses pretty much every special character somewhere so "reduce" is the best you can do). Choosing an explicit "start/stop" character sequence for recipes would make it simpler to write long recipes, possibly at the expense of some readability.
But that's not how it was done.
I am trying to write an automate process for AWS that requires some JSON processing and other things in bash script. I am following a few blogs for bash script and I found this:
a=b
with the following note:
There is no space on either side of the equals ( = ) sign. We
also leave off the $ sign from the beginning of the variable name when
setting it
This is ugly and very difficult to read and comparing to other scripting languages, it is easy for user to make a mistake when writing a bash script by leaving space in between. I think everyone like to write clean and readable code, this restriction for sure is bad for code readability.
Can you explain why? explanation with examples are highly appreciated.
It's because otherwise the syntax would be ambiguous. Consider this command line:
cat = foo
Is that an assignment to the variable cat, or running the command cat with the arguments "=" and "foo"? Note that "=" and "foo" are both perfectly legal filenames, and therefore reasonable things to run cat on. Shell syntax settles this in favor of the command interpretation, so to avoid this interpretation you need to leave out the spaces. cat =foo has the same problem.
On the other hand, consider:
var= cat
Is that the command cat run with the variable var set to the empty string (i.e. a shorthand for var='' cat), or an assignment to the shell variable var? Again, the shell syntax favors the command interpretation so you need to avoid the temptation to add spaces.
There are many places in shell syntax where spaces are important delimiters. Another commonly-messed-up place is in tests, where if you leave out any of the spaces in:
if [ "$foo" = "$bar" ]
...it will lead to a different meaning, which might cause an error, or might just silently do the wrong thing.
What I'm getting at is that shell syntax does not allow you to arbitrarily add or remove spaces to improve readability. Don't even try, you'll just break things.
What you need to understand is that the shell language and syntax is old. Really old. The first version of the UNIX shell with variables was the Bourne shell which was designed and implemented in 1977. Back then, there were few precedents. (AFAIK, just the Thompson shell, which didn't support variables according to the manual entry.)
The rationale for the design decisions in the 1970's are ... lost in the mists of time. The design decisions were made by Steve Bourne and colleagues working at Bell Labs on v6 UNIX. They probably had no idea that their decisions would still be relevant 40+ years later.
The Bourne shell was designed to be general purpose and simple to use ... compared with the alternative of writing programs in C. And small. It was an outstanding success in those terms.
However, any language that is successful has the "problem" that it gets widely adopted. And that makes it more difficult to fix any issues (real or perceived) that may arise. Any proposal to change a language needs to be balanced against the impact of that change on existing users / uses of the language. You don't want to break existing programs or scripts.
Irrespective of arguments about whether spaces around = should be allowed in a shell variable assignment, changing this would break millions of shell scripts. It is just not going to happen.
Of course, Linux (and UNIX before it) allow you to design and implement your own shell. You could (in theory) replace the default shell. It is just a lot of work.
And there is nothing stopping you from writing your scripts in another scripting language (e.g. Python, Ruby, Perl, etc) or designing and implementing your own scripting language.
In summary:
We cannot know for sure why they designed the shell with this syntax for variable assignment, but it is moot anyway.
Reference:
Evolution of shells in Linux: a history of shells.
It prevents ambiguity in a lot of cases. Otherwise, if you have a statement foo = bar, it could then either mean run the foo program with = and bar as arguments, or set the foo variable to bar. When you require that there are no spaces, now you've limited ambiguity to the case where a program name contains an equals sign, which is basically unheard of.
I agree with #StephenC, and here's some more context with sources:
Unix v6 from 1975 did not have an environment, there was just a exec syscall that took a program and a string array of arguments. The system sh, written by Thompson, did not support variables, only single digit numbered arguments like $1 (probably why $12 to this day is interpreted as ${1}2)
Unix v7 from 1979, emboldened by advances in hardware, added a ton of features including a second string array to the exec call. The man page described it like this, which is still how it works to this day:
An array of strings called the environment is made available by exec(2) when a process begins. By convention these strings have the form name=value
The system sh, now written by Bourne, worked much like v6 shell, but now allowed you to specify these environment strings in the same format in front of commands (because which other format would you use?). The simplistic parser essentially split words by spaces, and flagged a word as destined for a variable if it contained a = and all preceding characters had been alphanumeric.
Thanks to Unix v7's incredible popularity, forks and clones copied a lot of things including this behavior, and that's what we're still seeing today.
From what I've read so far, bash seems to fit the defintion of an interpreted language:
it is not compiled into a lower format
every statement ends up calling a subroutine / set of subroutines already translated into machine code (i.e. echo foo calls a precompiled executable)
the interpreter itself, bash, has already been compiled
However, I could not find a reference to bash on Wikipedia's page for interpreted languages, or by extensive searches on Google. I've also found a page on Programmers Stack Exchange that seems to imply that bash is not an interpreted language- if it's not, then what is it?
Bash is definitely interpreted; I don't think there's any reasonable question about that.
There might possibly be some controversy over whether it's a language. It's designed primarily for interactive use, executing commands provided by the operating system. For a lot of that particular kind of usage, if you're just typing commands like
echo hello
or
cp foo.txt bar.txt
it's easy to think that it's "just" for executing simple commands. In that sense, it's quite different from interpreted languages like Perl and Python which, though they can be used interactively, are mainly used for writing scripts (interpreted programs).
One consequence of this emphasis is that its design is optimized for interactive use. Strings don't require quotation marks, most commands are executed immediately after they're entered, most things you do with it will invoke external programs rather than built-in features, and so forth.
But as we know, it's also possible to write scripts using bash, and bash has a lot of features, particularly flow control constructs, that are primarily for use in scripts (though they can also be used on the command line).
Another distinction between bash and many scripting languages is that a bash script is read, parsed, and executed in order. A syntax error in the middle of a bash script won't be detected until execution reaches it. A Perl or Python script, by contrast, is parsed completely before execution begins. (Things like eval can change that, but the general idea is valid.) This is a significant difference, but it doesn't mark a sharp dividing line. If anything it makes Perl and Python more similar to compiled languages.
Bottom line: Yes, bash is an interpreted language. Or, perhaps more precisely, bash is an interpreter for an interpreted language. (The name "bash" usually refers to the shell/interpreter rather than to the language that it interprets.) It has some significant differences from other interpreted languages that were designed from the start for scripting, but those differences aren't enough to remove it from the category of "interpreted languages".
Bash is an interpreter according to the GNU Bash Reference Manual:
Bash is the shell, or command language interpreter, for the GNU operating system.
Looking for a command line code formatter that can be used for bash code. It must be configurable and preferably usable from command line.
I have a big project in bash, which I need to use Q in mind for. So far I am happy with a program written in python by Paul Lutus (a remake of his previous version in Ruby).
See http://arachnoid.com/python/beautify_bash_program.html (also cloned here https://github.com/ewiger/beautify_bash).
but I would like to learn any serious alternative to this tool if it exists. Requirements: it should provide robust enough performance and behavior of treating/parsing rather complicated code.
PS I believe full parsing of bash code is generally complicated because there exists no official language grammar (but please correct me if I am wrong about it).
You could give shfmt a try. It implements its own shell parser including Bash support, so it's more robust than plaintext-based tools.
And both the parser and printer are available as Go packages, so it should be easy to write a 20-line Go program to manipulate or play with shell code.
Please note that I'm the author, so the advice may be a bit biased :)
you can script vim to do: "gg=G" that means "indent all the file"
I discovered that the type builtin will print functions in a formatted manner.
#/usr/bin/env bash
source <(cat <(echo 'wrapper() {') - <(echo '}'));
type wrapper | tail -n +4 | head -n -1 | sed 's/^ //g'
https://github.com/bas080/flush
On the contrary the shell does have a rigorous grammar.
It is described both in English in the ISO standard and documentation for Bash and other shells, and in formal terms in the shell.y file in the Bash source tree.
What makes it "hard" is that where one normally thinks of, say, a quoted string as single lexical token, In the shell every meta character is a separate lexical token, so the meaning of a character can change depending on its grammatical context.
So the parsing tokens do not align with the "shell words" that a user thinks of, and a simple quoted string is at least 3 tokens.
The implementations typically take some shortcuts involving using multiple lexical analysers chosen by whether the grammar is inside quotes, inside numeric context, or outside both.
ALthough I've had to dabble in shell scripting and commands, I still consider myself a novice and I'm interested to hear from others what they consider to be crucial bits of knowledge.
Here's an example of something that I think is important:
I think understanding $PATH is crucial. In order to run psql, for instance, the PostgreSQL folder has to be added to the $PATH variable, a step easily over looked by beginners.
Concept of pipes. The fact that you can easily redirect output and divide complex task to several simple ones is crucial.
Do yourself a favor and get this book: Learning the Bash Shell
Read and understand:
The Official Bash FAQs
Greg Wooledge's Bash FAQs and Bash Pitfalls and everything else on that site
If you're writing shell scripts, an important habit to get into is to always put double quotes around variable substitutions. That is, always write "$myvariable" (and similarly "$(mycommand)"), never plain $myvariable or $(mycommand), unless you understand exactly why you need to leave them out. (Again, the question is not “should I use quotes?”, it's “why would I want to omit the quotes?”)
The reason is that the shell does nasty things when you leave a variable substitution unquoted. (Those nasty things are called field splitting and pathname expansion. They're good in some situations, but almost never on the result of a variable or command substitution.)
If you leave out the quotes, your script may appear to work at first glance. This is because nasty things only happen if the value of the variable contains some special characters (whitespace, \, *, ? and [). This sort of latent bug tends to be revealed the day you create a file whose name contains a space and your script ends up deleting your source tree/thesis/baby pictures/...
So for example, if you have a variable $filename that contains the name of a file you want to pass to a command, always write
mycommand "$filename"
and not mycommand $filename.