I use.?? Hackers or crackers, if you prefer break into computers and often use strange filenames, like ". You may not have noticed, but there was a space in these filenames. Refering to files with spaces in the names require quoting, which I will cover later.
Personally, all of my hidden files are matched by ". The slash is also special, as it is used to indicate a directory path. Filename expansion does not expand to match a slash, because a slash can never be part of the filename. Also, the same rules for filename expansion of hidden files applies if the pattern follows a slash.
If you want to match hidden files in a subdirectory, you must specify the explicit pattern. Table 2 lists some examples. The Bourne shell differs from the C shell if the meta-characters do not match any file. If this happens, the pattern is passed to the program unchanged. The C shell will either do this, or generate an error, depending on a variable. If you are not sure how something will expand, use the echo command to check. It generates output more compact than ls , and it will not list contents of directories like ls will.
You will also notice the output is sorted alphabetically. The shell not only expans filenames, but sorts them, for all applications. Finding the executable Once the shell expands the command line, it breaks up the line into words, and takes the first word as the command to be executed. The special Bourne variable "IFS" contains the characters used to "break up" the line. Normally, this variable contains a space and a tab.
Afterwards, the first word is used as the name of the program. If the command is specified without an explicit directory path, the shell then searches the different directories specified by the "PATH" variable, until it finds the program specified.
You forgot to specify the command! Click here to get file: 0. Provided the current directory was in your search path. So you see, filename expansion can be anywhere on a command line. You can execute programs with long names without typing the entire name. However, filename expansion only works if the file is in the directory you specify. Some people create a file called "-i" in a directory. Therefore rm runs with the interactive option, protecting programs from accidental deletion.
One last point. I'd like to say two things in defense. First, the above usage of filename expansion is an "unnatural act," as far as the UNIX philosophy is concerned. There are many advantages to the shell handling the filename expansion, and perhaps one disadvantage in one case.
I believe the advantages of the UNIX philosophy far outweight the disadvanges. Second, I don't believe it is a disadvantage anyway. Renaming files like that is wrong. The only reason to do so is because DOS does it that way, and you have to to this because you are limited to 11 characters. If you must rename them, append a string to the end instead of changing the original filename.
This is UNIX. You can have filenames characters long, so this approach isn't a problem. NEW done This alows you to undo what you did, and retains the original filename. Even better, move the files into another directory, letting them keep their original name.
The standard keyboard has three quotation marks. Each one has a different purpose, and only two are used in quoting strings. Why quote at all, and what do I mean by quoting? Well, the shell understands many special characters, called meta-characters. These each have a purpose, and there are so many, beginners often suffer from meta-itis. Example: The dollar sign is a meta-character, and tells the shell the next word is a variable. If you wanted to use the dollar sign as a regular character, how can you tell the shell the dollar sign does not indicate a variable?
Answer: the dollar sign must be quoted. Quoted characters do not have a special meaning. Let me repeat this with emphasis.
Quoted characters do not have a special meaning A surprising number of characters have special meanings. The lowly space, often forgotten in many books, is an extremely important meta-character. Consider the following: rm -i file1 file2 The shell breaks this line up into four words. The first word is the command, or program to execute.
The next three words are passed to the program as three arguments. The word "-i" is an argument, just like "file1. In other words, the program treats arguments starting with a hyphen as special. The shell doesn't much care, except that it follows the convention. In this case, rm looks at the first argument, realizes it is an option because it starts with a hyphen, and treats the next two arguments as filenames.
The program then starts to delete the files specifies, but firsts asks the user for permission because of the "-i" option. The use of the hyphen to indicate an option is a convention. There is no reason you can't write a program to use another character. You could use a forward slash, like DOS does, to indicate a hyphen, but then your program would not be able to distinguish between an option and a path to filename whose first characters is a slash.
Can a file have a space in the name? There are few limitations in filenames. As far as the operating system is concerned, You can't have a filename contain a slash or a null. The shell is a different story, and one I don't plan to discuss. Normally, a space delineates arguments. To include a space in a filename, you must quote it.
Another verb used in the UNIX documentations is "escape;" this typically refers to a single character. You "quote" a string, but "escape" a meta-character. In both cases, all special characters are treated as regular characters. Assume, for a moment, you had a file named "file1 file2," This is one file, with a space between the "1" and the "f.
Most people consider the quotation mark as something you place at the beginning and end of the string. A more accurate description of the quoting process is a switch, or toggle.
The following variations are all equivalent: rm 'file1 file2' rm file1' 'file2 rm f'ile1 file'2 In other words, when reading a shell script, you must remember the current "quoting state. Therefore if you see a quotation mark in the middle of a line, it may be turning the toggle on or off. You must start from the beginning, and group the quotation marks in pairs. There are two other forms or quoting. The double quotation mark is similar to the single quotes used above, but weaker.
I'll explain strong and weak quotation later on. Here is the earlier example, this time using the other forms of quoting: rm "file1 file2" rm file1 file2 rm file1" "file2 Nested quotations A very confusing problem is placing quotation marks within quotation marks.
It can be done, but it is not always consistent or logical. Quoting a double quote is perhaps the simplist, and does what you expect. A double quote is weaker, and does not quote a backslash. Single quotes are different again. In other words, the quoting function will stay toggled, and will continue until another single quote is found. If none is found, the shell will read the rest of the script, until an end of file is found.
Strong versus weak quoting Earlier I described single quotes as strong quoting, and double quotes as weak quoting. What is the difference? Strong quoting prevents characters from having special meanings, so if you put a character inside single quotes, what you see is what you get.
Therefore, if you are not sure if a character is a special character or not, use strong quotation marks. Weak quotation marks treat most characters as plain characters, but allow certain characters or rather meta-characters to have a special meaning.
As the earlier example illustrates, the backslash within double quotation marks is a special meta-character. It indicates the next character is not, so it can be used before a backslash and before a double quotation mark, escaping the special meaning.
There are two other meta-characters that are allowed inside double quotation marks: the dollar sign, and the back quote. Dollar signs indicate a variable. One important variable is "HOME" which specifies your home, or starting directory. The C shell is best suited for interactive sessions. Because of this, it assumes a quote ends with the end of a line, if a second quoute character is not found.
The Bourne shell makes no assumptions, and only stops quoting when you specify a second quotation mark. If you are using this shell interactively, and type a quotation mark, the normal prompt changes, indicating you are inside a quote.
This confused me the first time it happened. I used the C shell for my first scripts, but I soon realized how awkward the C shell was when I included a multi-line awk script insice the C shell script.
The Bourne shell's handling of awk scripts was much easier:! You will find that the easiest way to escape a quotation mark is to use the other form of quotation marks. Missing keyword: "end"' Quotes within quotes - take two Earlier I showed how to include a quote within quotes of the same kind.
As you recall, you cannot place a single quote within a string terminated by single quotes. The easiest solution is to use the other type of quotation marks. But there are times when this is not possible.
There is a way to do this, but it is not obvious to many people, especially those with a lot of experience in computer languages. Most languages, you see, use special characters at the beginning and end of the string, and has an escape to insert special characters in the middle of the string. The quotation marks in the Bourne shell are not used to define a string.
There are used to disable or enable interpretation of meta-characters. You should understand the following are equivalent: echo abcd echo 'abcd' echo ab'c'd echo a"b"cd echo 'a'"b"'c'"d" The last example protects each of the four letters from special interpretation, and switches between strong and weak quotation marks for each letter. Letters do not need to be quoted, but I wanted a simple example.
If I wanted to include a single quote in the middle of a string delineated by a single quote marks, I'd switch to the different form of quotes when that particular character is encountered. That is, I'd use the form 'string1'"string2"'string3' where string2 is a single quote character. Placing variables within strings Change the quoting mid-stream is also very useful when you are inserting a variable in the middle of a string. It cannot start with a number. Unlike the C shell, spaces are important when defining Bourne shell variables.
Whitespace spaces, tabs or newlines terminate the value. This terminates the value. The "date" command? Which introduces A subtle point Notice how two commands are executed on one line: the variable is changed, and the "date" program is executed.
It is not obvious that this is valid. The manual page doesn't mention this. Even stranger is some commands can be executed, while others cannot. The "date" command is an external program. That is, the command is not built into the shell, but is an external executable.
Other commands are internal command, built into the shell. Don't believe me? I'll give you an example, and you have to guess what the results will be. I suspect that that Put on your thinking caps. You'll need it. See below Ready? What does the following Bourne shell commands do?
I failed the test myself. Well, I got partial credit. But I wrote the quiz. The first line is simple: "one" is output to the screen. The second line behaves differently. The value of variable "a" is set to "two," but the echo command outputs "one. The shell treats built-in commands like external commands, and expands the meta-characters before executing the built-in commands.
Ready for a curve ball? What does the last line do? It creates a file. The file contains the word "two. For those to thought the answer is "two," I'm terrible sorry, you didn't win the grand price.
We do have a nice home version of this game, and a year's supply of toothpaste. The correct answer is "three. The C shell doesn't do this, by the way. On an old Sun system, it behaved as I noted. Note that this was the Bourne shell, and not Bash. I tried it using the Bash shell on a 2. The third line created a file called "three" but it contained the string "one"!. I also tried it on a Ubuntu Just consider this an example where the behavior is unpredictable. Now let's continue in the tutorial.
Perhaps Mr. Although the real reason is that the above command treats the "a" variable like an environment variable, and sets the variable, marks it for export, and then executes the command. Because the variable is "passed" to the command by the environment, the shell simply sets the standard output to the appropriate file, and then processes the line for variables, and lastly executing the command on the line.
More on this later. The "set" command is one way to determine which shell you are currently using. The C shell puts spaces between the variable and the value. Also note the assortment of variables already defined. These are environment variables. Environment Variables UNIX provides a mechanism to pass information to all processes created by a parent process by using environment variables.
When you log onto a system, you are given a small number of variables, predefined. You can add to this list in your shell start-up files.
Every program you execute will inherit these variables. But the information flow is one-way. New UNIX users find this confusing, and cannot understand why a shell script can't change their current directory. Picture it this way: suppose you executed hundreds of programs, and they all wanted to change their environment to a different value. It should be obvious that they can't all control the same variable. Imagine hundreds of programs trying to change the directory you are currently using!
Perhaps these variables ought to be called hereditary, and not environmental. Children processes inherit these values form the parents, but can never change how the parents were created. That would require time-travel, a feature not currentable available in commercial UNIX systems.
As I mentioned before, the shell command "export" is used to update environment variables. The command export a b c marks the variables "a" "b" and "c," and all child processes will inherit the current value of the variable. With no arguments, it lists those variables so marked.
The command "export" is necessary. Changing the value of an environment variable does not mean this change will be inherited. Another way to test this is to start a new copy of the shell, and execute the "export" command. No variables are reported. The command marks the variable. It does not copy the current value into a special location in memory. A variable can be marked for export before it is changed.
Special Environment Variables There are several special variables the shell uses, and there are special variables the system defined for each user. SunOS and Solaris systems use different environment variables. If in doubt, check the manual pages. I'll describe some the important Solaris variables. When you type an arbitrary command, the directories listed are searched in the order specified.
The colon is used to separate directory names. An empty string corresponds to the current directory. This is dangerous, as someone can create a program called "ls" and if you change your current directory to the one that contains this program, you will execute this trojan horse. If you must include the current directory, place it last in the searchpath. The HOME evvironment variable is set by the login process.
CDPATH - cd searchpath When you execute the "cd" command, and specify a directory, the shell searches for that directory inside the current working directory.
You can add additional directories to this list. If the shell can't find the directory in the current directory, it will look in the list of directories inside this variable. I discussed this briefly earlier.
Normally, whitespace separates words, and this variable contains a space, a tab and a new line. Hackers find this variable interesting, because it can be used to break into computer systems. If the hacker has placed a program called "bin" in the searchpath, then the hacker gains privileged access.
Well, some people make a joke, and tell a new user to place a period inside this variable. PS2 - Secondary Prompt The "PS2" environment variable defines the secondary prompt, This is the prompt you see when you execute a multi-line command, such as "for" or "if. It is set by the login process. The default value is seconds 10 minutes. If you set it to zero, every time the shell types a prompt, it will check for mail.
In addition, several environment variables are specified by the login process. Add a comment. Active Oldest Votes. From the bash manual: Each command in a pipeline is executed as a separate process i. Improve this answer. The question was about the Bourne Shell but your proposed solution does not work with the Bourne Shell. BTW: Jobcontrol needs to be manually enabled in the Bourne Shell, even when it is in interactive mode.
You are right, didn't know that there is a difference between the original Bourne Shell and sh. I'm on Alpine, not Solaris. Why does this correct answer get a downvote and an incorrect answer gets an upvote? My answer is easy to verify Quite right, I did not read that accurately. When you use the right side of a pipe, there are some limitations.
Empty in sh, bash, mksh, dash, ash, yash. Same as above for other shells. ImHere ImHere Sign up or log in Sign up using Google. Ashton Ashton 1 1 gold badge 2 2 silver badges 9 9 bronze badges.
I should elaborate on the first bit You mean the original bourne shell, not bash? Add a comment. Active Oldest Votes. I've tested this on Bash and Dash on my system sh is symlinked to dash.
Of course it works. The third line and subsequent lines of doc2 are discarded. Dennis Williamson Dennis Williamson k 87 87 gold badges silver badges bronze badges. Project and question is long gone, but I went back and checked the relevant shell The second example with the arrays fails Colin Macleod Colin Macleod 2, 15 15 silver badges 17 17 bronze badges.
Stefano Borini k 92 92 gold badges silver badges bronze badges. I'm just starting out with UNIX and have figured some stuff out. I just need some help with accepting user input on the command line.
For instance, I created a number counter that counts down from any positive hard coded number. But, I want the commnad line line to read "Countdown 20" where RedHat Commands.
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