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CS253 Arguments

The Argument Clinic

Program

Here is a program, args.cc, that displays all its arguments in “curly quotes”:

#include <iostream>
using namespace std;

int main(int argc, char *argv[]) {
    cout << "argc: " << argc << '\n';
    for (int i=0; i<argc; i++)
        cout << "argv[" << i << "]: “" << argv[i] << "”\n";
}

argc & argv

#include <iostream>
using namespace std;

int main(int argc, char *argv[]) {
    cout << "argc: " << argc << '\n';
    for (int i=0; i<argc; i++)
        cout << "argv[" << i << "]: “" << argv[i] << "”\n";
}

• argc is the length of argv.
• argv is a C array of C strings.
• If, for example, argc==3, then argv contains three strings:
  • argv[0]: the program name
  • argv[1]: the first real argument
  • argv[2]: the last real argument.
• Unlike most arrays, argv[argc] does exist, and contains a null pointer (nullptr). Not an zero-length string—a null pointer.

No arguments given

% g++ -Wall ~cs253/Examples/args.cc
% ./a.out
argc: 1
argv[0]: “./a.out”

• Even though no explicit arguments were given, argv always contains at least one entry, argv[0], the program name. Therefore, argc must be at least 1.
• It’s not a special case; argv is simply the all words typed, including the first one, the command itself.

Arguments given

% g++ -Wall ~cs253/Examples/args.cc
% ./a.out red white blue
argc: 4
argv[0]: “./a.out”
argv[1]: “red”
argv[2]: “white”
argv[3]: “blue”

Three arguments were given, but argc==4, because the program name is argv[0]. That counts for one.

Program name

The -o option to g++ names the resulting program something other than a.out:

% g++ -Wall -o war ~cs253/Examples/args.cc
% ls -l
total 20
-rwx------ 1 cs253 class 18744 Nov 21 09:07 war
% ./war famine pestilence death
argc: 4
argv[0]: “./war”
argv[1]: “famine”
argv[2]: “pestilence”
argv[3]: “death”

That’s why we use argv[0] in error messages—it’s always the name of the program.

Quoting

Normally, bash splits the argument string on whitespace. Quoting with "double quotes" or 'single quotes' protects the spaces:

% g++ -Wall ~cs253/Examples/args.cc
% ./a.out Earth "Air Fire" 'Water'
argc: 4
argv[0]: “./a.out”
argv[1]: “Earth”
argv[2]: “Air Fire”
argv[3]: “Water”

• Air Fire is a single argument, argv[2].
• The “curly quotes” were generated by the program itself.
• The "double" or 'single' quotes do not get sent to the program—bash removes them.

More Quoting

Backslash makes ordinary things special (\n) and makes special things ordinary (\"):

cout << "They call me \"Bonehead\".\n";

They call me "Bonehead".

Putting \ before a space prevents it from breaking arguments apart:

% g++ -Wall ~cs253/Examples/args.cc
% ./a.out Peter Flopsy\ Mopsy Cotton-tail
argc: 4
argv[0]: “./a.out”
argv[1]: “Peter”
argv[2]: “Flopsy Mopsy”
argv[3]: “Cotton-tail”

The \ prevented the following space from breaking up words. Instead, the space was treated as just another character. We call this quoting, also, even though no quote characters are involved.

Special characters

Many characters have meaning in bash: `~#$&*(){}[]\|;'"<>?. These are called special characters or metacharacters.

% g++ -Wall ~cs253/Examples/args.cc
% ./a.out >foo
% cat foo
argc: 1
argv[0]: “./a.out”

> sent the output of ./a.out to the file foo. a.out did not get an argument >foo.

% g++ -Wall ~cs253/Examples/args.cc
% ./a.out ">foo"
argc: 2
argv[0]: “./a.out”
argv[1]: “>foo”

Quoting prevented > from being treated specially.

Wildcards

% echo abcdefghijklmnopqrstuvwxyz >a-b-c
% g++ -Wall ~cs253/Examples/args.cc
% ./a.out *
argc: 3
argv[0]: “./a.out”
argv[1]: “a-b-c”
argv[2]: “a.out”
% ./a.out "*"
argc: 2
argv[0]: “./a.out”
argv[1]: “*”
% ./a.out zulu*
argc: 2
argv[0]: “./a.out”
argv[1]: “zulu*”

• Some characters, such as *, are wildcards for matching filenames.
• The * wildcard expands to the names of all the files in that directory.
• zulu* would match all files starting with zulu, but there are none, so it remains zulu*.

Redirection

bash uses <, >, and | to reroute standard input/output.

% date >now
% g++ -Wall ~cs253/Examples/args.cc
% ./a.out now
argc: 2
argv[0]: “./a.out”
argv[1]: “now”
% ./a.out <now
argc: 1
argv[0]: “./a.out”

Note the difference. In the first run, now is an argument. In the second run, bash runs ./a.out with its standard input attached to the file now, and does not pass now as an argument.

Pipe

% date >now
% g++ -Wall ~cs253/Examples/args.cc
% ./a.out <now
argc: 1
argv[0]: “./a.out”
% cat now | ./a.out
argc: 1
argv[0]: “./a.out”

• These two a.out runs are almost identical.
• The first is better: the file now is attached to a.out’s standard input.
• In the second, the cat program reads the file now, and writes it to its own standard output, which is attached to a pipe. a.out’s standard input is attached to other side of the same pipe. The two programs execute simultaneously, writing & reading.
  • On a single-CPU machine, they can’t really run simultaneously—they take turns, each a millisecond or so. It’s effectively simultaneous.