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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 is 1.
• It’s not a special case; argv is simply the all words typed, including the first one.

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 18688 Apr 25 20:24 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

In general, backslash makes ordinary things special (\n) and 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.

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”

The > 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.