Read a file using argv | 易学教程

问题

Currently the code is reading the file and sort the records, as shown below,

#include"fileIO/file.h"

#define MAX_RECORD_SIZE 256 // Bad style


typedef struct{
  int age;
  char *lastName;
  char *firstName;
}Person;

 .....



int main(int argc, char *argv[]){

  FILE *pFile = fopen("file.txt", "r");

  ....
  callInsertionSort(getItemList(cache), getSize(getItemList(cache)), less);

}

where, file.txt is,

Age,LastName,FirstName
50,B,A
30,A,B
20,X,D
10,F,A
10,A,B
90,V,E
60,N,M

Execution:

$./sort.exe
Before sorting
Age,LastName,FirstName
  50  B  A
  30  A  B
  20  X  D
  10  F  A
  10  A  B
  90  V  E
  60  N  M
After sorting

Age,LastName,FirstName
  10  F  A
  10  A  B
  20  X  D
  30  A  B
  50  B  A
  60  N  M
  90  V  E

Question:

Without using fopen(), is there a way to get the shell to pass the contents of file.txt as command-line arguments, one argument per line in the file, (through argv by passing shell command (sort.exe) parameters)?

回答1:

You can run the program as:

IFS=$'\n'
./sort.exe $(cat file.txt)

Each line of the file will then be an argument to the program (setting IFS to just newline prevents spaces in the file from being argument delimiters).

Then the program can loop over argv (except for argv[0], which contains the program name) to get all the lines to sort.

Note that there's a limit on the size of the arguments, so this method is not very scalable. Reading from the file is generally preferred.

FYI, $(cat file.txt) can also be written as $(< file.txt)

回答2:

If the objective is to pass each line of the file as a separate command line argument to the sort.exe program, then you will need Bash (which is in the tags) and you can use:

IFS=$'\n'; sort.exe $(cat file)

To demonstrate, take an alternative command, printf, and a data file like this, using ⧫ to represent a blank:

verbiage
spaced⧫out
⧫leading⧫and⧫trailing⧫
⧫⧫multiple⧫⧫spaces⧫⧫everywhere⧫⧫

Running the command yields:

$ ( IFS=$'\n'; printf '[%s]\n' $(cat file) )
[verbiage]
[spaced out]
[ leading and trailing ]
[  multiple  spaces  everywhere  ]
$

This uses the outer ( … ) to run the command in a sub-shell so that my interactive IFS is not messed up. They're unnecessary in some other contexts.

Note that there's an upper-bound on how long the command line arguments (plus environment variables) can be — it is sometimes as much as 256 KiB, and seldom larger. This means you can only afford to do this with small enough files. In most circumstances, you'd be better off following the Unix (POSIX) sort command and reading the data direct from the filename specified as an argument to the sort command.

来源：https://stackoverflow.com/questions/41687782/read-a-file-using-argv

标签

bash

argv