rep: A Tool for Repeating Commands

When I needed to run a command multiple times, I would wrap that command into a loop.

for i in {1..10}; do ...; done

But it's a bother to type that out just to repeat a command a few times, so I wrote a simple C script that simplifies the process.

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdbool.h>

char *join(char **strs, int len) {
    int lens = 0;
    for(int i=0; i<len; ++i) {
        lens += (strlen(strs[i])+1);
    }
    char *str = malloc(lens);
    for(int i=0; i<len; ++i) {
        strcat(str, strs[i]);
        strcat(str, " ");
    }
    return str;
}

int main(int argc, char **argv) {
    if(argc < 2) {
        fprintf(stderr, "usage: rep [n] cmd\n");
        exit(EXIT_FAILURE);
    }

    char *endptr;
    int repeat = strtol(argv[1], &endptr, 10);
    bool parsable = (*endptr == '\0');

    if(!parsable) {
        repeat = 20;
    }

    if(repeat <= 0) {
        fprintf(stderr, "cannot repeat %s times.\n", argv[1]);
        exit(EXIT_FAILURE);
    }

    /* you can change sh to bash or zsh or whatever other shell. */
    char *cmd[4] = {"sh", "-c", join(
        /* if the first argument is a parsable number, skip it. */
        argv + (parsable ? 2 : 1),
        argc - (parsable ? 2 : 1)
    ), NULL};

    while(repeat--) {
        pid_t pid;
        switch(pid=fork()) {
            case -1:
                perror("fork()");
                exit(EXIT_FAILURE);
            case 0:
                if(execvp(cmd[0], cmd) == -1) {
                    perror("execvp()");
                    exit(EXIT_FAILURE);
                }
                break;
            default: {
                int status;
                if(waitpid(pid, &status, 0) == -1) {
                    perror("waitpid()");
                    exit(EXIT_FAILURE);
                }
            }
        }
    }
}

For example,

$ rep ls # repeat ls 20 times.
$ rep 100 ./exe1 '$(./exe2)' # run exe1 100 times, with its first argument as the output of exe2.

If the first argument is fully parsable as an integer, it will be taken as the number of times to repeat. Otherwise, it's treated as the beginning of a command.