Change environment variables' values

-E option is used to change environment variables' values. Actually, strace just calls putenv() function (code is here). Check following code:

# cat env_test.c
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    char *p = NULL;
    if (p = getenv("HG"))
    {
        printf("HG enviromental variable value is %s\n", p);
    }
    else
    {
        printf("HG enviromental variable value is not set\n");
    }
    return 0;
}

Build and run it:

#  gcc env_test.c -o env_test
# ./env_test
HG enviromental variable value is /usr/bin/hg

Set HG to other value:

# strace -E "HG=git" ./env_test
......
write(1, "HG enviromental variable value i"..., 38HG enviromental variable value is git
) = 38
......

Remove HG:

# strace -E "HG" ./env_test
......
write(1, "HG enviromental variable value i"..., 42HG enviromental variable value is not set
) = 42
......