detecting a newline in fgets

Question

I am attempting to create a C program to be used as a simple command line interpreter for a UNIX system. I use fgets() to read user input which then stores input in a buffer to be parsed. If the only user input is pushing enter, I would like to reissue the prompt. Is there a way to detect if the return key was the only key entered at the prompt? Below is a snippet of code I have tried so far:

for (;;) {
    printf("$prompt$ ");
    fflush(stdout);

    fgets(commandBuffer, 200, stdin);

    /* remove trailing newline:*/
    ln = strlen(commandLine) - 1;
    if(commandLine[ln] == '\n')
        commandLine[ln] = '\0';

    /* attempt to handle if user input is ONLY return key:*/
    if(commandLine[0] == '\n')
        continue;

For the (rare) case of `fgets()` returning a zero-length string the code will fail miserably. — alk, Nov 25 '13 at 17:08
@chux I was responding to alk's statement, but now that I think about it, I think you want `strlen` instead. I was thinking of `sizeof` for something else. — jfa, Nov 25 '13 at 20:32

simonc · Answer 1 · 2013-11-25T17:12:59.777

1

You need to replace

 if(commandLine[0] == '\n')

with

if(commandLine[0] == '\0')

The code just above this replaces newlines with nuls.

edited Nov 25 '13 at 17:12

answered Nov 25 '13 at 17:02

simonc

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score 1 · Answer 2 · answered Nov 25 '13 at 18:59

1

ln = strlen(commandLine);
while (ln > 0 && commandLine[ln-1] == '\n')
    --ln;
commandLine[ln] = '\0';

is a more compact solution handling special cases like empty input.

answered Nov 25 '13 at 18:59

ensc

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chux - Reinstate Monica · Answer 3 · 2020-02-02T22:05:43.920

A popular one-liner to remove a potential new line is

if (fgets(commandBuffer, 200, stdin)) {
  commandBuffer[strcspn(commandBuffer, "\n")] = '\0';

The below suffers a problem when strlen(commandLine) returns 0. This happen when the input line read begins with a null character.

fgets(commandBuffer, 200, stdin);
ln = strlen(commandLine) - 1;  // ln could be outside 0...199  
if(commandLine[ln] == '\n')    // commandLine[ln] is then undefined behavior

Otherwise perform like @ensc.

if (fgets(commandBuffer, 200, stdin)) {
  ln = strlen(commandLine);
  while (ln > 0 && commandLine[ln-1] == '\n') {
    --ln;
  }
  commandLine[ln] = '\0';

detecting a newline in fgets

3 Answers3