Shell Programming and Scripting

> filename

typeset -i LogMax=5000
typeset -i LogLines=12
typeset -i LinesInFile=$(wc -l $LOGFILE)
typeset -i LinesToRemove=$(( LinesInFile - LogMax ))

if [ $LinesToRemove -gt 0 ] ; then
     (( LinesToRemove = ( ( LinesToRemove + LogLines - 1 ) / LogLines ) * LogLines ))
     tail -n $(( LinesInFile - LinesToRemove )) $LOGFILE > tempfile
     cat tempfile > $LOGFILE
fi

[g]cc -o trunc trunc.c

/* truncfile.c

function: truncfile(char *filename, size_t lines)
	      reduce the number of lines in a text file keep the newer lines,
	      discarding the older lines.  Makes changes to files using memory
	      without tmpfiles.
Parameters:
	      [filename] file path 
	      [lines] to keep - valid range 0 <-> n.
	      If lines > number of existing lines no truncation occurs.
	      If lines = 0 all lines are removed.
Return value: 0 = success -1 == failure.		      
Hist:
	01-13-02 - jmc  truncfile(), reset_file_start()
	07-27-06 - BH  move into standalone utility trunc
	               usage ./trunc <filename> <lines to keep>
*/	      

#include <sys/types.h>
#include <unistd.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

#define FAILURE (-1)
#define SUCCESS 0
void usage(void);

/* number of bytes in a file */
size_t filesize(int fd)
{
	struct stat st;	
	st.st_size=0;
	return (fstat(fd, &st)== FAILURE)?0 :st.st_size;
}

/* remove top lines of file to result in size_t lines in new file */
char *reset_file_start(char *p, char *buf, size_t lines, size_t *filebytes)
{
	size_t count=0;
	size_t left=0;

	p=buf;
	for(p+=*filebytes; p > buf && count < lines; p--, left++)
		count+=(*p=='\n') ?1 :0;
	*filebytes=left;
	
	return p;
}

/*  truncate a file in memory  to have n lines, keep newest lines*/
int truncfile(const char *filename, const size_t lines)
{
	int retval=0;
	FILE *fp=fopen(filename, "r");

	if (fp !=NULL)
	{
		size_t filebytes=filesize(fileno(fp));
		char *buf=malloc(filebytes+1);

		if(buf==NULL ) /* no memory */
		{
			perror("Error");
			exit(1);
		}
		memset(buf, 0x0, filebytes + 1 );
		if(fread(buf, 1, filebytes, fp) == filebytes)
		{
			char *p=reset_file_start(p, buf, lines, &filebytes);

			fclose(fp);
	        if ((fp=fopen(filename,"w")) == NULL)
				retval=FAILURE;
				
			if (retval == SUCCESS && filebytes > 0)
			{
				if(fwrite(p, 1, filebytes, fp) != filebytes)			   
			    	retval=FAILURE;
			}
			
			if(retval == SUCCESS && fclose(fp) == EOF)
				retval=FAILURE;
		}
		else
			retval=FAILURE;
		free(buf);
	}
	else
	{
		usage();
		retval=FAILURE;
    }
	return retval;
}

/* BH create utility wrappers: main(), usage() 07-27-06 
------------------------------------------------------------------------
*/

/* help to stderr */
void usage(void)
{
	fprintf(stderr, "%s%s",
		"trunc: \ttruncate a file removing lines from top of file\n",
		"Usage: \ttrunc <file> <lines of text to keep>  lines = 0 -> n\n");
	exit(EXIT_FAILURE);     
}

/* requires two arguments - filename and lines. */
int main(int argc, char **argv)
{
	int retval=SUCCESS;

	errno=0;
	if(argc==3)
	{
		ssize_t lines=atoi(argv[2]);
		if(lines > -1)
		{
		    if(access(argv[1],W_OK)==SUCCESS)		
				retval=truncfile(argv[1], lines);
			else
				retval=FAILURE;	
		}
		else
			errno=EINVAL;
	}
	else
		usage();

	if(errno || retval==FAILURE)
	{
		perror("Error");
		retval=EXIT_FAILURE;
	}
	return retval;
}