UNIX: Need to replace last line on the file and also remove extra blank line in the end

Question

I need to replace - WOR to VWOR in end of the file.

I have tried

sed s/WOR/VWOR/g    

But this does not work until I have new blank line in file.

So I used

( cat FileName && echo ) | sed 's/WOR/VWOR/g'  

This worked. But it is taking long time to replace as the sed is reading the entire file.

Answer 1

Make sure the file ends in a new line. That is a requirement of "text files".

This command will do exactly that:

[ -n $(tail -c1 FileName) ] && printf '\n' >> FileName

It will add a trailing new line only if one is needed, so there is no problem if it is used several times, also, it is very fast as it only checks the last character.

Then, do the change on the (now valid) last line ($):

sed -ie '$s/WOR/VWOR/'

Note: This command will convert a symlink to a real file.

Answer 2

Well, I don't know how to solve this with sed/awk or other. Here it is in C and it should be quite fast:

/*
by Hans Schou <chlor@schou.dk> 2016
Use on your own risk.
vim: ts=4 :
gcc -o replacelastline replacelastline.c && ./replacelastline WOR VWOR Filename V
*/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#define SEARCH 1
#define REPLACE 2
#define FILENAME 3
#define VERBOSE 4
int main(int argc, char *argv[]) {
    if (FILENAME >= argc) {
        printf("Error, too few args. Only found %d.\n", argc);
        return EXIT_FAILURE;
    }
    printf("argc %d.\n", argc);

    int V = 0;
    if (VERBOSE <= argc) {
        V = 'V' == argv[VERBOSE][0] ? 1 : 0;
        printf("Verbose: %d\n", V);
    }

    if (V) {
        printf("argv[SEARCH]: %d %s\n", strlen(argv[SEARCH]), argv[SEARCH]);
        printf("argv[REPLACE]: %d %s\n", strlen(argv[REPLACE]), argv[REPLACE]);
        printf("argv[FILENAME]: %s\n", argv[FILENAME]);
        printf("Open file: %s\n", argv[3]);
    }

    size_t newFileSize = 0;
    FILE *fp = fopen(argv[FILENAME],"r+");
    if (NULL == fp) {
        fprintf(stderr, "Error open file: %s\n", argv[FILENAME]);
        return EXIT_FAILURE;
    } else {
        char inpBuf[8192];
        /* Seek to end-of-file minus buffer size */
        if (0 == fseek(fp, -sizeof(inpBuf), SEEK_END)) {
            /* Read buffer with last part of file */
            if (sizeof(inpBuf) != fread(&inpBuf, 1, sizeof(inpBuf), fp)) {
                fprintf(stderr, "Error, could not read %d bytes from '%s'\n", sizeof(inpBuf), argv[FILENAME]);
                return EXIT_FAILURE;
            } else {
                /* In the inpBuf, search backwards where the last line start */
                int i = sizeof(inpBuf)-1-1; /* one extra minus-1 if last char is '\n' */
                while (i && '\n' != inpBuf[i-1]) {
                    --i;
                }
                newFileSize = ftell(fp) - sizeof(inpBuf) + i;
                if (V) printf("Last line number of chars: %d\n", sizeof(inpBuf)-i);
                if (V) printf("The line: '%*.*s'\n", 10, sizeof(inpBuf)-i, &inpBuf[i]);
                /* Seek back in the file to where the last line starts */
                if (0 == fseek(fp, -(sizeof(inpBuf)-i), SEEK_END)) {
                    if (V) printf("Pos start last line: %d\n", ftell(fp));
                    char outBuf[sizeof(inpBuf)];
                    int o = 0;
                    /* Read inpBuf and copy to outBuf. If SEARCH found replace with REPLACE */
                    while (sizeof(inpBuf) > i) {
                        if (0 != strncmp(&inpBuf[i], argv[SEARCH], strlen(argv[SEARCH]))) {
                            outBuf[o] = inpBuf[i];
                            ++i;
                            ++o;
                        } else {
                            memcpy(&outBuf[o], argv[REPLACE], strlen(argv[REPLACE]));
                            i += strlen(argv[SEARCH]);
                            o += strlen(argv[REPLACE]);
                        }
                    }
                    newFileSize += o;
                    if (V) printf("New line: '%*.*s'\n", o, o, &outBuf);
                    size_t bytesWritten = fwrite(&outBuf, 1, o, fp);
                    if (o != bytesWritten) {
                        fprintf(stderr, "Error, should write %d bytes, but wrote %zu bytes.\n", o, bytesWritten);
                        return EXIT_FAILURE;
                    }
                }
            }
        }
        fclose(fp);
        /* if file should be smaller than before then truncate */
        if (newFileSize && strlen(argv[SEARCH]) > strlen(argv[REPLACE])) {
            if (V) printf("New file size: %zu\n", newFileSize);
            truncate(argv[FILENAME], newFileSize);
        }
    }

    return EXIT_SUCCESS;
}

Answer 3

You can use the following command to remove blank lines in the end of a file and append some text in that file:

(sed -e :a -e '/^\n*$/{$d;N;ba' -e '}' "${FILE_NAME}" | sed '$d' && echo "Hello") > "${NEW_FILE_NAME}"

sed -e :a -e '/^\n*$/{$d;N;ba' -e '}': this will remove all the blank lines from end of the file.

sed '$d' : this will remove the last line.

echo "Hello": this will append text.