How do I replace a word in a file with another word using C?
For example, I have a file which contains:
my friend name is sajid
I want to replace the word friend with grandfather, such that the file is changed to:
my grandfather name is sajid
(I am developing on Ubuntu Linux.)
Update:
I am doing filing in C. I have created a .txt file and write some data into it, but as my program progresses I have to search some text and replace it with the other words. The problem I am facing is that suppose in my file I wrote
"I bought apple from the market"
If i replace apple with pineapples as apple has 5 char and pineapple has 9 char it will write it as
"I bought pineapple m the market"
It also has affected the words written after apple.
I have to do it using C, not a command line script.
How about using the exiting Linux sed program?
sed -i 's/friend/grandfather/' filename
That will replace friend with grandfather in the existing file. Make a copy first if you want to keep the original!
Edit:
Alternatively, load the file into an STL string, replace 'friend' with 'grandfather' using a technique such as this, and save the new string into a file.
As you've realized, you won't be able to make the change in place in the original file.
The easy solution is to read each string from the original file and write it to standard output, making the replacement as necessary. In pseudocode:
open original file
while not at end of original file
get next string from original file
if string == "friend"
write "grandfather" to standard output
else
write string to standard output
end while
You can then redirect the output to another file when you execute it, similar to how sed and awk work.
Alternately, you can create a destination file in the code and write to it directly. If you need to replace the original file with the new file, you can use the remove() and rename() library functions in stdio.
Related
steps:
Let's say I have a C program inputFileName.c
I run inputFileName with input redirection such as ./inputFileName < file
How can I print the name of the file in my C program that I have typed in the terminal as an input redirection file?
The input redirection is a function of the shell. Your inputFileName executable see this as standard input. Depending on the exact operating system, you may be able to use system-specific functions to get the information you want, but there is not a standard means of doing so.
Input redirection can be achieved not only with the '<' symbol, but also with '|'.
program < filename
is equivalent to
cat filename | program
From there, one could go to
cat file1 file2 file3 | program
You begin to see why the initial 'stdin' for an executable cannot and does not have a "filename" associated with it.
If input comes from a pipe, there can't be an associated filename. Also if the file has been deleted or moved before closing the file descriptor, there is no associated filename. A file can have multiple names. In that case there are multiple filenames.
Given that, the "associated filename" of a file descriptor doesn't really make much sense. And even if you could get that info, using the filename in any way might make race conditions an issue.
The linux kernel does try to track an associated filename if a file descriptor was created by opening a file. But the keyword here is "tries".
If you are running Linux, you can find the filenname for standard input as a symlink under "/proc/self/fd/0". Just remember that you should not rely on that name for anything more than debug or display purposes.
I am writing a simple encryption program, that takes any given file, writes encrypted data to a temporary file and I am now looking for the most efficient way to replace the original file with its encrypted counterpart.
I know I could just fopen the original with w and copy line by line the encrypted file, but I was wondering if there was any more efficient way to do it, like overwriting the original file hard-link to point to the ciphered file sparing me the need to rewrite the entirety of the file?
on linux, you could use mv.
And if the two files are not in the same directory, mv would be the better choice for several reasons, including that an option can be given to mv so no prompt output when a file is overwritten I.E
mv -f tempfile original_newfile
the result will be that tempfile no longer exists and the original file now contains the tempfile with the original name
Note: mv manipulates the 'hardlinks' to do its work
As suggested by #Chris-Turner and explained by #Jabberwocky using rename works fine
I am using trying to get specific information from a group of MP3 files, currently I am in the main cygwin64 that holds MP3 files and a .C file which simply contains
FILE * fp;
It contains that single line of code because when that line of code is in place and I type and run "thing.c" in the cygwin command line it outputs what seems the be the information of the contents of the folder. For example it outputs,
home: sticky, directory
lib: directory
sbin: directory
setup-x86_64.exe: PE32+ executable (GUI) x86-64 (stripped to external PDB), for MS Windows
song.mp3: Audio file with ID3 version 2.3.0, contains: MPEG ADTS, layer III, v1, 128 kbps, 44.1 kHz, JntStereo
song1.mp3: Audio file with ID3 version 2.3.0, contains: MPEG ADTS, layer III, v1, 128 kbps, 44.1 kHz, JntStereo
thing.c: ASCII text, with CRLF line terminators
thing.txt: empty
What I want to do is be able to pull that output into a string that I can then use in my C file and alter and then re print out the new altered information. However I'm not sure where the output really is coming from or how I might be able to get it or save the output as a .txt file or back into a C file.
Any advice is appreciated Thanks!
This file is not really a C file at all. Because you're in Cygwin, you're likely operating on a case-insensitive filesystem (NTFS). As such, Cygwin's file command is running when you run the .c file. The way you've attempted to declare a variable (apparently) just so happens to be doing a 'file * fp' command. I'm sure you're getting fp: Cannot open "fp" or something similar after the rest of your output.
This is not anything C-related at all but is just being interpreted as a script by your shell.
It sounds like you have a lot to learn if you want to do this in C. More likely, you can probably write a shell script to accomplish what you want. While I've never used it, mp3info (https://github.com/jaalto/cygwin-package--mp3info) exists for pulling tag information from MP3 files. You could possibly get the exact information you want from that, or pipe the output into sed, awk, or a number of other tools.
I've started using Lua for school, and I'm wondering if it is possible to save a string to a txt file?
i.e
I have a string called shroom and it equals mush.
I have a file in the same folder as the .lua file called output.txt.
I now want to save the string shroom to output.txt.
I then want to make a while loop and every 1 second save the string shroom to the output.txt file, overwriting the contents of output.txt each time i save the string.
Is this possible in Lua? I've never used Lua before and I'm really confused at the moment..
Thanks :)
You can read this chapter from Programming in Lua to see how you can work with files; you'll need to use "w" mode when you open file and then call f:write(shroom) to do the write.
For sleeping in a loop you may want to check suggestions in this SO question.
I am working with a text file, which contains a list of processes under my programs control, along with relevant data.
At some point, one of the processes will finish, and thus will need to be removed from the file (as its no longer under control).
Here is a sample of the file contents (which has enteries added "randomly"):
PID=25729 IDLE=0.200000 BUSY=0.300000 USER=-10.000000
PID=26416 IDLE=0.100000 BUSY=0.800000 USER=-20.000000
PID=26522 IDLE=0.400000 BUSY=0.700000 USER=-30.000000
So for example, if I wanted to remove the line that says PID=26416.... how could I do that, without writing the file over again?
I can use external unix commands, however I am not very familiar with them so please if that is your suggestion, give an example.
Thanks!
Either you keep the contents of the file in temporary memory and then rewrite the file. Or you could have a file for each of the PIDs with the relevant information in them. Then you simply delete the file when it's no longer running. Or you could use a database for this instead.
As others have already pointed out, your only real choice is to rewrite the file.
The obvious way to do that with "external UNIX commands" would be grep -v "PID=26416" (or whatever PID you want to remove, obviously).
Edit: It is probably worth mentioning that if the lines are all the same length (as you've shown here) and order doesn't matter, you could delete a line more efficiently by copying the last line into the space being vacated, then shorten the file so eliminate what had been the last line. This will only work if they really are all the same length though (e.g., if you got a PID of '1', you'd need to pad it to the same length as the others in the file).
The only way is by copying each character that comes after the deleted line down over the characters that are deleted.
It is far more efficient to simply rewrite the file.
how could I do that, without writing the file over again?
You cannot. Filesystems (perhaps besides more esoteric record based ones) does not support insertion or deletion.
So you'll have to write the lines to a temporary file up till the line you want to delete, skip over that line, and write the rest of the lines to the file. When done, rename/copy the temp file to the original filename
Why are you maintaining these in a text file? That's not the best model for such a task. But, if you're stuck with it ... if these lines are guaranteed to all be the same length (it appears that way from the sample), and if the order of the lines in the file doesn't matter, then you can write the last line over the line for the process that has died and then shorten the file by one line with the (f)truncate() call if you're on a POSIX system: see Jonathan Leffler's answer in How to truncate a file in C?
But note carefully netrom's answer, which gives three different better ways to maintain this info.
Also, if you stick with a text file (preferably written from scratch each time from data structures you maintain, as per netrom's first suggestion), and you want to be sure that the file is always well formed, then write the new data into a temp file on the same device (putting it in the same directory is easiest) and then do a rename() call, which is an atomic operation.
You can use sed:
sed -i.bak -e '/PID=26416/d' test
-i is for editing in place. It also creates a back-up file with the new extension .bak
-e is for specifying the pattern. The /d indicates all lines matching the pattern should be deleted.
test is the filename
The unix command for it is:
grep -v "PID=26416" myfile > myfile.tmp
mv myfile.tmp myfile
The grep -v part outputs the file without the rows with the search term.
The > myfile.tmp part creates a new temp file for this output.
The mv part renames the temp file to the original file.
Note that we are rewriting the file here, and moreover, we can lose data if someone write something to file between the two commands.