Append a record to the end of a text file

From Rosetta Code
Append a record to the end of a text file is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Many systems offer the ability to open a file for writing, such that any data written will be appended to the end of the file. Further, the file operations will always adjust the position pointer to guarantee the end of the file, even in a multitasking environment.

This feature is most useful in the case of log files, where many jobs may be appending to the log file at the same time, or where care must be taken to avoid concurrently overwriting the same record from another job.

Task: Given a two record sample for a mythical "passwd" file:

  • Write these records out in the typical system format.
    • Ideally these records will have named fields of various types.
  • Close the file, then reopen the file for append.
    • Append a new record to the file and close the file again.
    • Take appropriate care to avoid concurrently overwrites from another job.
  • Open the file and demonstrate the new record has indeed written to the end.

Finally: Provide a summary of the language's "append record" capabilities in a table.

Alternatively: If the language's appends can not guarantee its writes will always append, then note this restriction in the table. If possible, provide an actual code example (possibly using file/record locking) to guarantee correct concurrent appends.

C

Append Capabilities.
Data Representation IO
Library 		Append
Possible 		Automatic
Append 		Multi-tasking
Guaranteed
In core On disk
C struct CSV text file glibc/stdio ☑ ☑ ☑

From a C "struct" to CSV File

<lang C>#include <stdio.h>

  1. include <string.h>

/****************************

  • Create a passwd text file
                                                        • /

/* note that UID & gid are of type "text" */ typedef char *STRING; typedef struct{STRING fullname, office, extension, homephone, email; } gecos_t; typedef struct{STRING account, password; int uid, gid; gecos_t gecos; STRING directory, shell; } passwd_t;

  1. define GECOS_FMT "%s,%s,%s,%s"
  2. define PASSWD_FMT "%s:%s:%d:%d:"GECOS_FMT":%s:%s"

passwd_t passwd_list[]={ 

 {"jsmith","x",1001,1000,
   {"Joe Smith","Room 1007","(234)555-8917","(234)555-0077","jsmith@rosettacode.org"},
   "/home/jsmith","/bin/sh"},
 {"jdoe","x",1002,1000,
   {"Jane Doe","Room 1004","(234)555-8914","(234)555-0044","jdoe@rosettacode.org"},
   "/home/jsmith","/bin/sh"}

};

main(){ 

 FILE *passwd_text=fopen("passwd.txt","w");
 int rec_num;
 for(rec_num=0; rec_num < sizeof passwd_list/sizeof(passwd_t); rec_num++){
   fprintf(passwd_text, PASSWD_FMT"\n", passwd_list[rec_num]);
 }
 fclose(passwd_text);
 
 /********************************
 * Load text ready for appending
 ********************************/
 passwd_text=fopen("passwd.txt","a+");
 passwd_t new_rec = 
     {"xyz","x",1003,1000,
         {"X Yz","Room 1003","(234)555-8913","(234)555-0033","xyz@rosettacode.org"},
         "/home/xyz","/bin/sh"};
 fprintf(passwd_text, PASSWD_FMT"\n", new_rec);
 fclose(passwd_text);
 
 /*********************************************
 * Finally refopen and check record was appended
 *********************************************/
 char passwd_buf[512]; /* warning: fixed length */
 passwd_text=fopen("passwd.txt","r");
 while(!feof(passwd_text))
   fscanf(passwd_text, "%s\n", passwd_buf);
 if(strstr(passwd_buf,"xyz"))
   printf("Appended record: %s\n",passwd_buf);

}</lang> Output: 

Appended record: 1003,(234)555-8913,(234)555-0033:xyz@rosettacode.org:/home/xyz

Java

This example is incomplete. It does not have a table of append record capabilities. Please ensure that it meets all task requirements and remove this message.

<lang java>import java.io.BufferedReader; import java.io.FileNotFoundException; import java.io.FileReader; import java.io.FileWriter; import java.io.IOException; import java.io.PrintWriter;

public class Append{ public static void main(String[] args){ try{ System.out.println("Pre-loading file..."); //the "true" here is for autoflush PrintWriter out = new PrintWriter(new FileWriter("passwd"), true); out.println("user1 password1"); out.println("user2 password2"); out.close(); showFile("passwd");

System.out.println("\nAppending new line to file..."); //the first "true" is for append, the second is for autoflush out = new PrintWriter(new FileWriter("passwd", true), true); out.println("user3 password3"); out.close(); showFile("passwd"); }catch(IOException e){ System.err.println("Error in writing file: " + e.getMessage()); } }

private static void showFile(String filename){ try{ BufferedReader in = new BufferedReader(new FileReader(filename)); while(in.ready()){ System.out.println(in.readLine()); } }catch(FileNotFoundException e){ System.err.println("File " + filename + " not found."); }catch(IOException e){ System.err.println("Error in reading file: " + e.getMessage()); } } }</lang> Output: 

Pre-loading file...
user1 password1
user2 password2

Appending new line to file...
user1 password1
user2 password2
user3 password3

Python

Append Capabilities.
Data Representation IO
Library 		Append
Possible 		Automatic
Append 		Multi-tasking
Guaranteed
In core On disk
dict CSV text file builtin ☑ ☑ ☑
instance CSV text file builtin ? ? ?

From a "dict" to a CSV File

<lang python>#############################

  1. Create a passwd text file
  2. note that UID & gid are of type "text"

passwd_list=[ 

 dict(account='jsmith', password='x', UID=1001, GID=1000, # UID and GID are type int
      GECOS=dict(fullname='Joe Smith', office='Room 1007', extension='(234)555-8917',
                 homephone='(234)555-0077', email='jsmith@rosettacode.org'),
                 directory='/home/jsmith', shell='/bin/sh'),
 dict(account='jdoe', password='x', UID=1002, GID=1000,
      GECOS=dict(fullname='Jane Doe', office='Room 1004', extension='(234)555-8914',
                 homephone='(234)555-0044', email='jdoe@rosettacode.org'),
      directory='/home/jsmith', shell='/bin/sh')

]

passwd_fields="account password UID GID GECOS directory shell".split() GECOS_fields="fullname office extension homephone email".split()

def passwd_text_repr(passwd_rec):

  1. convert individual fields to string type
 passwd_rec["GECOS"]=",".join([ passwd_rec["GECOS"][field] for field in GECOS_fields])
 for field in passwd_rec: # convert "int" fields
   if not isinstance(passwd_rec[field], str):
     passwd_rec[field]=`passwd_rec[field]`
 return ":".join([ passwd_rec[field] for field in passwd_fields ])

passwd_text=open("passwd.txt","w") for passwd_rec in passwd_list: 

 print >> passwd_text,passwd_text_repr(passwd_rec)

passwd_text.close()

del passwd_list, passwd_text

  2. Load text ready for appending

passwd_text=open("passwd.txt","a+") new_rec=dict(account='xyz', password='x', UID=1003, GID=1000, 

            GECOS=dict(fullname='X Yz', office='Room 1003', extension='(234)555-8913',
                       homephone='(234)555-0033', email='xyz@rosettacode.org'),
            directory='/home/xyz', shell='/bin/sh')

print >> passwd_text, passwd_text_repr(new_rec) passwd_text.close()

del new_rec, passwd_text

  2. Finally reopen and check record was appended

passwd_list=list(open("passwd.txt","r")) if "xyz" in passwd_list[-1]: 

 print "Appended record:",passwd_list[-1][:-1]</lang>

Output: 

Appended record: xyz:x:1003:1000:X Yz,Room 1003,(234)555-8913,(234)555-0033,xyz@rosettacode.org:/home/xyz:/bin/sh
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