Generate files using content from a master text file

Shell Programming and Scripting
1

Environment: Bash on RHEL 8.9

I have a 'master file' like below with hundreds of lines.
In this file, I have the file names with the syntax file<n>.prm.code. The immediate line below each file name (like file1.prm.code) should
become the content of that file.

$ cat master_file.txt
file1.prm.code
Hello world

file2.prm.code
Hello Universe

file3.prm.code
Some random content for file3

file4.prm.code
Hello galaxy

So, using the above 'master file', is there a way, I can generate files like below ?

$ cat file1.prm.code
Hello world
$
$ cat file2.prm.code
Hello Universe
$
$ cat file3.prm.code
Some random content for file3
$
$ cat file4.prm.code
Hello galaxy
1 Like
2

Most likely than not.
Where exactly are you stuck?

3

Always a single line to go to the file? It would not be hard to deal with multiple lines for each output file, terminated by either the first blank line, or the next appropriate filename.

4

The following relies on a closing empty line (and would take multi-line contents).

#!/bin/bash
# Read a line from stdin (redirected), it is the filename
while read fn
do
  # Read further lines and print to stdout (redirected)
  # until the line is empty
  until
    IFS= read -r line; [ -z "$line" ]
  do
    printf "%s\n" "$line"
  done > "$fn"
  # Simply continue, the next line will be a filename
done < master_file.txt
3 Likes
5

Please Explain this :

IFS= read -r line; [ -z "$line" ]

and why the first line itself (i.e the name of file to be created) is not added as data to file.

Thanks

6

You are an angel, MadeInGermany. Thank You !

I was trying some stuff with grep but had no idea how to loop through line by line.
Plus, I didn't think of simple solution of output redirection to generate the files. Very clever.

One more question:
How does [ -z "$line" ] test work with the UNTIL clause ?

7

[ -z "$line" ]
tests if $line is empty.
The loop repeats until $line is empty.
The read command is stuffed before it; it is just executed; the exit status is taken from the [ ] command (the last command before the do).

until is like while not
You can also write

  while
    IFS= read -r line; [ -n "$line" ]
  do

While $line is not empty ...

The outer loop has the input redirected; everything up to the closing done is redirected.
The first read (the first line) goes to the fn variable. Then the inner loop does the next reads to the line variable, until an empty line has been read.

3 Likes
8

The file name is not written to its output file because that line is safely held in fn and is not printed at all. Your original example data does not show that as output either.

Because the redirection is owned by the until loop, you would need to wrap another block around both a single printf "%s\n" "$fn" before the until, and the until ... done loop inself, to reflect the fact that you are sending one header line and several contents lines to the same redirection.

{
    printf "%s\n" "$fn"
    until IFS= read -r line; [ -z "$line" ]; do
        printf "%s\n" "$line"
    done;
} > "$fn"
4 Likes
9

Still , one point is not understood.

Until loop is a different inner (subshell) loop and has different variable(line)
When this loop acts and takes the control to the blank line, then how this is communicated to the outer loop so that the control of the outer do-while loop also shifts to the line after this blank line.

This means outer loop is not executing independently and the control of inner loop is communicated to outer parent loop (but how).

Hope i have communicated my question with clarity.

Thanks.

10

Each read reads one line (the next line from the stdin stream), regardless if the read is located in the outer or inner loop.
The "communication" is the file pointer.

The stdin is redirected by the outer loop, and everything in it, including the inner loop, gets this redirected stdin (unless it were again redirected).

1 Like
11

Thanks buddy for increasing my knowledge.. :smile:

Sincerely,