read line basic question

ofer4 · 06-17-2012, 01:54 PM

hi

i have a question

when i write:

Code:

while read line;do

...

done

from where am i read the line?

also, if i want to read a line from the file: "ttt.dat"

what is the code to do that?

thanks alot

firstfire · 06-17-2012, 02:17 PM

Hi.

If you type in the console `help read | less', you'll probably see the following:

Quote:

$ help read
read: read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
Read a line from the standard input and split it into fields.

Reads a single line from the standard input, or from file descriptor FD
if the -u option is supplied. The line is split into fields as with word
splitting, and the first word is assigned to the first NAME, the second
word to the second NAME, and so on, with any leftover words assigned to
the last NAME. Only the characters found in $IFS are recognized as word
delimiters.
....

So, `read' reads from standard input. If you want to read from file, just redirect it to standard input:

Code:

while read line;do

...

done < input_file

ofer4 · 06-17-2012, 02:25 PM

Quote:

Originally Posted by firstfire

Hi.

If you type in the console `help read | less', you'll probably see the following:

So, `read' reads from standard input. If you want to read from file, just redirect it to standard input:

Code:

while read line;do

...

done < input_file

hi

thanks you

can i ask you more question?

if i have a file named: "aaa-bbb.dat"

and i want to seperate into two variables the words: "aaa" and "bbb"

how can i do it?

i tried to use

cut -f -d- "file"

however it doesnt work for me

PTrenholme · 06-17-2012, 02:38 PM

Did you look at man cut or even cut --help?

In order to get output from cut, you need to specify the fields to output.

Example:

Code:

$ echo "abc-def.ghi" | cut -d- -f1
abc
$ echo "abc-def.ghi" | cut -d- -f2
def.ghi
$ echo "abc-def.ghi" | cut -d- -f2 | cut -d. -f1
def
$ echo "abc-def.ghi" | cut -d- -f2 | cut -d. -f2
ghi
$ echo "abc-def.ghi" | cut -d. -f1 | cut -d- -f1,2 --output-delimiter=" "
abc def

ofer4 · 06-17-2012, 02:45 PM

Quote:

Originally Posted by PTrenholme

Did you look at man cut or even cut --help?

In order to get output from cut, you need to specify the fields to output.

Example:

Code:

$ echo "abc-def.ghi" | cut -d- -f1
abc
$ echo "abc-def.ghi" | cut -d- -f2
def.ghi
$ echo "abc-def.ghi" | cut -d- -f2 | cut -d. -f1
def
$ echo "abc-def.ghi" | cut -d- -f2 | cut -d. -f2
ghi

1. i didnt understand this example "$ echo "abc-def.ghi" | cut -d- -f2 | cut -d. -f1" ,i didnt understand why do i need the middle cut? and what is the input to this cut?

2.how can i refer as input to cut the file-name?

p.s i read about cut here:

http://ss64.com/bash/cut.html

PTrenholme · 06-17-2012, 03:45 PM

Quote:

Originally Posted by ofer4

1. i didnt understand this example "$ echo "abc-def.ghi" | cut -d- -f2 | cut -d. -f1" ,i didnt understand why do i need the middle cut? and what is the input to this cut?

The pipe (|) takes standard output of the command on the left and makes it the standard input to the command on the right. So, in the example, the echo created the input to the first cut, that first cut split the input into two parts, abc and def.ghi, and passed the def.ghi down the pipe to the second cut. Then the second cut split the def.ghi and printed the first part, the def.

Quote:

2.how can i refer as input to cut the file-name?

That, of course, depends on the script you are writing. If you are reading the file names from a file (As suggested by the question with which you started this thread.), something like this might be what you want:

Code:

#!/bin/bash
while read line
do
 for p in $(echo line | cut -d. -f1 | cut -d- -f1,2 --output-delimiter=" ")
 do
   echo ${p}
 done
done < input_file_name

(Where, of course, you would replace the echo ${p} with whatever you wanted to do with each part of the file names from the input file.)

Quote:

p.s i read about cut here:http://ss64.com/bash/cut.html

ofer4 · 06-17-2012, 03:56 PM

Quote:

Originally Posted by PTrenholme

The pipe (|) takes standard output of the command on the left and makes it the standard input to the command on the right. So, in the example, the echo created the input to the first cut, that first cut split the input into two parts, abc and def.ghi, and passed the def.ghi down the pipe to the second cut. Then the second cut split the def.ghi and printed the first part, the def.

That, of course, depends on the script you are writing. If you are reading the file names from a file (As suggested by the question with which you started this thread.), something like this might be what you want:

Code:

#!/bin/bash
while read line
do
 for p in $(echo line | cut -d. -f1 | cut -d- -f1,2 --output-delimiter=" ")
 do
   echo ${p}
 done
done < input_file_name

(Where, of course, you would replace the echo ${p} with whatever you wanted to do with each part of the file names from the input file.)

HI

first of all thanks you all for your help...

second, the solution that you suggested it is not solve my problem...

because the file locted in a library and their names are not locaed in a spesific file...

is there any solution for this problem?

thanks in advance

PTrenholme · 06-17-2012, 04:52 PM

Quote:

Originally Posted by ofer4

HI

first of all thanks you all for your help...

second, the solution that you suggested it is not solve my problem...

because the file locted in a library and their names are not locaed in a spesific file...

is there any solution for this problem?

thanks in advance

Sure. In place of < input_file_name use something like < $(ls -1 folder*-*.*) where folder is the name of the library containing the files. To see what the input would be, just run the command inside the $() expression from a command line. (Note that the option is -"number one" not -"letter L".)

firstfire · 06-17-2012, 11:15 PM

Hi.

To break a string (say, file name) into pieces and assign them to variables in one pass, one may use something like

Code:

$ fname="abc-def.ghi"
$ read a b c <<< ${fname//[-.]/ }
$ echo $a + $b + $c
abc + def + ghi

Here-string "<<<" and pattern substitution ${parameter/pattern/string} are used here to minimize the number of forks (additional processes) required.