I have a text file that contains something like this:
abc 123, comma
the quick brown fox
jumped over the lazy dog
comma, comma
I wrote a script
for i in `cat file`
do
echo $i
done
For some reason, the output of the script doesn't output the file line by line but breaks it off at the commas, as well as the newline. Why is cat or "for blah in cat xyz
" doing this and how can I make it NOT do this? I know I can use a
while read line
do
blah balh blah
done < file
but I want to know why cat or the "for blah in" is doing this to further my understanding of unix commands. Cat's man page didn't help me and looking at for or looping in the bash manual didn't yield any answers (http://www.gnu.org/software/bash/manual/bashref.html). Thanks in advance for your help.
The problem is not in cat
, nor in the for
loop per se; it is in the use of back quotes. When you write either:
for i in `cat file`
or (better):
for i in $(cat file)
or (in bash
):
for i in $(<file)
the shell executes the command and captures the output as a string, separating the words at the characters in $IFS
. If you want lines input to $i
, you either have to fiddle with IFS
or use the while
loop. The while
loop is better if there's any danger that the files processed will be large; it doesn't have to read the whole file into memory all at once, unlike the versions using $(...)
.
IFS='
'
for i in $(<file)
do echo "$i"
done
The quotes around the "$i"
are generally a good idea. In this context, with the modified $IFS
, it actually isn't critical, but good habits are good habits even so. It matters in the following script:
old="$IFS"
IFS='
'
for i in $(<file)
do
(
IFS="$old"
echo "$i"
)
done
when the data file contains multiple spaces between words:
$ cat file
abc 123, comma
the quick brown fox
jumped over the lazy dog
comma, comma
$
Output:
$ sh bq.sh
abc 123, comma
the quick brown fox
jumped over the lazy dog
comma, comma
$
Without the double quotes:
$ cat bq.sh
old="$IFS"
IFS='
'
for i in $(<file)
do
(
IFS="$old"
echo $i
)
done
$ sh bq.sh
abc 123, comma
the quick brown fox
jumped over the lazy dog
comma, comma
$
You can use IFS
variable to specific you want a newline as the field separator:
IFS=$'\n'
for i in `cat file`
do
echo $i
done
the for loop coupled with a change of the internal field separator(IFS) will read file as intended
for an input
abc 123, comma
the quick brown fox
jumped over the lazy dog
comma, comma
For loop coupled with an IFS change
old_IFS=$IFS
IFS=$'\n'
for i in `cat file`
do
echo $i
done
IFS=$old_IFS
results in
abc 123, comma
the quick brown fox
jumped over the lazy dog
comma, comma
IFS - Internal field separator can be set to get what you want.
To read a whole line at once, use:
IFS=""