Joe Chung
Monmouth U. Homepage

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====== Reading Lines From Files ======

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Links & references:

  * [[http://mywiki.wooledge.org/BashFAQ/001|Bash FAQ, Question #1]]
  * [[http://askubuntu.com/questions/344407/how-to-read-complete-line-in-for-loop-with-spaces|An askubuntu page on this topic]]
  * [[http://en.tldp.org/LDP/abs/html/internalvariables.html|Bash Internal Field Separator variable]]
  * [[http://en.kioskea.net/faq/1757-how-to-read-a-file-line-by-line|Summarizes the above]]

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===== While Read =====

The shell [[cs_370_-_shell_scripting#input_in_shell_scripts|read]] command inputs entire lines.

We can do the following to send a file, line by line, to a // while read// loop:

<code>
# Non-ideal method.
# File we are going to process is represented by $filename.
cat $filename |
while read line; do
   echo $line   # simple echo of each line
done
</code>

But [[http://linuxpoison.blogspot.com/2012/08/bash-script-how-read-file-line-by-line.html|some people]] have a problem with this. They say that you should instead do the following, which represents the "BEST" way to read a file line by line in a shell script:

<code>
# BEST method.
# File we are going to process is represented by $filename.
while read line; do
   echo $line   # simple echo of each line
done < $filename
</code>

===== For Loop =====

For files that contain only one word or one field per line, where it is guaranteed that no white space characters will appear, a //for// loop can be used like this:

<code>
# WORST method.
# File we are going to process is represented by $filename.
for line in $(cat $filename); do
   echo $line   # simple echo of each line
done
</code>

This is considered the absolute [[http://linuxpoison.blogspot.com/2012/08/bash-script-how-read-file-line-by-line.html|"WORST"]] way to try to read a file, line by line. But a Bash internal variable [[http://en.tldp.org/LDP/abs/html/internalvariables.html|($IFS)]] (internal field separator) can mitigate this for files that contain more than one word per line. See examples at http://en.kioskea.net/faq/1757-how-to-read-a-file-line-by-line.

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