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====== BASH - Files - Read a file - Read fields from a file ======

To read fields within each line of the file, additional variables may be used with the read:

----

===== Fields are separated with white-space (space or tab characters only) =====

If an input file has 3 columns separated by white-space (space or tab characters only).

<code bash>
while read -r first_name last_name phone; do
  # Only print the last name (second column).
  printf '%s\n' "$last_name"
done < "$file"
</code>

----

===== Fields are NOT separated with white-space =====

If the field delimiters are not whitespace, set the IFS (internal field separator):

<code bash>
# Extract the username and its shell from /etc/passwd:
while IFS=: read -r user pass uid gid gecos home shell; do
  printf '%s: %s\n' "$user" "$shell"
done < /etc/passwd
</code>

<WRAP info>
**NOTE:**  IFS is set to a colon, **:**, as every field in the passwd file is separated by a colon.
</WRAP>

----

===== Tab-delimited files =====

<WRAP info>
**NOTE:**  For tab-delimited files, use **IFS=$'\t'**.

<WRAP important>
**WARNING:**  Multiple tab characters in the input will be considered as one delimiter (and the **IFS=$'\t\t'** workaround does not work in Bash).

</WRAP>

</WRAP>

----

===== Not knowing how many fields a line contains =====

You do not necessarily need to know how many fields each line of input contains.

  * If you supply more variables than there are fields, the extra variables will be empty.
  * If you supply fewer, the last variable gets "all the rest" of the fields after the preceding ones are satisfied. 

For example:

<code bash>
read -r first last junk <<< 'Bob Smith 123 Main Street Saint Helier Jersey'
</code>

<WRAP info>
**NOTE:**

  * **first**:   will contain "Bob"
  * **last**:  will contain "Smith".
  * **junk**:  holds everything else.

</WRAP>

----

===== Throwaway variable =====

<code bash>
read -r _ _ first middle last _ <<< "$record"
</code>


<WRAP info>
**NOTE:**  The throwaway variable **_** can be used as a "junk variable" to ignore fields.

  * It, and any other variable, can be used more than once in a single read command, if we don't care what goes into it.

  * The first two fields are skipped.
  * The next three fields are reading into variables.
  * The final **_** will absorb any remaining fields on the line.
    * It does not need to be repeated there.

<WRAP important>
**WARNING:**  This usage of **_** is only guaranteed to work in Bash.

  * Many other shells use **_** for other purposes that will at best cause this to not have the desired effect, and can break the script entirely.
  * It is better to choose a unique variable that isn't used elsewhere in the script, even though **_** is a common Bash convention.

</WRAP>

</WRAP>

----