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

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).

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

Fields are NOT separated with white-space

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

# 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

NOTE: IFS is set to a colon, :, as every field in the passwd file is separated by a colon.

Tab-delimited files

NOTE: For tab-delimited files, use IFS=$'\t'.

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

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:

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

NOTE:

first: will contain “Bob”
last: will contain “Smith”.
junk: holds everything else.

Throwaway variable

read -r _ _ first middle last _ <<< "$record"

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.

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.