Classic Shell Scripting

Reading Lines with read

The read command is one of the most important ways to get information into a shell program:

$ x=abc ; printf "x is now '%s'. Enter new value: " $x ; read x
x is now 'abc'. Enter new value: PDQ
$ echo $x
PDQ

read

Usage

read [ -r ] variable ...

Purpose

To read information into one or more shell variables.

Major options

 

-r

Raw read. Don't interpret backslash at end-of-line as meaning line continuation.

Behavior

Lines are read from standard input and split as via shell field splitting (using $IFS). The first word is assigned to the first variable, the second to the second, and so on. If there are more words than variables, all the trailing words are assigned to the last variable. read exits with a failure value upon encountering end-of-file.

If an input line ends with a backslash, read discards the backslash and newline, and continues reading data from the next line. The -r option forces read to treat a final backslash literally.

Caveats

When read is used in a pipeline, many shells execute it in a separate process. In this case, any variables set by read do not retain their values in the parent shell. This is also true for loops in the middle of pipelines.

read can read values into multiple variables at one time. In this case, characters in $IFS separate the input line into individual words. For example:

printf "Enter name, rank, serial number: "
read name rank serno

A typical use is processing the /etc/passwd file. The standard format is seven colon-separated fields: username, ...