Read Command

The read command is a Bash builtin utility that reads a single line of text from standard input (stdin) or a specified file descriptor, splits the line into fields based on the Internal Field Separator (IFS), and assigns those fields to specified shell variables.

read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

Core Mechanics

Field Splitting and Assignment When read processes an input string, it evaluates the IFS shell variable (which defaults to space, tab, and newline). It strips leading and trailing IFS whitespace characters, then splits the remaining string into discrete words.

These words are assigned sequentially to the variable identifiers provided in the [name ...] arguments:

• Exact Match: If the number of words equals the number of variables, each variable receives one word.
• Overflow: If there are more words than variables, the final variable in the sequence receives all remaining unparsed words, including their intervening IFS characters.
• Underflow: If there are fewer words than variables, the remaining variables are assigned an empty string ("").

The Default Variable If no name arguments are supplied, read assigns the input line (excluding the terminating newline) to the default shell variable $REPLY. Although the input is not split into discrete fields, backslash escape processing still occurs and will mutate the string unless the -r flag is invoked.

Parsing Behavior and Escape Characters

By default, read interprets the backslash (\) as an escape character. A backslash followed by a newline acts as a line continuation, instructing read to ignore the newline and continue reading the next line of input as part of the current string.

To disable this behavior and treat backslashes as literal characters, the -r (raw) option must be invoked.

# Default behavior (backslash escapes the newline)
read var1 var2


# Raw behavior (backslash is a literal character)
read -r var1 var2

Flag Specifications

• -r (Raw Input): Disables backslash escape processing. This is the strictly recommended default for almost all programmatic implementations to prevent unintended data mutation.
• -a array (Array Assignment): Assigns the split words to sequential indices of an indexed array named array, starting at index 0. Unsets the array before assignment.
• -d delim (Delimiter): Instructs read to terminate upon encountering the first character of the string delim, rather than the default newline (\n).
• -e (Readline): Uses the Readline library to obtain the line of input whenever standard input is a terminal. This allows for line editing and history traversal, and operates successfully even within non-interactive shells (such as standard executed Bash scripts).
• -i text (Initial Text): Inserts the string text into the Readline buffer as the initial text before reading input. This option is only evaluated if the -e flag is also invoked.
• -p prompt (Prompt): Outputs the string prompt to standard error (stderr) without a trailing newline before attempting to read input. The prompt is only displayed if input is coming from a terminal.
• -t timeout (Timeout): Causes read to time out and return a failure exit status if a complete line of input is not read within timeout seconds. timeout accepts fractional seconds.
• -n nchars (Character Limit): Returns after reading exactly nchars characters, rather than waiting for a delimiter. It will return earlier if a delimiter is encountered.
• -N nchars (Strict Character Limit): Returns only after reading exactly nchars characters, ignoring delimiters entirely (including newlines).
• -s (Silent Mode): Disables terminal echoing of characters as they are typed. This is achieved by temporarily disabling the ECHO flag in the terminal’s termios settings.
• -u fd (File Descriptor): Reads input from the specified file descriptor fd instead of standard input (0).

Exit Status

The read command returns an exit status of 0 upon successful execution. Success occurs when a delimiter is reached, or when the specified character limits of the -n or -N flags are fulfilled, even if no delimiter is encountered. It returns a non-zero exit status (>0) under the following conditions:

• End-of-file (EOF) is encountered before a delimiter or character limit is reached.
• A timeout specified by -t is exceeded (returns exit status > 128).
• An invalid file descriptor is supplied to -u.
• An invalid variable name is provided.

Master Bash with Deep Grasping Methodology!

Learn More