18.1. Here Strings

A here string can be considered as a stripped-down form of a here document. It consists of nothing more than COMMAND <<<$WORD, where $WORD is expanded and fed to the stdin of COMMAND.

As a simple example, consider this alternative to the echo-grep construction.

# Instead of: if echo "$VAR" | grep -q txt # if [[ $VAR = *txt* ]] # etc. # Try: if grep -q "txt" <<< "$VAR" then # ^^^ echo "$VAR contains the substring sequence \"txt\"" fi # Thank you, Sebastian Kaminski, for the suggestion.

Or, in combination with read:

String="This is a string of words." read -r -a Words <<< "$String" # The -a option to "read" #+ assigns the resulting values to successive members of an array. echo "First word in String is: ${Words[0]}" # This echo "Second word in String is: ${Words[1]}" # is echo "Third word in String is: ${Words[2]}" # a echo "Fourth word in String is: ${Words[3]}" # string echo "Fifth word in String is: ${Words[4]}" # of echo "Sixth word in String is: ${Words[5]}" # words. echo "Seventh word in String is: ${Words[6]}" # (null) # Past end of $String. # Thank you, Francisco Lobo, for the suggestion.

Example 18-13. Prepending a line to a file

#!/bin/bash # prepend.sh: Add text at beginning of file. # # Example contributed by Kenny Stauffer, #+ and slightly modified by document author. E_NOSUCHFILE=65 read -p "File: " file # -p arg to 'read' displays prompt. if [ ! -e "$file" ] then # Bail out if no such file. echo "File $file not found." exit $E_NOSUCHFILE fi read -p "Title: " title cat - $file <<<$title > $file.new echo "Modified file is $file.new" exit 0 # from 'man bash': # Here Strings # A variant of here documents, the format is: # # <<<word # # The word is expanded and supplied to the command on its standard input.

Example 18-14. Parsing a mailbox

#!/bin/bash # Script by Francisco Lobo, #+ and slightly modified and commented by ABS Guide author. # Used in ABS Guide with permission. (Thank you!) # This script will not run under Bash versions < 3.0. E_MISSING_ARG=67 if [ -z "$1" ] then echo "Usage: $0 mailbox-file" exit $E_MISSING_ARG fi mbox_grep() # Parse mailbox file. { declare -i body=0 match=0 declare -a date sender declare mail header value while IFS= read -r mail # ^^^^ Reset $IFS. # Otherwise "read" will strip leading & trailing space from its input. do if [[ $mail =~ "^From " ]] # Match "From" field in message. then (( body = 0 )) # "Zero out" variables. (( match = 0 )) unset date elif (( body )) then (( match )) # echo "$mail" # Uncomment above line if you want entire body of message to display. elif [[ $mail ]]; then IFS=: read -r header value <<< "$mail" # ^^^ "here string" case "$header" in [Ff][Rr][Oo][Mm] ) [[ $value =~ "$2" ]] && (( match++ )) ;; # Match "From" line. [Dd][Aa][Tt][Ee] ) read -r -a date <<< "$value" ;; # ^^^ # Match "Date" line. [Rr][Ee][Cc][Ee][Ii][Vv][Ee][Dd] ) read -r -a sender <<< "$value" ;; # ^^^ # Match IP Address (may be spoofed). esac else (( body++ )) (( match )) && echo "MESSAGE ${date:+of: ${date[*]} }" # Entire $date array ^ echo "IP address of sender: ${sender[1]}" # Second field of "Received" line ^ fi done < "$1" # Redirect stdout of file into loop. } mbox_grep "$1" # Send mailbox file to function. exit $? # Exercises: # --------- # 1) Break the single function, above, into multiple functions, #+ for the sake of readability. # 2) Add additional parsing to the script, checking for various keywords. $ mailbox_grep.sh scam_mail MESSAGE of Thu, 5 Jan 2006 08:00:56 -0500 (EST) IP address of sender: 196.3.62.4

Exercise: Find other uses for here strings.