How to use read command in bash?

Question

When I try to use the read command in Bash as follows:

 echo hello | read str echo $str 

Nothing happened again, although I think str should contain the string hello . Can someone help me understand this behavior?

Answer 1

read in your command script is ok. However, you execute it in the pipeline, which means that it is in a subshell, so the variables that it reads are not visible in the parent shell. You can either

• also move the rest of the script to a subshell:

   echo hello | { read str echo $str } 

• or use command substitution to get the value of a variable from a subshell

   str=$(echo hello) echo $str 

  or a slightly more complex example (capturing the second ls element)

   str=$(ls | { read a; read a; echo $a; }) echo $str 

Answer 2

Other bash alternatives that do not include a subshell:

 read str <<END # here-doc hello END read str <<< "hello" # here-string read str < <(echo hello) # process substitution 

Answer 3

A typical use might look like this:

 i=0 echo -e "hello1\nhello2\nhello3" | while read str ; do echo "$((++i)): $str" done 

and conclusion

 1: hello1 2: hello2 3: hello3 

Answer 4

The value disappears as the read command runs in a separate subshell: Bash FAQ 24

Answer 5

To place my two cents here: on KSH, read ing, as well as on a variable, will work, because according to the IBM AIX documentation , KSH read affects the current shell environment:

Setting shell variables using a read command affects the current shell runtime.

This led me to spend a lot of minutes figuring out why the single-line finale with read , which I used a million times before I didn't work on AIX on Linux ... is because KSH saves to the current environment and BASH does not work!

Answer 6

I really use read only with "while" and do loop:

 echo "This is NOT a test." | while read -rabc theRest; do echo "$a" "$b" "$theRest"; done 

This is a test.
For what it's worth, I saw a recommendation to always use -r with a read command in bash.

Answer 7

Another alternative is to use the printf function.

 printf -v str 'hello' 

In addition, this design, combined with the use of single quotes, where appropriate, helps to avoid problems with multiple shoots of subshells and other forms of interpolation citation.

Answer 8

Do you need a pipe?

 echo -ne "$MENU" read NUMBER