How to use the read command in Bash?

问题:

When I try to use the read command in Bash like this:

echo hello | read str
echo $str

Nothing echoed, while I think str should contain the string hello. Can anybody please help me understand this behavior?

回答1:

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

• move the rest of the script in the subshell, too:

  echo hello | { read str
    echo $str
  }
  

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

  str=$(echo hello)
  echo $str
  

  or a slightly more complicated example (Grabbing the 2nd element of ls)

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

回答2:

Other bash alternatives that do not involve a subshell:

read str <<END             # here-doc
hello
END

read str <<< "hello"       # here-string

read str < <(echo hello)   # process substitution

回答3:

Typical usage might look like:

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

and output

1: hello1
2: hello2
3: hello3

回答4:

The value disappears since the read command is run in a separate subshell: Bash FAQ 24

回答5:

To put my two cents here: on KSH, reading as is to a variable will work, because according to the IBM AIX documentation, KSH's read does affects the current shell environment:

The setting of shell variables by the read command affects the current shell execution environment.

This just resulted in me spending a good few minutes figuring out why a one-liner ending with read that I've used a zillion times before on AIX didn't work on Linux... it's because KSH does saves to the current environment and BASH doesn't!

回答6:

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

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

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

回答7:

Another alternative altogether is to use the printf function.

printf -v str 'hello'

Moreover, this construct, combined with the use of single quotes where appropriate, helps to avoid the multi-escape problems of subshells and other forms of interpolative quoting.

回答8:

Do you need the pipe?

echo -ne "$MENU"
read NUMBER