Home / ubuntu / Questions / 1197400
In Process
An Ignorant Wanderer
Asked: 2019-12-20 18:12:41 +0800 CST

here strings versus input redirection

  • 772

The following example is taken from William Shott's The Linux command line. It is a shell script which gathers user info from the /etc/passwd file. The specific user for which this is done is read from stdin.

My question is about the "here string" used in line 7. Just for kicks, I tried using the redirection operator instead, but it did not work. Why?

PS: I have some background in C++ so I expected the string file_info to act as a stringstream. 

     1  #!/bin/bash

     2  # read-ifs: read fields from a file

     3  FILE=/etc/passwd

     4  read -p "Enter a username > " user_name

     5  file_info="$(grep "^$user_name:" $FILE)"

     6  if [ -n "$file_info" ]; then
     7       IFS=":" read user pw uid gid name home shell <<< "$file_info"
     8       echo "User = '$user'"
     9       echo "UID = '$uid'"
    10       echo "GID = '$gid'"
    11       echo "Full Name = '$name'"
    12       echo "Home Dir. = '$home'"
    13       echo "Shell = '$shell'"

    14  else
    15      echo "No such user '$user_name'" >&2
    16      exit 1
    17  fi
command-line bash redirect

1 Answers

  1. If you do something like

    grep "^$user_name:" $FILE | IFS=":" read user pw uid gid name home shell
echo "User = '$user'"

    the right side of the pipeline (the read command) is run in its own subshell process and the variables it sets are gone when it ends so echo just displays empty strings.

    But you can do something like

    grep "^$user_name:" $FILE | ( IFS=":" read user pw uid gid name home shell
    echo "User = '$user'" )

    In this case the whole ( ... ) part is run in the same subshell and the variables are available for the echo call.

    • 0