使用 sed 在 txt 文件的第 1 行插入一个变量

Question

I have the following bash:

#!/bin/bash
if ["$#" -ne "1"]; then
   echo "Usage: `basename $0` <HOSTNAME>"
   exit 1
fi

IPADDR=`ifconfig | head -2 | tail -1 | cut -d: -f2 | rev | cut -c8-23 | rev`
sed -i -e '1i$IPADDR   $1\' /etc/hosts

But when I cat /etc/hosts :

$IPADDR

How can I deal with such issues?

Answer 1

Your problem is that variables inside single quotes ' aren't expanded by the shell, but left unchanged. To quote variables you want expanded use double quotes " or just leave off the quotes if they are unneeded like here, eg

sed -i -e '1i'$IPADDR'   '$1'\' /etc/hosts

In above line $IPADDR and $1 are outside of quotes and will be expanded by the shell before the arguments are being feed to sed .

Answer 2

The single quotes mean the string isn't interpolated as a variable.

#!/bin/bash
IPADDR=$(/sbin/ifconfig | head -2 | tail -1 | cut -d: -f2 | rev | cut -c8-23 | rev)
sed -i -e "1i${IPADDR}   ${1}" /etc/hosts

I also did the command in $(...) out of habit!

Answer 3

Refer to sed manual: https://www.gnu.org/software/sed/manual/sed.html

As a GNU extension, the i command and text can be separated into two -e parameters, enabling easier scripting:

The formal usage should be:

sed -i -e "1i$IPADDR\\\\" -e "$1" /etc/hosts