Pete's Linux Advent Calendar 2007

The 8th day

Stupid Shell Tricks or What to do in an emergency

What happens when you execute this command: /bin/ls* (including the "*")? On my system it gives:

/bin/lsmod  /bin/lsmod.modutils  /bin/lspci

And you probably know what has happened. The Shell expanded the pattern /bin/ls* to /bin/ls /bin/lsmod /bin/lsmod.modutils /bin/lspci and executed this resulting command line. How can that be useful? Imagine a situation where you have no more or less (pun indendet :-) nor vi nor ls but you are running bash. Maybe somebody has deleted these files. How can you substitute these commands by only using shell builtins?

ls

just type echo *

cat

type:

while read; do echo "$REPLY"; done < filename

append a line

echo new line >> filename

prepend a line

oldfile=$(while read; do echo "$REPLY"; done < filename)
{ echo new line ; echo "$oldfile" ; } > filename

modify a line in a file

Say, you want to modify the IP address of an entry in /etc/hosts. Let us assume you want to replace the address 192.168.0.1 to 192.168.0.2:

modified_hosts=$(while read ip rest; do
    if [ "$ip" = 192.168.0.1 ]; then
	ip=192.168.0.2
    fi
    echo -e "$ip\t$rest"
done < /etc/hosts)
echo "$modified_hosts" > /etc/hosts

Note that this is not very strict in preserving whitespace, but it does what we wanted.

Pete's Linux Advent Calendar 2007

Comments? send mail to pkruse@arcor.de