Different versions of UNIX handle the stripping of newlines in a string differently. Obviously if you echo a string, the shell automatically expands newlines “\n”.

Solution

sed and tr don’t appear to like performing these substitutions. But with Perl its a steal.

Example

% echo 'hello world\nhello world\ntest it' | perl -ane 's#\\n# #g; print,"";'
hello world hello world test it

Reference

Technorati Tags: , Unix Coding School

• Check out my perl blog

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You want to resize images from the command line.

Solution

Use image magik mogrify command.

mogrify -resize “percentage” image_na

Also you can view current sizing - like this:

% identify IMG_3864.JPG
IMG_3864.JPG JPEG 563×422 563×422+0+0 DirectClass 8-bit 32.8887kb

Example

mogrify -resize 20% IMG_3705.JPG

Reference

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• mogrify man page - resize an image

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You want to listen on a pipe and perform commands, based on the text sent to the pipe.

This could be useful for triggering action based on events;
Running code as different users - i.e. allow root or a functional user to run something - just need to allow access to group of users, via permission to write to the pipe.

Solution

1. mknod /tmp/.restart_crd

2. chmod a+w /tmp/.restart_crd

3. nohup tail -f /tmp/.restart_crd| while read line
do
case “$line” in
‘restart’) cmd=”./stop >> /tmp/restart.log 2>&1; sleep 5; ./start >> /tmp/restart.log 2>&1″;;
’stop’) cmd=”./stop >> /tmp/restart.log 2>&1″;;
’start’) cmd=”./start >> /tmp/restart.log 2>&1″;;
*) cmd=”no match ${line}.”;;
esac

echo “command is $cmd”
eval $cmd

sleep 5

done &

4. disown

Example

$ echo "restart" > /tmp/.restart_crd

$ echo "start" > /tmp/.restart_crd

$ echo "stop" > /tmp/.restart_crd

Watch output in by tail -f /tmp/restart.log

Reference

Technorati Tags: , Unix Coding School

• Linux Man Pages - mknod command
• Linux Man Pages - bash command
• Linux Man Pages - chmod command

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