How to use pipes to connect programs

• Use the vertical bar (|) between two commands. In this example, send ls command output to grep command i.e. find out if data.txt file exits or not in the current working directory):

ls | grep "data.txt"

• All command line arguments (parameters) listed after command name, but before the the vertical bar:

ls -al | grep "data.txt"

• There is no need to put spaces between command names and vertical bars, it is optional:

ls -al|grep "data.txt"

• However, I recommend putting white spacing between the command names and vertical bars to improve the readability.
• You can redirect pipe output to a file (output redirection with > symbol):

ps aux | grep httpd > /tmp/ps.output.log

Examples

Common shell pipe examples:

Pause ls command output

Send output of the ls command as input to the more command. So that output is printed one screen full page at a time:

ls -l | more

Show a sorted list of logged on users

Output of who command is given as input to sort command. So that it will print a sorted list of users:

who | sort
who | sort > sorted_list.txt

Count logged in users

Output of who command is given as input to wc command, so that it will number of user who logged on to the system:

who | wc -l

Find out if user vivek logged in or not

who | grep -i vivek

Count total files in current directory

ls -l | wc  -l

Execute a shutdown command at a given time

echo "shutdown -h now" | at 12am tomorrow

Format output of mount command

Display mount command output in a nice format

mount | column -t

Backup (tar over ssh)

Use tar command over secure ssh session to backup local /home file system:

tar zcvf - /home | ssh user@server "cat > /backup/home_fs.workstation_sep_21_09.tar.gz"

Case conversion

v="Unix Philosophy"; echo $v | tr '[:lower:]' '[:upper:]' 
echo 'tHIs IS A TeSt' | tr '[:upper:]' '[:lower:]'

Birthday Email Reminder

echo "/usr/bin/mail -s 'Birthday gift for Julia' vivek@gite.in < /dev/null" | at 17:45

Create An ISO CD Image

Create an ISO cdrom image from contents of /home/vivek/photos directory:

mkisofs -V Photos -r /home/vivek/photos | gzip -9 > /tmp/photos.iso.cd.gz

You can burn an ISO cdrom image using the following syntax:

gzip -dc /tmp/photos.iso.cd.gz | cdrecord -v dev=/dev/dvdrw -

It is also possible to create an ISO image and burn it directly to cd:

mkisofs -V Photos -r /home/vivek/photos | cdrecord -v dev=/dev/dvdrw -

Create a random password

tr -dc A-Za-z0-9_ < /dev/urandom  | head -c12 | xargs