Tag Archives: seq

bash random number generator using seq and sort

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Create the sequence between 0 and 9 and then do a random sort and get the first one.

# seq 0 9 | sort -R | head -1

You can count up the instances of each one and see the distribution looks normal to a human.

# for i in `seq 1 100000`; do seq 0 9 | sort -R | head -1 >> /tmp/rando; done;

# cat /tmp/rando | sort -n | uniq -c | sort -nk2
   9896 0
  10140 1
   9928 2
   9975 3
   9929 4
  10129 5
   9951 6
  10007 7
   9882 8
  10163 9

TODO: test with chi-square? the -R flag can’t be truly random?

remove IPv6 address from interface

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If you don’t want to have an IPv6 address on an interface, you can quickly remove it:

# ip addr show dev eth0 
2: eth0:  mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:0c:29:c2:fc:59 brd ff:ff:ff:ff:ff:ff
    inet 10.210.0.141/16 brd 10.210.255.255 scope global eth0
    inet6 fe80::20c:29ff:fec2:fc59/64 scope link 
       valid_lft forever preferred_lft forever

Only get the address:

# ip addr show dev eth0 | grep inet6 | awk '{print $2}'
fe80::20c:29ff:fec2:fc59/64

Remove it:

# ip -6 addr del `ip addr show dev eth0 | grep inet6 | awk '{print $2}'` dev eth0

Or do it to many hosts:

for i in `seq 201 272`; do ssh www$i "ip -6 addr del \`ip addr show dev eth0 | grep inet6 | awk '{print \$2}'\` dev eth0"; done;

make many directories with Bash sequences

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Sometimes it’s necessary to do things with sequences in Bash. If you want to create a bunch of directories that will be mount points for NFS servers you could do this:


# mkdir /mnt/fs42
# mkdir /mnt/fs42/vol0
# mkdir /mnt/fs42/vol1
# mkdir /mnt/fs42/vol2
# mkdir /mnt/fs42/vol3

But, even the best experts at the “up arrow key” wouldn’t want to do this for 50 file servers. A nested for loop would work, but it’s not necessarily the easiest way to go. The command seq makes a sequence of integers bounded by the numbers specified.

ok:

for i in `seq 1 50`; do for j in `seq 0 3`; do mkdir -p /mnt/fs$i/vol$j; done; done;

Using Bash sequences, you can tell the shell to interpret this {1..50} as a list of integers between 1 and 50. This also works with letters like {a..t}.

# echo {a..t}
a b c d e f g h i j k l m n o p q r s t
# echo {0..50}
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

way better:


# mkdir -p /mnt/fs{1..50}/vol{0..3}

of course if you have non-sequential lists, you may have to specify each element like this:

{1,3,4,5,6,9}