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List only the directories

to include hidden dirs use: tree -adL 1 (with ls, requires 'ls -ad */ .*/')
This is sample output - yours may be different.
3

tree -dL 1

Escher · 2009-10-11 23:20:17 5
Remount a usb disk in Gnome without physically removing and reinserting

Remounts a usb disk /dev/sdb, without having to physically remove and reinsert. (Gnome desktop)
This is sample output - yours may be different.
7

eject /dev/sdb; sleep 1; eject -t /dev/sdb

Escher · 2009-10-11 23:16:49 8
download all the presentations from UTOSC2010

miss a class at UTOSC2010? need a refresher? use this to curl down all the presentations from the UTOSC website. (http://2010.utosc.com) NOTE/WARNING this will dump them in the current directory and there are around 37 and some are big - tested on OSX10.6.1 Show Sample Output
This is sample output - yours may be different.
```
http://2009.utosc.com/static/slides/Crowdsourcing_UTOSC.ppt
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 4819k  100 4819k    0     0   300k      0  0:00:16  0:00:16 --:--:--  250k
```
2

b="http://2010.utosc.com"; for p in $( curl -s $b/presentation/schedule/ | grep /presentation/[0-9]*/ | cut -d"\"" -f2 ); do f=$(curl -s $b$p | grep "/static/slides/" | cut -d"\"" -f4); if [ -n "$f" ]; then echo $b$f; curl -O $b$f; fi done

danlangford · 2009-10-11 17:28:46 3
convert filenames in current directory to lowercase

The simplest way I know. Show Sample Output
This is sample output - yours may be different.
```
gfagreaa renamed as GFAGREAA
gfdfhf renamed as GFDFHF
req renamed as REQ
rewqr renamed as REVQR
```
22

rename 'y/A-Z/a-z/' *

Karakh · 2009-10-11 11:36:07 16
Create an SSH connection (reverse tunnel) through your firewall.

Allows you to establish a tunnel (encapsulate packets) to your (Server B) remote server IP from your local host (Server A). On Server B you can then connect to port 2001 which will forward all packets (encapsulated) to port 22 on Server A. -- www.fir3net.com --
This is sample output - yours may be different.
8

ssh -R 2001:localhost:22 [username]@[remote server ip]

felix001 · 2009-10-11 09:51:04 4
convert filenames in current directory to lowercase
This is sample output - yours may be different.
2

find my_root_dir -depth -exec rename 's/(.*)\/([^\/]*)/$1\/\L$2/' {} \;

evalinux · 2009-10-11 07:09:06 5
autossh + ssh + screen = super rad perma-sessions

Only useful for really flakey connections (but im stuck with one for now). Though if youre in this situation ive found this to be a good way to run autossh and it does a pretty good job of detecting when the session is down and restarting. Combined with the -t and screen commands this pops you back into your working session lickety split w/ as few headaches as possible. And if autossh is a bit slow at detecting the downed ssh connection, just run this in another tab/terminal window to notify autossh that it should drop it and start over. Basically for when polling is too slow. kill -SIGUSR1 `pgrep autossh` Show Sample Output
This is sample output - yours may be different.
```
<Your running screen session>
```
11

AUTOSSH_POLL=1 autossh -M 21010 hostname -t 'screen -Dr'

iamarchimedes · 2009-10-11 06:04:29 7

Sort all running processes by their memory & CPU usage

you can also pipe it to "tail" command to show 10 most memory using processes. Show Sample Output

This is sample output - yours may be different.

root        21  0.0  0.0      0     0 ?        S<   Oct10   0:11 [ata/0]
68        1417  0.0  0.2   6772  2900 ?        Ss   Oct10   0:14 hald   
root      1586  0.0  0.1  17184  1836 ?        Ssl  Oct10   0:14 NetworkManager --pid-file=/var/run/NetworkManager/NetworkManager.pid
root      1554  0.0  0.0   3516   564 ?        S    Oct10   0:19 hald-addon-storage: polling /dev/sr0 (every 2 sec)                  
ali       2013  0.0  0.9 247540 10068 ?        S    Oct10   0:24 kwin -session 10dad4d46f000125200430400000021020032_1255123367_509351
dbus      1377  0.0  0.1  13492  1484 ?        Ssl  Oct10   0:25 dbus-daemon --system                                                 
root        47  0.0  0.0      0     0 ?        S<   Oct10   0:29 [scsi_eh_1]                                                          
ali       2047  0.0  1.5 123976 15816 ?        Rl   Oct10   0:44 yakuake -session 10dad4d46f000125165081500000020800011_1255123367_401521 -name Qt-subapplication
ali       2323  0.1  0.2 1610188 2528 ?        Ss   Oct10   0:52 C:\windows\system32\explorer.exe /desktop                                   
ali       2021  0.1  2.2 287952 23152 ?        Sl   Oct10   1:37 /usr/bin/plasma-desktop
ali       2299  0.2  0.0   5404   992 ?        Ss   Oct10   2:24 /usr/bin/wineserver
root      6341  0.5  7.0 122288 72160 pts/3    S+   00:33   0:33 /usr/bin/python /usr/bin/yum groupinstall GNOME Desktop Environment
ali       2296  0.6  0.7 1694604 8076 ?        Sl   Oct10   5:03 C:\Program Files\www.cproxy.com\cproxy.exe                                  
ali       2191  0.8  0.2 100880  2160 ?        Ssl  Oct10   7:07 /usr/bin/pulseaudio --start --log-target=syslog
ali       6164  1.2  3.2 165432 33084 ?        Sl   00:30   1:20 /usr/bin/kopete -caption Kopete
ali       4854  2.6  2.2 147408 22468 ?        Sl   Oct10   6:07 /usr/lib/nspluginwrapper/npviewer.bin --plugin /usr/lib/mozilla/plugins/libflashplayer.so --connection /org/wrapper/NSPlugins/libflashplayer.so/4759-1
ali       4546  2.9  5.9 228028 61048 ?        Sl   Oct10   7:24 /usr/lib/songbird-1.2.0/./songbird-bin
root      1840  5.9  2.7  55864 28172 tty1     Ss+  Oct10  49:50 /usr/bin/X -br -nolisten tcp :0 vt1 -auth /var/run/xauth/A:0-lhNFjm
ali       4759 16.6 18.4 652080 187640 ?       Sl   Oct10  38:56 /usr/lib/firefox-3.5.3/firefox
ali       7231  2.0  0.0   4764   988 pts/0    R+   02:16   0:00 ps aux --sort=%mem,%cpu

ps aux --sort=%mem,%cpu

mrwill · 2009-10-10 22:48:51 7

Functions to display, save and restore $IFS

You can display, save and restore the value of $IFS using conventional Bash commands, but these functions, which you can add to your ~/.bashrc file make it really easy. To display $IFS use the function ifs shown above. In the sample output, you can see that it displays the characters and their hexadecimal equivalent. This function saves it in a variable called $saveIFS: sifs () { saveIFS=$IFS; } Use this function to restore it rifs () { IFS=$saveIFS; } Add this line in your ~/.bashrc file to save a readonly copy of $IFS: declare -r roIFS=$IFS Use this function to restore that one to $IFS rrifs () { IFS=$roIFS; } Show Sample Output
This is sample output - yours may be different.
```
' '     '\t'    '\n'
0x20    0x09    0x0a
```
2

ifs () { echo -n "${IFS}"|hexdump -e '"" 10/1 "'\''%_c'\''\t" "\n"' -e '"" 10/1 "0x%02x\t" "\n\n"'|sed "s/''\|\t0x[^0-9]//g; $,/^$/d"

dennisw · 2009-10-10 22:41:35 8
Directory Tree

tree has lots of parms - man is your friend Show Sample Output
This is sample output - yours may be different.
```
[17:44][~/junk]$ tree -d
.
|-- dira
|   |-- f
|   |   `-- r
|   `-- s
|       |-- l
|       |-- q
|       `-- w
|           `-- d
|-- dirb
|   |-- h
|   |   `-- k
|   |-- i
|   `-- j
`-- dirc

14 directories
```
2

tree -d

bsussman · 2009-10-10 21:40:56 3
Simple Find
This is sample output - yours may be different.
-2

find / -name FILENAME

theIndianMaiden · 2009-10-10 20:12:33 3
Directory Tree
This is sample output - yours may be different.
1

find . -type d -print | sed -e 's;[^/]*/;..........;g'|awk '{print $0"-("NR-1")"}'

theIndianMaiden · 2009-10-10 20:08:40 4
VIM: when Ctrl-D and Ctrl-U only scroll one line, reset to default

Resets the scroll parameter to the default (half the rows in the current window). The scroll parameter can be inadvertently set to 1, e..g., if you type '1 Ctrl-D' or '1 Ctrl-U' in normal mode.
This is sample output - yours may be different.
1

:set scroll=0

tmsh · 2009-10-10 07:38:27 6
Schedule Nice Background Commands That Won't Die on Logout - Alternative to nohup and at

Check out the usage of 'trap', you may not have seen this one much. This command provides a way to schedule commands at certain times by running them after sleep finishes sleeping. In the example 'sleep 2h' sleeps for 2 hours. What is cool about this command is that it uses the 'trap' builtin bash command to remove the SIGHUP trap that normally exits all processes started by the shell upon logout. The 'trap 1' command then restores the normal SIGHUP behaviour. It also uses the 'nice -n 19' command which causes the sleep process to be run with minimal CPU. Further, it runs all the commands within the 2nd parentheses in the background. This is sweet cuz you can fire off as many of these as you want. Very helpful for shell scripts.
This is sample output - yours may be different.
2

( trap '' 1; ( nice -n 19 sleep 2h && command rm -v -rf /garbage/ &>/dev/null && trap 1 ) & )

AskApache · 2009-10-10 04:43:44 7
Count the total number of files in each immediate subdirectory
This is sample output - yours may be different.
-5

ps -ef | grep pmon

Sriregati · 2009-10-10 03:13:46 15
Count the total number of files in each immediate subdirectory

counts the total (recursive) number of files in the immediate (depth 1) subdirectories as well as the current one and displays them sorted. Fixed, as per ashawley's comment Show Sample Output
This is sample output - yours may be different.
```
>find . -type f -printf "%h\n" | cut -d/ -f-2 | sort | uniq -c | sort -rn
    694 Source
    312 Defaults
     50 Makeinfo
      5 CVS
      1 .
```
6

find . -type f -printf "%h\n" | cut -d/ -f-2 | sort | uniq -c | sort -rn

ivancho · 2009-10-09 23:49:53 5
Determine MAC address of remote host when you know its IP address

arping sends ARP requests to a neighboring host. This won't work if there is an ARP subnet gateway in the middle. If there is, you'll just get the gateway's MAC address not the host's address you are really trying to get to. Show Sample Output
This is sample output - yours may be different.
```
ARPING 192.168.1.2 from 192.168.1.3 eth0
Unicast reply from 192.168.1.2 [0B:DE:36:AF:C1:C8]  0.968ms
```
5

arping 192.168.1.2

stinkerweed999 · 2009-10-09 21:39:35 3
Monitor a file's size

use "watch" instead of while-loops in these simple cases
This is sample output - yours may be different.
9

watch -n60 du /var/log/messages

matrixguy · 2009-10-09 18:37:45 6
Monitor a file's size

Got a file you're generating and you want the size without typing in 'ls -l file' all the time? Use this instead. Show Sample Output
This is sample output - yours may be different.
```
2380 messages
2396 messages
2401 messages
```
0

while [ 1 ]; do du /var/log/messages;sleep 60; done

savanik · 2009-10-09 17:22:19 3
Outputs a 10-digit random number

Makes use of $RANDOM environment variable.
This is sample output - yours may be different.
0

head -c10 <(echo $RANDOM$RANDOM$RANDOM)

jgc · 2009-10-09 15:09:02 6
Check if variable is a number

Using the standard numeric comparison but suppressing the STDERR output acts as the simplest way to check a value is numeric. See sample output for some examples. Show Sample Output
This is sample output - yours may be different.
```
$ export testnum=123
$ if [ "$testnum" -eq "$testnum" 2>/dev/null ]; then echo It is numeric; fi
It is numeric
$ [ "$testnum" -eq "$testnum" 2>/dev/null ] && echo It is numeric
It is numeric
$ export testnum=123xxx
$ if [ "$testnum" -eq "$testnum" 2>/dev/null ]; then echo It is numeric; fi
$ [ "$testnum" -eq "$testnum" 2>/dev/null ] && echo It is numeric
$ 
```
0

if [ "$testnum" -eq "$testnum" 2>/dev/null ]; then echo It is numeric; fi

jgc · 2009-10-09 14:57:27 9
Check if variable is a number
This is sample output - yours may be different.
-1

echo $X | egrep "^[0-9]+$"

sharfah · 2009-10-09 13:59:47 6
Outputs a 10-digit random number
This is sample output - yours may be different.
2

tr -c -d 0-9 < /dev/urandom | head -c 10

hfs · 2009-10-09 11:58:30 3
calulate established tcp connection of local machine

If you want prepend/append text just wrap in echo: echo Connected: `netstat -an|grep -ci "tcp.*established"` Show Sample Output
This is sample output - yours may be different.
```
39
```
3

netstat -an|grep -ci "tcp.*established"

romulusnr · 2009-10-09 01:08:18 3
print crontab entries for all the users that actually have a crontab

Lists crontab for all users on system that have crontabs. Show Sample Output
This is sample output - yours may be different.
```
=== crontab for username ===
MAILTO="server@servername.com"

=== crontab for username2 ===
15 0 0 0 0 /some/command/to/run
```
-4

for USER in `ls /var/spool/cron`; do echo "=== crontab for $USER ==="; echo $USER; done

jemmille · 2009-10-08 23:51:43 3
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Do one ping to a URL, I use this in a MRTG gauge graph to monitor connectivity

Drop or block attackers IP with null routes

Someone might attack on your system. You can drop attacker IP using IPtables. However, you can use route command to null route unwanted traffic. A null route (also called as blackhole route) is a network route or kernel routing table entry that goes nowhere. Matching packets are dropped (ignored) rather than forwarded, acting as a kind of very limited firewall. The act of using null routes is often called blackhole filtering.

Which processes are listening on a specific port (e.g. port 80)

swap out "80" for your port of interest. Can use port number or named ports e.g. "http"

Which processes are listening on a specific port (e.g. port 80)

swap out "80" for your port of interest. Can use port number or named ports e.g. "http"

Timer with sound alarm

Which processes are listening on a specific port (e.g. port 80)

swap out "80" for your port of interest. Can use port number or named ports e.g. "http"

Use tee to process a pipe with two or more processes

Tee can be used to split a pipe into multiple streams for one or more process to work it. You can add more " >()" for even more fun.

Short one line while loop that outputs parameterized content from one file to another

The above is an example of grabbing only the first column. You can define the start and end points specifically by chacater position using the following command: $ while read l; do echo ${l:10:40}; done < three-column-list.txt > column-c10-c40.txt Of course, it doesn't have to be a column, or extraction, it can be replacement $ while read l; do echo ${l/foo/bar}; done < list-with-foo.txt > list-with-bar.txt Read more about parameter expansion here: http://wiki.bash-hackers.org/syntax/pe Think of this as an alternative to awk or sed for file operations

Files extension change

Changing a file extension to a new one for all files in a directory.

Which processes are listening on a specific port (e.g. port 80)

swap out "80" for your port of interest. Can use port number or named ports e.g. "http"

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