What does 'set -e' do, and why might it be considered dangerous?

Source: http://serverfault.com/questions/143445/what-does-set-e-do-and-why-might-it-be-considered-dangerous

set -e causes the shell to exit if any subcommand or pipeline returns a non-zero status.

The answer the interviewer was probably looking for is:

It would be dangerous to use "set -e" when creating init.d scripts:
From http://www.debian.org/doc/debian-policy/ch-opersys.html 9.3.2 --

Be careful of using set -e in init.d scripts. Writing correct init.d scripts requires accepting various error exit statuses when daemons are already running or already stopped without aborting the init.d script, and common init.d function libraries are not safe to call with set -e in effect. For init.d scripts, it's often easier to not use set -e and instead check the result of each command separately.
This is a valid question from an interviewer standpoint because it gauges a candidates working knowledge of server-level scripting and automation

How to identify Linux kernel verion ?

In kernel source tree, check top makefile.
For ex:

VERSION = 3
PATCHLEVEL = 1
SUBLEVEL = 0
EXTRAVERSION = -pax
NAME = Custom Pax Version

Or,

$ make kernelversion

In distribution shell,

$ uname -a

How to run 32-bit app in Ubuntu 64-bit?

When an executable exists but shell replies "No such file or directory",
check the executable by
    $ source
check the Linux architecture by
    $ uname -a

How to run 32-bit app in Ubuntu 64-bit?

To run a 32-bit executable file on a 64-bit multi-architecture Ubuntu system, you have to add the i386 architecture and install the three library packages libc6:i386, libncurses5:i386, and libstdc++6:i386:

    $ sudo dpkg --add-architecture i386

Or if you are using Ubuntu 12.04 LTS (Precise Pangolin) or below, use this:
    $ sudo echo "foreign-architecture i386" > /etc/dpkg/dpkg.cfg.d/multiarch
 Then:
    $ sudo apt-get update 
    $ sudo apt-get install libc6:i386 libncurses5:i386 libstdc++6:i386

After these steps, you should be able to run the 32-bit application.

Linux 'find' handbook

#Exclude a directory in find . command

find . -path ./misc -prune -o -name '*.txt'

find . -type d \( -path dir1 -o -path dir2 -o -path dir3 \) -prune -o -print

find / -xdev -name p2p_supplicant.conf

#List all .bin (case ignored) and .txt (case sensitivity) files in /fooA and /fooB folders.

find /fA /fB -type f -iname "*.bin" -name "*.txt"
find -name "[a-z]*[4-9].log"
# Copy all .txt files in /foo to ~/txt folder.

find /foo -type f -name "*.txt" -exec cp {} ~/txt \;
# Remove all .git folders in /foo 

find /foo -type d -name ".git" -exec rm -fr {} \;    

# List all hidden folders/files in /foo 

find /foo -name ".*" -ls    

# Find files and tar them (with spaces)
# It will:
#   + deal with files with spaces, newlines, leading dashes, and other funniness

#   + handle an unlimited number of files

#   + won't repeatedly overwrite your backup.tar.gz like 

#       using tar -c with xargs will do when you have a large number of files.

find . -type f -print0 | tar -czvf backup.tar.gz --null -T -

# Find files and open them in gedit.

find . -name '*.foo' -exec gedit {} +

# Find symbolic links but those inside .git folders and count.

find . ! -path "*/.git/*" -type l | wc -l

find . ! -path "*/.git/*" -type f -name "*.js" ! -name "*-min-*" ! -name "*console*"

Find files by text in the file (find + grep)

----------------------------------------------------------------------------------------------
# Find StringBuffer in all *.java files 

find -type f -name "*.java" -exec grep -l StringBuffer {} \;
# ignore case with -i option 

find -type f -name "*.java" -exec grep -il string {} \;
# search for a string in gzip'd files

find -type f -name "*.gz" -exec zgrep 'GET /foo' {} \;        

Find files by modification time

---------------------------------------------------------------------------------------------- 

find -newer A.log ! -newer B.log     # newer than A.log but no newer than B.log
find . -mtime 1                                 # 24 hours 

find . -mtime -7                                # last 7 days

touch -t YYMMDDhhmm.SS /tmp/timestamp
find directory -type f -newer /tmp/timestamp
rm /tmp/timestamp

touch –date "2012-11-07" /tmp/find_after
find / -newer /tmp/find_after
rm /tmp/find_after

Find files by user/group

---------------------------------------------------------------------------------------------- 

find /etc -user peida

find /home -nouser

find /apps -group sudo

find /apps -nogroup

find -perm 755                                # List all files/folders of permission 755

find /dir -type d -iname ".*" -ls      # List all files/folders in hidden folders.

find \! -user foo -print                     # List items NOT owned by user foo.

Find files by size,folder depth,partition

---------------------------------------------------------------------------------------------- 

find -size +1000000c                      # list items of size larger than 10^6 bytes.

find -name A*.txt -depth                 # list in the same depth and then subfolders.

find -name *.XC -mount                  # only for the same partition.

Error handling

----------------------------------------------------------------------------------------------

#ERROR find: paths must precede expression
find . -name "*.log"                     
#Handle file with space in its name.  ('find' treats SPACE, TAB, and NEWLINE as delimiter by default)
find . -name "*.log" -print0 | xargs -0 cat           #'-print0' append 0 rather than NEWLINE to the output string.  '-0' takes only 0 as delimiter.
find . -name "*.log" -print0 | xargs -d "\\n" cat  # ' -d "\\n" ' takes only NEWLINE as delimiter.

#Skip “Permission denied” errors
source: https://unix.stackexchange.com/questions/42841/how-to-skip-permission-denied-errors-when-running-find-in-linux

find / -name art  2>&1 | grep -v "Permission denied"

Explanation:

In short, all regular output goes to standard output (stdout). All error messages to standard error (stderr).

grep usually finds/prints the specified string, the -v inverts this, so it finds/prints every string that doesn't contain "Permission denied". All of your output from the find command, including error messages usually sent to stderr (file descriptor 2) go now to stdout(file descriptor 1) and then get filtered by the grep command.

This assumes you are using the bash/sh shell.

Under tcsh/csh you would use

 find / -name art |& grep ....

To Be Sorted

---------------------------------------------------------------------------------------------- 

删除以html结尾的10天前的文件，包括带空格的文件：

find /usr/local/backups -name "*.html" -mtime +10 -print0 |xargs -0 rm -rfv
find /usr/local/backups -mtime +10 -name "*.html" -exec rm -rf {} \;

find -print 和 -print0的区别:
-print 在每一个输出后会添加一个回车换行符，而-print0则不会。

当前目录下文件从大到小排序（包括隐藏文件），文件名不为"."：
find . -maxdepth 1 ! -name "." -print0 | xargs -0 du -b | sort -nr | head -10 | nl
nl：可以为输出列加上编号,与cat -n相似，但空行不编号
以下功能同上，但不包括隐藏文件：
for file in *; do du -b "$file"; done|sort -nr|head -10|nl

xargs结合sed替换：
find . -name "*.txt" -print0 | xargs -0 sed -i 's/aaa/bbb/g'

xargs结合grep：
find . -name '*.txt' -type f -print0 |xargs -0 grep -n 'aaa' #“-n”输出行号

Find things by name

 # find /path/to/search -name filename

Example

 # find /etc -name hosts
  /etc/hosts

Find things by name (case-insensitive)

 # find /path/to/search -iname filename

Example

 # find /etc -iname HOSTS
  /etc/hosts

Find only files by name

 # find /path/to/search -name filename -type f

Example

 # find /etc -name network* -type f
  /etc/init/networking.conf

Find only directories by name

 # find /path/to/search -name dirname -type d

Example

 # find /etc -name network* -type d
  /etc/apparmor/init/network-interface-security

Find all symlinks

 # find /path/to/search -type l

Example

 # find /etc -type l
  /etc/vtrgb

Find things by owner

 # find /path/to/search -user owner

Example

 # find ./ -user root
  ./
  ./log.file

Find executable files

 # find /path/to/search -type f -executable

Example

 # find ./ -type f -executable
 ./4/2651.file

Find SUID files

 # find /path/to/search -perm -4000

Example

 # find /sbin -perm -4000
 /sbin/mount.ecryptfs_private

Find things changed today

 # find /path/to/search -daystart -ctime -1

Example

 # find ./ -daystart -ctime -1
  ./

Find things changed in the last 24 hours

 # find /path/to/search -ctime -1

Example

 # find ./ -ctime -1
  ./

Counting how many things you find

 # find /path/to/search | wc -l

Example

 # find ./ | wc -l
  14674

Deleting things you find

 # find /path/to/search -delete

Deleting things you find (with xargs)

 # find /path/to/search | xargs rm

Deleting things you find (with exec)

 # find /path/to/search -exec rm {} ;

Printing Type of file, Filename & Inode

 # find /path/to/search -printf "%y %i %prn"

Example

 # find ./ -printf "%y %i %prn"
  d 4852409 ./

Finding Broken Symlinks

 # find /path/to/search -follow -lname "*"

Example

 # find ./ -follow -lname "*"
  ./bad_link

Find files older than 31 days and delete them

 # find /path/to/search -mtime +31 -delete

Example

 # find ./ -mtime +31
  ./sudoers.new
  ./file.symlink
  ./somedirectory
  ./play/list.txt2
  ./tar.tgz
  # find ./ -mtime +31 -delete
  # find ./ -mtime +31

Remove Empty Directories

 # find /path/to/search -type d -exec rmdir --ignore-fail-on-non-empty {} + ;

Tar files changed today

 # tar -cvzf ../files_created_today.tgz `find /path/to/search -type f -daystart -ctime -1`

Find files bigger than X Size

 # find /path/to/search -size +

Example

 # find ./ -size +100M
 ./madflojo/Downloads/ubuntu-12.04-server-amd64.iso

Using Regex with find

 # find /path/to/search -regex 'regex pattern (full path)'

Example

 # find /var -regex '.*/tmp/.*[0-9]*.file'
 /var/tmp/testing/2/914.file

Ubuntu下的中文輸入(ibus)

System Settings > Language Support > Keyboard input method system > ibus

If there is no keyboard icon is shown in the system tray, do "ibus-daemon -x -r -d"

Click on this keyboard icon > Preferences > Input Method > Check Customize active input methods ...

Key to toggle the input method : CTRL + SPACE.
Key to select: ALT +
Key to select the highlighted term: SPACE