Category Archives: Coding

Write commands with multiple lines in Dockerfile

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There are several ways to write commands with multiple lines in Dockerfile, for example you wanna echo a bash file entrypoint.sh with content:

#!/bin/bash
echo 3
echo 2
echo 1
echo run

You could:

Using printf

RUN printf '#!/bin/bash\necho 3\necho 2\necho 1\necho run' > /entrypoint.sh

Using cat

RUN sh -c "$(/bin/echo -e "cat > /entrypoint.sh <<EOF\
\n#!/bin/bash\
\necho 3\
\necho 2\
\necho 1\
\necho run\
\nEOF\n")"

Using echo -e

RUN echo -e " #!/bin/bash\n\
echo 3\n\
echo 2\n\
echo 1\n\
echo run" > /entrypoint.sh

Using $'...'

The $’…’ feature is known as "ANSI-C quoting" but it’s not a POSIX shell > feature. According to unix.stackexchange.com/a/371873/109111 it was > originally a ksh93 feature but it is now available in bash, zsh, mksh, > FreeBSD sh and in busybox’s ash

RUN echo $'#!/bin/bash\n\
echo 3\n\
echo 2\n\
echo 1\n\
echo run' > /entrypoint.sh

echo -e & $'...' are both similar in that they support the following escape sequences:

\a     alert (bell)
\b     backspace
\e
\E     an escape character
\f     form feed
\n     new line
\r     carriage return
\t     horizontal tab
\v     vertical tab
\\     backslash
\0nnn  the eight-bit character whose value is the octal value nnn (zero to three octal digits)
\xHH   the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
     the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)

They do have differences. In addition to the above, echo -e supports:

\c     suppress further output
\0nnn  the eight-bit character whose value is the octal value nnn (zero to three octal digits)

By contrast, $'....' supports:

 \'     single quote
 \"     double quote
 \nnn   the eight-bit character whose value is the octal value nnn (one to three digits)
 \cx    a control-x character

How to bash multithread?

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I do think using xargs to run code multithread in bash is a better way, you still could use fifo. Just like the sample below: 

#!/bin/bash
function a_sub
{
    sleep 3
}
tmp_fifofile="/tmp/$$.fifo"
mkfifo $tmp_fifofile
exec 6<>$tmp_fifofile

rm $tmp_fifofile

thread=15

for((i=0;i<$thread;i++));do
    echo
done >&6

for((i=0;i<50;i++));do
    read -u 6
    {
        a_sub && {
            echo "a_sub is finished"
        } || {
            echo "sub error"
        }
        echo >&6
    }&
done

exec 6>&-

wait

Useful bash functions

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Get bash file absolute path

realpath(){
  path="$1"
  while [ -h "$path" ] ; do path="$(readlink "$path")"; done
  echo "$(cd "$(dirname "$path")"; echo -n "$(pwd)/$(basename "$path")")";
}

Log

log() {
  if [[ -n "$VERBOSE" ]]; then echo -e "$@"; else test 1; fi
}

error() {
  echo "$@" >&2
  exit 1
}
warning() {
  echo "$@" >&2
}

function check_status {
  if [ $? -ne 0 ];then
    error ${@:-"Encountered an error, aborting!"}
  fi
}

Loop find

find . -type f -iname "*.txt" -print0 | while IFS= read -r -d $'\0' line; do
  echo "$line"
  ls -l "$line"
done

Process Substitution <(LIST) >(LIST)

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Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files. It takes the form of

<(list)
or
>(list)

The process list is run with its input or output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an argument to the current command as the result of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list. Note that no space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection.

For example:
$ cat < <(ls)
$ (echo "YES")> >(read str; echo "1:${str}:first";)> >(read sstr; echo "2:$sstr:two")> >(read ssstr; echo "3:$ssstr:three")

See also:

Bash Reference Manual
Bash Hackers Wiki

Weighted Round Robin In Nodejs

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Round-robin (RR) is one of the algorithms employed by schedulers in computing. Jobs are assigned to each worker in circular order, It’s simple but useful.

I have a web server writing by node just for front-end display, APIs are most holding by back-end servers. So I have 2 functions supporting my structure. The first is health check helping me check whether the back-end server is ready; and the other one is round robin scheduling, to ensure an even distribution.
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XBase64

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Basics

xbase64 is a fast javascript base64 encoder and decoder;

Is it the fastest?

As far as I know it’s fast enough. Although it depends on far too many variables to enumerate, I wrote a test, the result below is for reference.

Tell me if you know a faster one. Continue reading →

Webp-test

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Webp-test is a javascript tool to check whether the browser support webp.

Why Webp-test?

WebP is an image format that employs both lossy and lossless compression. The degree of compression is adjustable so a user can choose the trade-off between file size and image quality. WebP typically achieves an average of 39% more compression than JPEG and JPEG 2000, without loss of image quality.
But a few browsers support WebP right now (as of April 2010, Google Chrome 9+ and Opera 11.10 beta), how to use it?
Now you can use webptest to test browser whether it supports.
Continue reading →