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

Removing alpha channel of pngs using Imagemagick

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Apple has released new version of iTunes Connect and you would get an error message when try to send images on iTunes Connect for apps.

It would tell you that “Invalid Icon” balabala “xxx.png with an alpha channel. Icons should not have an alpha channel.” and so on.

You can check your pngs to see whether it has an alpha channel.

find . -type f -name "*.png" -exec identify -format '"%d/%f" %[channels]\n' {} \;

And you can remove their alpha channel to make apple happier.

find . -type f -name "*.png" -exec mogrify -background white -alpha off -flatten {} \;

You’d better run it in the specific directory so that you can limit the affect.

That’s all.

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