Tag Archives: shell

Write commands with multiple lines in Dockerfile

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?

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

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 "[email protected]"; else test 1; fi
}

error() {
  echo "[email protected]" >&2
  exit 1
}
warning() {
  echo "[email protected]" >&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

Invisible characters mess js and css up

Some invisible characters may cause program fault, and it’s hard to locate.

For example UTF-8 BOM in the middle of css file may interrupt the css parser, you can find it by
find . -name '*.js' -type f -print0 | xargs -0 grep -r $'\xEF\xBB\xBF'

UTF-8 Line Separator may cause the debugger misunderstand the right line, you can find it by
find . -name '*.js' -type f -print0 | xargs -0 grep -r $'\xe2\x80\xa8'

And you also can cat to display all non-printing characters
BSD cat
cat -evt file
GNU cat
cat -A file

Or sed (better for multi-byte characters)
sed -n "l" file