Wait until md5sum command is completed

Question

#!/bin/bash

cd /path-to-directory
md5=$(find . -type f -exec md5sum {} \; | sort -k 2 | md5sum)

zenity --info \
--title= "Calculated checksum" \
--text= "$md5"

The process of a recursive checksum calculation for a directory takes a while. The bash script doesn´t wait until the process is finished it just moves to the next command which is the dialog box, that displays the calculated checksum. So the dialog box shows a wrong checksum.

Is there an option to tell the script to wait until the calculation of the checksum is finished? Furthermore, is there an option to pipe the progress of the checksum calculation to some kind of progress bar like in zenity for example?

Answer 1

As written, it would be waited for. For a pulsating progress bar:

#! /bin/sh -
export LC_ALL=C
cd /path/to/dir || exit
{
  md5=$(
    find . -type f -print0 |
      sort -z |
      xargs -r0 md5sum |
      md5sum
  )
  exec >&-
  zenity --info \
         --title="Checksum" \
         --text="$md5"
} | zenity --progress \
           --auto-close \
           --auto-kill \
           --pulsate \
           --title="${0##*/}" \
           --text="Computing checksum"

For an actual progress bar, you'd need to know the number of files to process in advance.

With zsh:

#! /bin/zsh -
export LC_ALL=C
autoload zargs
cd /path/to/dir || exit
{
  files=(.//**/*(ND.))
} > >(
  zenity --progress \
         --auto-close \
         --auto-kill \
         --pulsate \
         --title=$0:t \
         --text="Finding files"
)
md5=(
  $(
   zargs $files -- md5sum \
      > >(
        awk -v total=$#files '/\/\// {print ++n * 100 / total}' | {
          zenity --progress \
            --auto-close \
            --title=$0:t \
            --text="Computing checksum" || kill -s PIPE $$
        }) \
      | md5sum
  )
)
zenity --info \
       --title=$0:t \
       --text="MD5 sum: $md5[1]"

Note that outside of the C locale, on GNU systems at least, filename order is not deterministic, as some characters sort the same and also filenames are not guaranteed to be made of valid text, hence the LC_ALL=C above.

The C locale order is also very simple (based on byte value) and consistent from system to system and version to version.

Beware that means that error messages if any will be displayed in English instead of the user's language (but then again the Computing checksum, Finding files, etc are not localised either so it's just as well).

Some other improvements over your approach:

• Using -exec md5sum {} + or -print0 | xargs -r0 md5sum (or zargs equivalent) minimises the number of md5sum invocations, each md5sum invocation being passed a number of files. -exec md5sum {} \; means running one md5sum per file which is very inefficient.
• we sort the list of files before passing to md5sum. Doing sort -k2 in general doesn't work as file names can contain newline characters. In general, it's wrong to process file paths line-based. You'll notice we use a .// prefix in the zsh approach for awk to be able to count files reliable. Some md5sum implementations also have a -z option for NUL-delimited records.

Answer 2

That code will wait for the find and md5sum commands to finish. That's just the normal behavior, unless you have a & to send the commands to the background.

However, your zenity command is malformed: you can't have a space after the =. So I am guessing that you are seeing an empty zenity window and that's why you think it isn't waiting. Try again, but remove the spaces:

#!/bin/bash

cd /path-to-directory
md5=$(find . -type f -exec md5sum {} \; | sort -k 2 | md5sum)

zenity --info \
--title="Calculated checksum" \
--text="$md5"

You can also avoid the need to cd and make it a bit more concise if you do:

#!/bin/bash

zenity --info \
--title="Calculated checksum" \
--text="$(find /path-to-directory -type f -exec md5sum {} \; | sort -k 2 | md5sum)"

If you need this to always return the same result, no matter what the parent path of the directory is, you can use this to remove the file names from the output of the find ... md5sum command before passing to the second md5sum:

#!/bin/bash

zenity --info \
--title="Calculated checksum" \
--text="$(find /path-to-directory -type f -exec md5sum {} \; | cut -d ' ' -f1 | sort -k 2 | md5sum)"