sed/Awk/cut... How to decide which to use to parse Docker output?

Your input seems to have a fixed width of 20 chars for each field, so you can make use of gawk's FIELDWIDTHS feature.

$ awk -v FIELDWIDTHS="20 20 20 20 20" '{ print $3 }' fileIMAGE ID806f56c844440da05d84b1fe$$ awk -v FIELDWIDTHS="20 20 20 20 20" '{ printf "%20s%20s\n", $1, $3 }' fileREPOSITORY          IMAGE IDjenkins/jenkins     806f56c84444mongo               0da05d84b1fe

From man gawk:

If the FIELDWIDTHS variable is set to a space-separated list of numbers, each field is expected to have fixed width, and gawk splits up the record using the specified widths. Each field width may optionally be preceded by a colon-separated value specifying the number of characters to skip before the field starts. The value of FS is ignored. Assigning a new value to FS or FPAT overrides the use of FIELDWIDTHS.

You have to "squeeze" the space padding in the default output to single space.

1 2 == 1-space-space-2 == Field 1 before 1st space, Field between 1st and 2nd space, Field 3 after 2nd space.

cut -d' ' -f1 ==> '1'

cut -d' ' -f2 ==> '' empty field between 1st and 2nd delimiter

cut -d' ' -f3 ==> '2'

So, in your case use sed to replace consecutive spaces with 1:

docker images | sed 's/ */ /g' | cut -d " " -f1,3

If the output is fixed columns widths, then you can use this variant of cut:

docker images | cut -c1-20,41-60

This will cut out columns 41 to 60, where we find the Image ID.

If ever the output uses TAB for padding, you should use expand -t n to make the output consistently space padded then apply the appropriate cut -cx,y, e.g. (numbers may need adjusting):

docker images | expand -t 4 | cut -c1-20,41-60

Try this:

docker images | tr -s ' ' | cut -f3 -d' '

The command tr -s ' ' convert multiple spaces into a single one and after with cut you can grab your field. This work fine if values in your field haven't spaces.