Replace values in fifth column

Okay, unfortunally, sed doesn't know that 10 is John. So if you don't want to write another substitution for each user, you might want to use a second file users.txt with a translation table like this:

10 John
20 Robert
30 Jennifer
40 Mary

Then you feed both files into this sed command:

sed '/^[0-9]* [[:alnum:]]*$/{H;d;};G;s/ \([0-9]*\)\n.*\n\1 \([[:alnum:]]*\)/ \2/;P;d' users.txt yourfile.log

The translation table is read first and stored into the hold space. For the other files, the hold space with the translations is appended and the replacement is performed if possible. Then P prints the line only to the first line break, so no mess is printed if a user ID is unknown.

If you are interested in a more detailed explanation, see How to perform replacements defined in one file on another file or feel free to ask.

Two more awk solutions.

$ awk '{$5="user"substr($5,1,1)}1' file
Name 1:10:34 date short_id user1
Name 1:10:45 date short_id user1
Name 1:20:54 date short_id user2
Name 1:30:43 date short_id user3
Name 1:40:43 date short_id user4
Name 1:40:13 date short_id user4
Name 1:20:01 date short_id user2
Name 1:10:01 date short_id user1
$

$ awk '{$5="user"$5/10}1' x1
Name 1:10:34 date short_id user1
Name 1:10:45 date short_id user1
Name 1:20:54 date short_id user2
Name 1:30:43 date short_id user3
Name 1:40:43 date short_id user4
Name 1:40:13 date short_id user4
Name 1:20:01 date short_id user2
Name 1:10:01 date short_id user1
$

awk 'gsub(/0/, "", $5) { print $1, $2, $3, $4, "user" $5 }' INPUT

gsub removes number "0" in the 5th column. Then awk prints column 1-4, and combines word "user" with 5th column (there is no "," between "user" and $5).

Just use awk:

$ awk '!($NF in users){ users[$NF]="user"(++cnt) } { $NF=users[$NF] } 1' file
Name 1:10:34 date short_id user1
Name 1:10:45 date short_id user1
Name 1:20:54 date short_id user2
Name 1:30:43 date short_id user3
Name 1:40:43 date short_id user4
Name 1:40:13 date short_id user4
Name 1:20:01 date short_id user2
Name 1:10:01 date short_id user1

The above assumes you want each user to get a unique ID based on the order they appear in the input. If that's not what you need then edit your question to clarify your requirements.

Using sed

sed 's/\(.*\) \(.\).*/\1 user\2/' input_file
Name 1:10:34 date short_id user1
Name 1:10:45 date short_id user1
Name 1:20:54 date short_id user2
Name 1:30:43 date short_id user3
Name 1:40:43 date short_id user4
Name 1:40:13 date short_id user4
Name 1:20:01 date short_id user2
Name 1:10:01 date short_id user1

If all you want to change is the last column you don't need the g modifier for global and you will want to add the $ anchor to limit it to the last column.

I.e.: sed 's/\b10$/user1/'

That is assuming you'll want to change only the 10.

If the 10, 20 etc. match directly to user 1,2 etc. something like the following may work:

sed 's/\b\([1-4]\)0$/user\1/'

Using Raku (formerly known as Perl_6)

raku -ne 'given .words -> $w {put "$w.[0..3] ", (S/ (\d+) /user{$0.substr(0,1)}/ with $w.[4]) };'

OR

raku -ne 'put .[0..3], " ", (S/ (\d+) /user{$0.substr(0,1)}/ with .[4]) given .words;' 

The Raku code here breaks lines into whitespace-separated words. The first 4 columns (index .[0..3]) are output. Then digits in the 5th column are matched, and substitute with the word user followed by the first digit.

[A nicety of the with .[4] clause in the column 5 substitution is that it tolerates missing values in that column].

Sample Input:

Name 1:10:34 date short_id 10 
Name 1:10:45 date short_id 10
Name 1:20:54 date short_id 20
Name 1:30:43 date short_id 30
Name 1:40:43 date short_id 40
Name 1:40:13 date short_id 40
Name 1:20:01 date short_id 20
Name 1:10:01 date short_id 10

Sample Output:

Name 1:10:34 date short_id user1
Name 1:10:45 date short_id user1
Name 1:20:54 date short_id user2
Name 1:30:43 date short_id user3
Name 1:40:43 date short_id user4
Name 1:40:13 date short_id user4
Name 1:20:01 date short_id user2
Name 1:10:01 date short_id user1

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