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Sed query

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# 1  
Old 02-11-2010
Sed query
Hello Im fairly familiar with using the sed command for substitution, however I have been passed a script which checks the logged on username and directory type with a sed section which I cant figure out. The sed function has me baffled and I cant find out from the man page what its trying to do with the output.

directoryType=`dscacheutil -cachedump -entries user | sed -n "/$username/{g;1!p;};h" | awk '{print $9}'`

Any pointers gratefully recieved.
# 2  
Old 02-21-2010
Hi.

It tries to match a username:
Code:
/$username/

If it matches, swaps the pattern space with the contents of the hold buffer, and prints it (if it's not the first line of the file, in which case the hold buffer would be empty):
Code:
{g;1!p;}

It then puts the current line (the pattern space) into the hold space:
Code:
/$username/{g;1!p;};h

Basically, it remembers the previous line, and prints that when username matches.

This is a bit long-winded, but you work through what the sed is doing:

Code:
$ dscacheutil -cachedump -entries user

1 Cache entries (ordered as stored in the cache):
2
3       Category         Best Before         Last Access      Hits    Refs       TTL    Neg  DS Node
4     ----------  ------------------  ------------------  --------  ------  --------  -----  ---------
5           User   02/21/10 20:45:00   02/21/10 19:56:00        96       1      3600         /Local/Default
6                      Key: pw_name:nobody
7
8           User   02/21/10 20:45:00   02/21/10 19:56:00        35       1      3600         /Local/Default
9                      Key: pw_name:scott
10
11          User   02/21/10 20:43:52   02/21/10 19:56:20        32       1      3600         /Local/Default
12                     Key: pw_uid:501
13
14          User   02/21/10 20:45:00   02/21/10 19:56:00       145       1      3600         /Local/Default

$ username=scott
$ dscacheutil -cachedump -entries user | sed -n "/$username/{g;1!p;};h" | awk '{print $9}'

Sed on Line:   1
Pattern space: Cache entries (ordered as stored in the cache):
Hold space:    <empty> (can't be any previous line in the hold buffer)

Sed on Line:   2
Pattern space: <empty> (line 2 is blank)
Hold space:    Cache entries (ordered as stored in the cache): (previous line)

Sed on Line:   3
Pattern space: Category         Best Before         Last Access      Hits    Refs       TTL    Neg  DS Node
Hold space:    <empty>

Sed on Line:   4
Pattern space: ----------  ------------------  ------------------  --------  ------  --------  -----  ---------
Hold space:    Category         Best Before         Last Access      Hits    Refs       TTL    Neg  DS Node

Sed on Line:   5
Pattern space: User   02/21/10 20:45:00   02/21/10 19:56:00        96       1      3600         /Local/Default
Hold space:    ----------  ------------------  ------------------  --------  ------  --------  -----  ---------

Sed on Line:   6
Pattern space: Key: pw_name:nobody
Hold space:    User   02/21/10 20:45:00   02/21/10 19:56:00        96       1      3600         /Local/Default

Sed on Line:   7
Pattern space: <blank>
Hold space:    Key: pw_name:nobody

Sed on Line:   8
Pattern space: User   02/21/10 20:45:00   02/21/10 19:56:00        35       1      3600         /Local/Default
Hold space:    <blank>

Sed on Line:   9 <<--- Match found, print hold space, to awk, print $9 - /Local/Default
Pattern space: Key: pw_name:scott
Hold space:    User   02/21/10 20:45:00   02/21/10 19:56:00        35       1      3600         /Local/Default

(the hold space for each line is the pattern space of the previous line)

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