Bash Output Grouping output differ on similar installs.

[faramirza]

Using Centos 7.8 I group `head -n1` and `sort` to keep the header from the output and still get a sorted output. As long as the user is not root the sort works as expected.

Example 1:

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# ps -ef | { head -n 1 ; sort ; } | head -n4
UID        PID  PPID  C STIME TTY          TIME CMD
0:00 /usr/sbin/smartd -n -q never
apache   12937  1019  0 Oct11 ?        00:00:00 /usr/sbin/httpd -DFOREGROUND
apache   12938  1019  0 Oct11 ?        00:00:01 /usr/sbin/httpd -DFOREGROUND

As you can see the line following the header is missing entries from the UID to the TTY

When I grep for the cut off entry I can see all the entries as expected:

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# ps -ef | grep smartd
root       575     1  0 Aug26 ?        00:00:00 /usr/sbin/smartd -n -q never
root     26011  1326  0 17:36 pts/0    00:00:00 grep --color=auto smartd

When I run the same on another Centos 7.8 install or a bit later on the same host I get the expected output as seen in Example 2.

Example 2:

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# ps -ef | { head -n 1 ; sort ; } | head -n4
UID        PID  PPID  C STIME TTY          TIME CMD
apache   10248 30267  0 17:05 ?        00:00:01 /usr/local/Striata/site/w/ap/apollofcgi
apache   19126 30267  0 17:18 ?        00:00:00 /usr/local/Striata/site/w/ap/apollofcgi
apache   19169 30267  0 16:31 ?        00:00:03 /usr/local/Striata/site/w/ap/apollofcgi

Anyone any ideas why the output differ on 2 very similar CentOS 7.8 installs?

[jlehtone]

Example 1:

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# ps -ef | { head -n 1 ; sort ; } | head -n4

I'm not familiar with list like that.

Should it output the header line and then the rest sorted, like:

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ps -ef | head -n 1 ; ps -ef --no-headers | sort | head -n 3

Or, with ps' own sort:

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ps -ef --sort=euser | head -n 4

[faramirza]

Hi,

Correct. Output Grouping will run the two consecutive commands on the data available. It should be best to do it the way you suggested but it looks like there is a race condition and therefore I'm thinking it is a minor bug.

It seems to be an issue when the process was owned by root. root would be cut off but for the Apache user everything was kept in place.

[pjsr2]

Look at the option -o of the ps command and use that to format the output of ps. It lets you select the output values to show and whether or not to display a header line for the columns in the output.