Month: December 2007

[problem]

You want to ascertain how long your script takes to run, displaying thousands of seconds and including CPU utilization.

[/problem]

[solution]

Just use the time command.

Really simple and an excellent way to see how long your scripts take. Plus showing the anticipated load on the system.

[/solution]

[example]

[marcus@bree marcus]$ time ps
PID TTY TIME CMD
27973 pts/0 00:00:00 bash
28120 pts/0 00:00:00 ps

real 0m0.025s
user 0m0.010s
sys 0m0.020s

[/example]

[reference]

[tags], Unix Coding School[/tags]

• Linux Man Pages – time command
• Linux Man Pages – ps command

[/reference]

[problem]

You want to display processes, commands within the process tree.

[/problem]

[solution]

There are a number of ways to search the process tree.

Here are a few:

Show all processes for user apache.


ps -fu apache


Show all process on a given terminal (try command tty to see your own)


ps -ftpts/0


[/solution]

[example]

Show a given process


ps -fpPID


[/example]

[reference]

[tags], Unix Coding School[/tags]

• Linux Man Pages – ps command

[/reference]

[problem]

You want to create and use an UNIX FIFO.

[/problem]

[solution]

FIFO means first in, first out.

To create a FIFO (also called a named pipe), UNIX supplies the mknod command.

[/solution]

[example]

This is the code to generate a FIFO:


$ mknod myFIFO p


Then the resulting named pipe:


$ ls -ld myFIFO
prw-r--r-- 1 marcus adm 0 Jun 13 21:15 myFIFO


To use the FIFO, you need to have a process that is reading from it. This could be a bit of Perl, C code, etc.

A perfect way to demonstrate using FIFOs, is tail -f:


$ tail -f myFIFO


Then a test:


$ echo "
Hello there
This is a test of FIFOs
" > myFIFO


From the tail -f when now see following output:

Hello there
This is a test of FIFOs

[/example]

[reference]

– as always an excellent description on wikipedia

[/reference]

[reference]

[tags]FIFO, Named Pipes, mknod, Unix Coding School[/tags]

• Linux Man Pages – mknod command
• Linux Man Pages – tail command
• Linux Man Pages – base command

[/reference]

[problem]

You need to find a pattern in a file. Using regular expression.

[/problem]

[solution]

Normal command to display something in a file, is grep.


grep "pattern" filename


[/solution]

[example]

You can also display the line number it occured on (-n) and ignore case (-i):


grep -n -i "pattern" filename


Also you can match more than one pattern:


egrep "patternA|patternB" filename


To match the beginning of a line (^) or the end of line ($) – this will only match lines with this specific pattern on:


grep "^patternA$" filename


We can also use wildcards and ranges – this will match any line with PATTERN in it followed by any amount of numbers:


grep "^.*PATTERN[0-9]*$" filename


[/example]

[reference]

[tags]grep, regular expression, Unix Coding School[/tags]

• Linux Man Pages – grep command

[/reference]

[problem]

You want to extract a specific line number from a file.

[/problem]

[solution]

Once you have your line number – see previous post on grep, we can extract lines around the pattern.

To do this sed (stream editor) can be used to print just desired lines – this says don’t print all lines (-n); start at line 456 and finish at line 466 – print:


sed -n 456,466p filename


[/solution]

[example]

Also with sed, we can say delete specific lines – in this case remove lines 5 to 10:


sed 5,10d filename


That’s not all sed can accept patterns, as start/end identifiers:


sed /start_pattern/,/end_pattern/d filename


[/example]

[reference]

[tags], Unix Coding School[/tags]

• Linux Man Pages – sed command

[/reference]

[problem]

Following on from the last post on removing lines with sed, how do we use sed to substitute output on the fly?

[/problem]

[solution]

This says substitute occurences of PATT with REPLACE global (or all occurences).


sed 's/PATT/REPLACE/g' filename


[/solution]

[example]

So to replace all occurences of Unix with UNIX:


sed 's/Unix/UNIX/g' filename


We can also replace patterns, with variable values, like this:


sed "s/PATT/${THEVAR}/g" filename


[/example]

[reference]

[tags]SED, Unix Coding School[/tags]

• Linux Man Pages – sed command

[/reference]

[problem]

Needing to view current activity of users on the system.

[/problem]

[solution]

Most flavours of UNIX provide a number of tools, to tell you who is logged in or when they last accessed the system.

[/solution]

[example]

A favorite of mine, is who -Hu – really good for seeing who is idle, quite literally! 🙂

Also you can just run last – although beware of rolled logs. Generally in /var/adm/wtmp* on Solaris and /var/log/wtmp* on Linux.

If you want to run last against an older file (by default it just uses wtmpx or wtmp) – just specify minus f – like this: last -f /var/log/wtmp.1

Additionally the finger command will quite often show last login time.

[/example]

[reference]

[tags]who, last, finger, Unix Coding School[/tags]

• Linux Man Pages – who command
• Linux Man Pages – last command
• Linux Man Pages – finger command

[/reference]

[problem]

You want to allow a user to run one command, as root user.

[/problem]

[solution]

To allow a user access to run stuff as root, just login as root and then add the following line to /etc/sudoers.


userid hostname=command


[/solution]

[example]

For example to allow user marcus access on bree, to restart, stop, start or check apache:


marcus bree=/etc/init.d/httpd


To allow all access on all boxes:


marcus (ALL)=(ALL)


[/example]

[reference]

[tags]sudo, switch user do, temporary UNIX authorisation, Unix Coding School[/tags]

• Linux Man Pages – sudo command

[/reference]

[problem]

Want to kill all processes, for a specific user.

[/problem]

[solution]

This command will send a -KILL signal, equivalent to minus 9:


pkill -KILL -U username


[/solution]

[example]

So to kill all processes owned by username marcus:


pkill -KILL -U marcus


If you issue kill -l – that is kill minus L, it will show all the symbol to numeric codes. Although you can just use numeric with pkill:


pkill -9 -U marcus


[/example]

[reference]

[tags]pkill, proc kill, Unix Coding School[/tags]

• Linux Man Pages – pkill command

[/reference]