The other day I wanted to create a simple bash script that would receive the name of a process and print out its pid. I wanted to run ps and then use awk to filter out the correct process and print out its pid. So I created the following bash script (called getPIDOf):
#! /bin/bash
command=”ps -o pid,command -C $1″
for (( i=1; i<=$#; i+=1 )); do
curval=${!i}
command=”$command | grep \”$curval\”"
done
command=”$command | awk ‘!/awk/ && !/getPIDOf/ {print \$1}’”echo $command
$command
I was quite pleased with myself that I had even added in the option to specify multiple identifiers for the process. But when I ran the script I got the following output:
yuval@obt:$ ./getPIDOf nc
ps -o pid,command -C nc | grep “nc” | awk ‘!/awk/ && !/getPIDOf/ {print $1}’
ERROR: Garbage option.
********* simple selection ********* ********* selection by list *********
-A all processes -C by command name
-N negate selection -G by real group ID (supports names)
-a all w/ tty except session leaders -U by real user ID (supports names)
-d all except session leaders -g by session OR by effective group name
-e all processes -p by process ID
T all processes on this terminal -s processes in the sessions given
a all w/ tty, including other users -t by tty
g OBSOLETE — DO NOT USE -u by effective user ID (supports names)
r only running processes U processes for specified users
x processes w/o controlling ttys t by tty
*********** output format ********** *********** long options ***********
-o,o user-defined -f full –Group –User –pid –cols –ppid
-j,j job control s signal –group –user –sid –rows –info
-O,O preloaded -o v virtual memory –cumulative –format –deselect
-l,l long u user-oriented –sort –tty –forest –version
-F extra full X registers –heading –no-heading –context
********* misc options *********
-V,V show version L list format codes f ASCII art forest
-m,m,-L,-T,H threads S children in sum -y change -l format
-M,Z security data c true command name -c scheduling class
-w,w wide output n numeric WCHAN,UID -H process hierarchy
I thought that I must have something wrong with the escaping. But when I ran the command that was printed out, it worked fine:
yuval@obt:$ ps -o pid,command -C nc | grep “nc” | awk ‘!/awk/ && !/getPIDOf/ {print $1}’
892
I even tried the following simplified script:
#! /bin/bash
command=”ps -o pid,command -C $1″
command2=”awk ‘!/awk/ && !/getPIDOf/ {print \$1}’”echo “$command | $command2″
$command | $command2
And got the following output:
yuval@obt:$ ./getPIDOf nc
ps -o pid,command -C nc | awk ‘!/awk/ && !/getPIDOf/ {print $1}’
awk: 1: unexpected character ”’
By this time I was quite puzzled. My simple getPIDOf script had become full of strange behaviors. So I decided to take the quick route out and use Perl instead of awk.
Here is my final version of the getPIDOf script:
#!/usr/bin/perl -w
use strict;die “Usage: $0 <identifier1> [identifier2] …\n” unless (@ARGV);
my $command = “/bin/ps -o pid,command -C \”$ARGV[0]\”";
my $output = `$command`;
foreach my $line (split(/\n/, $output))
{
if ($line =~ /^\s*(\d+)\s+(.+?)$/)
{
my $pid = $1;
my $command = $2;
my $found_mismatch = 0;
foreach my $filter (@ARGV)
{
if ($command !~ /$filter/)
{
$found_mismatch = 1;
last;
}
}if ($found_mismatch){next;
}print “$pid\n”;
}
}
And the output, as desired, is:
yuval@obt:$ ./getPIDOf nc
892
