How to get all running process ids only?
Use ps Output Formatting:
ps -A -o pid
Output formatting of the command is the best option. The o option controls the output formatting. I listed some of arguments below below, see 'man ps' for the rest ( to use multiple it would be -o pid,cmd,flags
).
KEY LONG DESCRIPTION
c cmd simple name of executable
C pcpu cpu utilization
f flags flags as in long format F field
g pgrp process group ID
G tpgid controlling tty process group ID
j cutime cumulative user time
J cstime cumulative system time
k utime user time
o session session ID
p pid process ID
Awk or Cut Would be Better to get Columns:
Generally you wouldn't want a regex for selecting the first column, you would want to pipe it to cut or awk to cut out the first column like:
ps ax | awk '{print $1}'
Regex is an Option, if not the best:
If you were to use regex, it could be something like:
ps ax | perl -nle 'print $1 if /^ *([0-9]+)/'
$1 prints only what was matched in the parenthesis. ^ anchors the to the start of the line. Space asterisk means allow for optional space characters before the number. [0-9]+ means one or more digits. But I wouldn't recommend regex for this particular task, see why? :-)
ps ax | awk '{ print $1; }'
Use the -o switch to have a cust format output
ps -o pid
The bad way using sed, as you explicitly asked may be
ps -ax | sed 's#^\( *[0-9]\+\) .*$#\1#'