How to display processes in Linux in a tree-like fashion

The simplest way to see operating system processes and their sub-processes is to use

ps aufx

Output will look something like the following, where I have used the Oracle agent and its child proceesses to illustrate the formatted process tree:

oracle   33037  0.0  0.0 162892 15600 ?        S    Jan29   0:06 /u01/oracle/product/agent13c/agent_13.2.0.0.0/perl/bin/perl /u01/oracle/product/agent13c/agent_13.2.0.
oracle   33127  0.3  0.9 2714968 313308 ?      Sl   Jan29   5:01  \_ /u01/oracle/product/agent13c/agent_13.2.0.0.0/oracle_common/jdk/bin/java -Xmx128M -XX:MaxPermSize=
oracle   48285  0.0  0.0 331652 23220 ?        S    15:41   0:00      \_ /u01/oracle/product/agent13c/agent_13.2.0.0.0/perl/bin/perl /u01/oracle/product/agent13c/agent

Alternatively, use pstree:

pstree -p
pstree -p 12345

where 12345 is the process id.

For example, the Oracle agent is started via a perl script, and spawns multiple java child processes:

 ps -ef |grep agent | grep perl
oracle    12345      1  0 08:53 ?        00:00:00 /sw/oracle/product/agent13c/GoldImage/agent_13.4.0.0.0/perl/bin/perl /sw/oracle/product/agent13c/GoldImage/agent_13.4.0.0.0/bin/emwd.pl agent /sw/oracle/product/agent13c/agent_inst/sysman/log/emagent.nohup

Use the process id as an argument to pstree to see all the child processes process 12345 has spawned (output abbreviated):

pstree -p 12345
perl(12345)───java(32690)─┬─{java}(32691)
                          ├─{java}(32692)