The ps command on a Debian system supports both BSD and SystemV features and helps to identify process activity statically.

Style

Style	Description
BSD	Supports BSD-style options and output.
System V	Supports SystemV-style options and output.

Typical Command

• ps aux
• ps -efH

Features

Feature	Description
Display %CPU %MEM	Displays the CPU usage and memory usage of processes.
Display PPID	Displays the parent process ID (PPID) of a process.

Table 9.10: List of ps Command Styles

For zombie (defunct) child processes, you can kill them by their parent process ID identified in the “PPID” field.

The top Command

The top command on a Debian system has rich features and helps to identify what process is acting funny dynamically. It is an interactive full-screen program. You can get its usage help by pressing the “h”-key and terminate it by pressing the “q”-key.

Listing Files Opened by a Process

You can list all files opened by a process with a process ID (PID), e.g., 1, using the following command:

$ sudo lsof -p 1

The PID=1 is usually the init program.

Tracing Program Activities

You can trace program activity with strace, ltrace, or xtrace for system calls and signals, library calls, or communication between X11 client and server. You can trace system calls of the ls command as follows:

$ sudo strace ls

Tip: Use the strace-graph script found in /usr/share/doc/strace/examples/ to make a nice tree view.

Identification of Processes using Files or Sockets

You can identify processes using files by fuser, e.g., for /var/log/mail.log:

$ sudo fuser -v /var/log/mail.log

This displays the process ID (PID) and command name of the process that has an open file descriptor to the specified file.

Repeating a Command with a Constant Interval

The watch command executes a program repeatedly with a constant interval while showing its output in full-screen mode. For example:

$ watch w

This displays who is logged on to the system updated every 2 seconds.

Repeating a Command Looping Over Files

There are several ways to repeat a command looping over files matching some condition, e.g., matching glob pattern “*.ext”:

• Shell for-loop method (see Section 12.1.4):

for x in *.ext; do if [ -f "$x" ]; then command "$x"; fi; done

• find and xargs combination:

find . -type f -maxdepth 1 -name '*.ext' -print0 | xargs -0 -n 1 command

• find with -exec option with a command:

find . -type f -maxdepth 1 -name '*.ext' -exec command {} \;

• find with -exec option with a short shell script:

find . -type f -maxdepth 1 -name '*.ext' -exec sh -c "command {} && echo 'successful'" \;

The above examples are written to ensure proper handling of funny file names such as ones containing spaces. See Section 10.1.5 for more advanced uses of find.