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strace cheat sheet

The strace utility is very powerful to learn what a new or running process is doing. Due to its diversity of monitoring options, the tool is less accessible at first. This strace cheat sheet helps with getting the best out of this tool.

Normally cheat sheets come in a single 1 page PDF. In this case, we combined it all within a blog post. First section shows an explanation per area, the bottom of the post contains all useful commands for quick reference.

Troubleshooting with strace

One of options of the strace utility is to help as a troubleshooting utility. If you want to know what a process is doing, or why it hangs, strace will definitely help. By running strace without any parameters, it will already show why a process is doing. You can trace a running process, or instruct strace to start it for you.

screenshot of strace utility with -c parameter

All syscall listed by amount of time

Monitoring file activity

Strace can monitor file related activity. There are two useful parts. The first is file, which shows file interactions. The other one allows tracing file descriptors. Both can be used to monitor for actions like opening files, reading/writing and closing. Usually using “trace=file” provides enough insights. If you really need more insights in the way a program deals with file descriptors, then use the second one.

  • Monitor opening of files: strace -e open -p 1234
  • See all file activity: strace -e trace=file -p 1234 or strace -e trace=desc -p 1234

If you want to track specific paths, use 1 or more times the -P parameter, following by the path.

# strace -P /etc/cups -p 2261  
Process 2261 attached  
- SIGHUP {si_signo=SIGHUP, si_code=SI_USER, si_pid=6149, si_uid=0} -  
lstat("/etc/cups", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0  
getdents(7, /* 11 entries */, 32768) = 336  
getdents(7, /* 0 entries */, 32768) = 0  
close(7) = 0  
getdents(7, /* 11 entries */, 32768) = 336  
getdents(7, /* 0 entries */, 32768) = 0  
close(7) = 0

Common calls:

SyscallIntended goal
accessChecks whether the calling process can access the pathname, dereferenced when it is a symbolic link
closeClose file descriptor
fchmod Same as chmod, but used file by open file descriptor fd
fchown  Changes ownership of file, referred to by open file descriptor (fd)
fstat Similar to stat(), but uses file descriptor fd
lseek Reposition file offset for read/write
openOpens file specified by pathname
readRead from file descriptor
statfsReturns information about mounted file system

See the syscalls overview for others.

A related example screen output:

Screenshot of strace monitoring file access and activity

Monitoring file access and activity with strace

Monitoring the network

Strace definitely can be useful for revealing more details about network traffic. Very useful to determine what network related connections are used, like when building your Docker image.

strace -e trace=network

Common syscalls:

  • accept
  • bind
  • getsockopt
  • listen
  • socket
  • setsockopt

Monitoring memory calls

To get better insights on the memory usage and system calls, strace can monitor for these as well. They are nicely grouped in the memory group.

strace -e trace=memory

Common syscalls:

  • mmap
  • munmap

Strace Cheat Sheet - Overview

Useful options and examples

Strace optionAction performed
-cSee what time is spend and where (combine with -S for sorting)
-fTrack process including forked child processes
-o my-process-trace.txtLog strace output to a file
-p 1234Track a process by PID
-P /tmpTrack a process when interacting with a path
-TDisplay syscall duration in the output

Track by specific system call group

Strace syscall groupAction performed
-e trace=ipcTrack communication between processes (IPC)
-e trace=memoryTrack memory syscalls
-e trace=networkTrack network syscalls
-e trace=processTrack process calls (like fork, exec)
-e trace=signalTrack process signal handling (like HUP, exit)
-e trace=fileTrack file related syscalls

Want to trace multiple syscalls instead of a full group? Combine them by specifying them directly instead of the syscall group.

strace -e open,close

Got other clever stracing tips? Let it know!

Relevant articles using strace command

The following articles include an example on how to use strace and might be worth further exploring.

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    This article has been written by our Linux security expert Michael Boelen. With focus on creating high-quality articles and relevant examples, he wants to improve the field of Linux security. No more web full of copy-pasted blog posts.

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