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ptail

ptail (process tail) is a simple tool for viewing the stdout and stderr of a process. Think of it as tail -f for the output of other processes.

Usage

There are two ways to use ptail (similar to strace): ptail a new process (with a given program), or ptail an exiting process (with PID).

# new process
./zig-out/bin/ptail PROGRAM [ARGS...]
# exiting process
sudo ./zig-out/bin/ptail -p PID

When would it be useful?

  1. There are situations in which the stdout and stderr of a process are captured and not shown. For example, zig build test with .test_server_mode captures stdout and stderr of processes and does not show them until the end of tests. If a test is blocked, nothing is shown and you cannot do printf debugging. You can use the following command to see the output of the tests as they are running:
./zig-out/bin/ptail /usr/bin/bash -c 'zig build test'
  1. The process is running in the background and its stdout is either not written to disk, or you don't know where it is. For example, you can inspect what your language server (eg zls) is printing with the following command:
sudo ./zig-out/bin/ptail -p $(ps -C "zls" -o pid= | head -1)

Example

Monitor new process

./zig-out/bin/ptail /usr/bin/bash -c 'ping localhost > /dev/null'

Monitor existing process

# terminal 1
ping localhost > /dev/null
# terminal 2
sudo ./zig-out/bin/ptail -p $(ps -C "ping" -o pid= | head -1)
64 bytes from localhost (::1): icmp_seq=1 ttl=64 time=0.058 ms
64 bytes from localhost (::1): icmp_seq=2 ttl=64 time=0.067 ms
...

Asciinema

asciicast

Build

zig build
  • Requires Linux kernel 5.3 or later, but let me know if you need support for older kernels. We just need to bring back the code removed in commit/ceedab19 for systems where PTRACE_SYSCALL_INFO_EXIT is not available.
  • Tested with Zig version 0.12.0-dev.3496+a2df84d0f

Test

zig build test