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Useful commands⚓︎

Kill unresponsive process⚓︎

An unresponsive processe/service ignores the kill command. In order to shut down the process immediately use

kill -9 PID

where PID is the process ID.

This is equivalent to

kill -SIGKILL PID

The kill -SIGKILL command bypasses the standard shutdown routine and unsaved data will be lost.

Note

kill sends the SIGTERM signal by default, which allows the process to terminate gracefully.

find <DIR> -xtype l -delete

Sycalls ID to name mapping⚓︎

The following command shows the mapping of syscall IDs to their names:

ausyscall --dump
Installation

ausyscall is part of the audit package.

sudo apt install auditd

Tracing system calls⚓︎

strace allows to trace system calls and signals made by a process.

By default strace does not follow child processes, therefore the -f is often useful.

strace -f -o <STRACE_OUTPUT_FILE> <COMMAND>

-e can be used to filter the traced syscalls: -e trace=process traces only process-related syscalls.

strace -f -e trace=process -s 1024 -o <STRACE_OUTPUT_FILE> <COMMAND>

The -s option sets the maximum string size, which by default is relatively small.

Tip

strace-process-tree allows to visualize the strace -f output as a process tree.

strace-process-tree output 
$ strace-process-tree /tmp/trace.out
25510 make binary-package
  ├─25511 /bin/sh -c 'dpkg-parsechangelog | awk '\''$1 == "Source:" { print $2 }'\'''
  │   ├─25512 dpkg-parsechangelog
  │   │   └─25514 tail -n 40 debian/changelog
  │   └─25513 awk '$1 == "Source:" { print $2 }'
  ├─25515 /bin/sh -c 'dpkg-parsechangelog | awk '\''$1 == "Version:" { print $2 }'\'''
  │   ├─25516 dpkg-parsechangelog
  │   │   └─25518 tail -n 40 debian/changelog
  │   └─25517 awk '$1 == "Version:" { print $2 }'
  ├─25519 /bin/sh -c 'dpkg-parsechangelog | grep ^Date: | cut -d: -f 2- | date --date="$(cat)" +%Y-%m-%d'
  │   ├─25520 dpkg-parsechangelog
  │   │   └─25525 tail -n 40 debian/changelog
  │   ├─25521 grep ^Date:
  │   ├─25522 cut -d: -f 2-
  │   └─25523 date --date=" Thu, 18 Jan 2018 23:39:51 +0200" +%Y-%m-%d
  │       └─25524 cat
  └─25526 /bin/sh -c 'dpkg-parsechangelog | awk '\''$1 == "Distribution:" { print $2 }'\'''
      ├─25527 dpkg-parsechangelog
      │   └─25529 tail -n 40 debian/changelog
      └─25528 awk '$1 == "Distribution:" { print $2 }'

git-like patches⚓︎

The diff command can be used to create git-like patches between two files or directories:

diff -uNr <OLD> <NEW> > changes.patch

This can be useful to generate patches for Spack.

bwrap⚓︎

bwrap (bubblewrap) allows to substitute a directory with another, which can be useful to test changes without modifying the original files.

bwrap --dev-bind / / --bind <NEW_DIR> <OLD_DIR> <COMMAND>

This is expecially useful to patch system files without modifying the original ones, or temporarily modify Spack-installed files without modifying the Spack installation.

Warning

--dev-bind / / allows the process to access the entire filesystem. This is necessary for commands to work properly, especially the shell.

Withou --dev-bind / /, one would get errors like the following:

bwrap: execvp bash: No such file or directory
Patch Spack-installed CUDA

On Ubuntu 25.10, glibc’s <math.h> declares rsqrt with noexcept(true), but CUDA’s headers declare it without noexcept.

To test a patch that adds noexcept to CUDA’s headers, one can use bwrap to substitute the original CUDA headers with modified the original Spack-installed ones.

cp $(spack location cuda)/include/crt/* ~/tmp/cuda-include/crt
# Modify the files in ~/tmp/cuda-include/crt/, such as math_functions.h

bwrap --dev-bind / / --bind ~/tmp/cuda-include/crt $(spack location cuda)/include/crt -- zsh
# The shell will have the modified files in place of the original ones
Modifying a squashfs image

To modify a squashfs image that has to be mounted, one can use bwrap to substitute the original image with a modified one.

# Extract the original image
unsquashfs <IMAGE> -d <DIRECTORY>

# Modify the files in <DIRECTORY> as needed

# Mount the modified image using bwrap
bwrap --dev-bind / / --bind <DIRECTORY> <DIRECTORY> -- bash

This avoids the need to repack the modified directory into a new squashfs image, which can be time-consuming.

flock⚓︎

flock allows to acquire a lock on a file. It is useful when multiple processes might be writing to it

flock myfile.txt -c 'echo "Hello, World!" > myfile.txt'