339 lines
8.8 KiB
Markdown
339 lines
8.8 KiB
Markdown
---
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type: reference
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---
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# Linux Cheat Sheet
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This is the GitLab Support Team's collection of information regarding Linux, that they
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sometimes use while troubleshooting. It is listed here for transparency,
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and it may be useful for users with experience with Linux. If you are currently
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having an issue with GitLab, you may want to check your [support options](https://about.gitlab.com/support/)
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first, before attempting to use this information.
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CAUTION: **CAUTION:**
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If you are administering GitLab you are expected to know these commands for your distribution
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of choice. If you are a GitLab Support Engineer, consider this a cross-reference to
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translate `yum` -> `apt-get` and the like.
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Note: **Note:**
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Most of the commands below have not been labeled as to which distribution they work
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on. Contributions are welcome to help add them.
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## System Commands
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### Distro Information
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```shell
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# Debian/Ubuntu
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uname -a
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lsb_release -a
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# CentOS/RedHat
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cat /etc/centos-release
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cat /etc/redhat-release
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# This will provide a lot more information
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cat /etc/os-release
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```
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### Shut down or Reboot
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```shell
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shutdown -h now
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reboot
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```
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### Permissions
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```shell
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# change the user:group ownership of a file/dir
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chown root:git <file_or_dir>
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# make a file executable
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chmod u+x <file>
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```
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### Files & Dirs
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```shell
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# create a new directory and all subdirectories
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mkdir -p dir/dir2/dir3
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# Send a command's output to file.txt, no STDOUT
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ls > file.txt
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# Send a command's output to file.txt AND see it in STDOUT
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ls | tee /tmp/file.txt
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# Search and Replace within a file
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sed -i 's/original-text/new-text/g' <filename>
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```
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### See all set environment variables
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```shell
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env
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```
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## Searching
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### File names
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```shell
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# search for a file in a filesystem
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find . -name 'filename.rb' -print
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# locate a file
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locate <filename>
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# see command history
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history
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# search CLI history
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<ctrl>-R
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```
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### File contents
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```shell
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# -B/A = show 2 lines before/after search_term
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grep -B 2 -A 2 search_term <filename>
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# -<number> shows both before and after
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grep -2 search_term <filename>
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# Search on all files in directory (recursively)
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grep -r search_term <directory>
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# search through *.gz files is the same except with zgrep
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zgrep search_term <filename>
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# Fast grep printing lines containing a string pattern
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fgrep -R string_pattern <filename or directory>
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```
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### CLI
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```shell
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# View command history
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history
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# Run last command that started with 'his' (3 letters min)
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!his
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# Search through command history
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<ctrl>-R
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# Execute last command with sudo
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sudo !!
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```
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## Managing resources
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### Memory, Disk, & CPU usage
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```shell
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# disk space info. The '-h' gives the data in human-readable values
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df -h
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# size of each file/dir and its contents in the current dir
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du -hd 1
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# or alternative
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du -h --max-depth=1
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# find files greater than certain size(k, M, G) and list them in order
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# get rid of the + for exact, - for less than
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find / -type f -size +100M -print0 | xargs -0 du -hs | sort -h
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# Find free memory on a system
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free -m
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# Find what processes are using memory/CPU and organize by it
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# Load average is 1/CPU for 1, 5, and 15 minutes
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top -o %MEM
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top -o %CPU
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```
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### Strace
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```shell
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# strace a process
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strace -tt -T -f -y -s 1024 -p <pid>
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# -tt print timestamps with microsecond accuracy
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# -T print the time spent in each syscall
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# -f also trace any child processes that forked
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# -y print the path associated with file handles
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# -s max string length to print for an event
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# -o output file
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# run strace on all unicorn processes
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ps auwx | grep unicorn | awk '{ print " -p " $2}' | xargs strace -tt -T -f -y -s 1024 -o /tmp/unicorn.txt
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```
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See the [strace zine](https://wizardzines.com/zines/strace/) for a quick walkthrough.
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Brendan Gregg has a more detailed explanation of [how to use strace](http://www.brendangregg.com/blog/2014-05-11/strace-wow-much-syscall.html).
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Be aware that strace can have major impacts to system performance when it is running.
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### The Strace Parser tool
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Our [strace-parser tool](https://gitlab.com/wchandler/strace-parser) can be used to
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provide a high level summary of the `strace` output. It is similar to `strace -C`,
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but provides much more detailed statistics.
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MacOS and Linux binaries [are available](https://gitlab.com/gitlab-com/support/toolbox/strace-parser/-/tags),
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or you can build it from source if you have the Rust compiler.
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#### How to use the tool
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First run the tool with no arguments other than the strace output file name to get
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a summary of the top processes sorted by time spent actively performing tasks. You
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can also sort based on total time, # of syscalls made, PID #, and # of child processes
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using the `-S` or `--sort` flag. The number of results defaults to 25 processes, but
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can be changed using the `-c`/`--count` option. See `--help` for full details.
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```shell
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$ ./strace-parser strace.txt
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Top 25 PIDs
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-----------
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pid active (ms) wait (ms) total (ms) % active syscalls
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---------- ---------- --------- --------- --------- ---------
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8795 689.072 45773.832 46462.902 16.89% 23018
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13408 679.432 55910.891 56590.320 16.65% 28593
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6423 554.822 13175.485 13730.308 13.60% 13735
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...
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```
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Based on the summary, you can then view the details of syscalls made by one or more
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procsses using the `-p`/`--pid` for a specific process, or `-s`/`--stats` flags for
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a sorted list. `--stats` takes the same sorting and count options as summary.
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```shell
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$ ./strace-parse strace.text -p 6423
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PID 6423
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13735 syscalls, active time: 554.822ms, total time: 13730.308ms
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syscall count total max avg min errors
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(ms) (ms) (ms) (ms)
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--------------- -------- ---------- ---------- ---------- ---------- --------
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epoll_wait 628 13175.485 21.259 20.980 0.020
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clock_gettime 7326 199.500 0.249 0.027 0.013
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stat 2101 110.768 19.056 0.053 0.017 ENOENT: 2076
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...
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---------------
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Parent PID: 495
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Child PIDs: 8383, 8418, 8419, 8420, 8421
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Slowest file access times for PID 6423:
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open (ms) timestamp error file name
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----------- --------------- --------------- ----------
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29.818 10:53:11.528954 /srv/gitlab-data/builds/2018_08/6174/954448.log
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12.309 10:53:46.708274 /srv/gitlab-data/builds/2018_08/5342/954186.log
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0.039 10:53:49.222110 /opt/gitlab/embedded/service/gitlab-rails/app/views/events/event/_note.html.haml
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0.035 10:53:49.125115 /opt/gitlab/embedded/service/gitlab-rails/app/views/events/event/_push.html.haml
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...
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```
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In the example above, we can see that file opening times on `/srv/gitlab-data` are
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extremely slow, about 100X slower than `/opt/gitlab`.
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When nothing stands out in the results, a good way to get more context is to run `strace`
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on your own GitLab instance while performing the action performed by the customer,
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then compare summaries of both results and dive into the differences.
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#### Stats for the open syscall
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Rough numbers for calls to `open` and `openat` (used to access files) on various configurations.
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Slow storage can cause the dreaded `DeadlineExceeded` error in Gitaly.
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Also [see this entry](../operations/filesystem_benchmarking.md)
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in the handbook for quick tests customers can perform to check their filesystem performance.
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Keep in mind that timing information from `strace` is often somewhat inaccurate, so
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small differences should not be considered significant.
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|Setup | access times |
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|:--------------|:--------------|
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| EFS | 10 - 30ms |
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| Local Storage | 0.01 - 1ms |
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## Networking
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### Ports
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```shell
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# Find the programs that are listening on ports
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netstat -plnt
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ss -plnt
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lsof -i -P | grep <port>
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```
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### Internet/DNS
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```shell
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# Show domain IP address
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dig +short example.com
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nslookup example.com
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# Check DNS using specific nameserver
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# 8.8.8.8 = google, 1.1.1.1 = cloudflare, 208.67.222.222 = opendns
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dig @8.8.8.8 example.com
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nslookup example.com 1.1.1.1
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# Find host provider
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whois <ip_address> | grep -i "orgname\|netname"
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# Curl headers with redirect
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curl --head --location https://example.com
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```
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## Package Management
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```shell
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# Debian/Ubuntu
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# List packages
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dpkg -l
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apt list --installed
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# Find an installed package
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dpkg -l | grep <package>
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apt list --installed | grep <package>
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# Install a package
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dpkg -i <package_name>.deb
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apt-get install <package>
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apt install <package>
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# CentOS/RedHat
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# Install a package
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yum install <package>
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dnf install <package> # RHEL/CentOS 8+
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rpm -ivh <package_name>.rpm
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# Find an installed package
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rpm -qa | grep <package>
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```
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## Logs
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```shell
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# Print last lines in log file where 'n'
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# is the number of lines to print
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tail -n /path/to/log/file
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```
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