--- stage: Enablement group: Distribution info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments type: reference --- # Linux Cheat Sheet This is the GitLab Support Team's collection of information regarding Linux, that they sometimes use while troubleshooting. It is listed here for transparency, and it may be useful for users with experience with Linux. If you are currently having an issue with GitLab, you may want to check your [support options](https://about.gitlab.com/support/) first, before attempting to use this information. WARNING: If you are administering GitLab you are expected to know these commands for your distribution of choice. If you are a GitLab Support Engineer, consider this a cross-reference to translate `yum` -> `apt-get` and the like. Most of the commands below have not been labeled as to which distribution they work on. Contributions are welcome to help add them. ## System Commands ### Distribution Information ```shell # Debian/Ubuntu uname -a lsb_release -a # CentOS/RedHat cat /etc/centos-release cat /etc/redhat-release # This will provide a lot more information cat /etc/os-release ``` ### Shut down or Reboot ```shell shutdown -h now reboot ``` ### Permissions ```shell # change the user:group ownership of a file/dir chown root:git # make a file executable chmod u+x ``` ### Files and directories ```shell # create a new directory and all subdirectories mkdir -p dir/dir2/dir3 # Send a command's output to file.txt, no STDOUT ls > file.txt # Send a command's output to file.txt AND see it in STDOUT ls | tee /tmp/file.txt # Search and Replace within a file sed -i 's/original-text/new-text/g' ``` ### See all set environment variables ```shell env ``` ## Searching ### File names ```shell # search for a file in a filesystem find . -name 'filename.rb' -print # locate a file locate # see command history history # search CLI history -R ``` ### File contents ```shell # -B/A = show 2 lines before/after search_term grep -B 2 -A 2 search_term # - shows both before and after grep -2 search_term # Search on all files in directory (recursively) grep -r search_term # search through *.gz files is the same except with zgrep zgrep search_term # Fast grep printing lines containing a string pattern fgrep -R string_pattern ``` ### CLI ```shell # View command history history # Run last command that started with 'his' (3 letters min) !his # Search through command history -R # Execute last command with sudo sudo !! ``` ## Managing resources ### Memory, Disk, & CPU usage ```shell # disk space info. The '-h' gives the data in human-readable values df -h # size of each file/dir and its contents in the current dir du -hd 1 # or alternative du -h --max-depth=1 # find files greater than certain size(k, M, G) and list them in order # get rid of the + for exact, - for less than find / -type f -size +100M -print0 | xargs -0 du -hs | sort -h # Find free memory on a system free -m # Find what processes are using memory/CPU and organize by it # Load average is 1/CPU for 1, 5, and 15 minutes top -o %MEM top -o %CPU ``` ### Strace ```shell # strace a process strace -tt -T -f -y -yy -s 1024 -p # -tt print timestamps with microsecond accuracy # -T print the time spent in each syscall # -f also trace any child processes that forked # -y print the path associated with file handles # -yy print socket and device file handle details # -s max string length to print for an event # -o output file # run strace on all unicorn processes ps auwx | grep unicorn | awk '{ print " -p " $2}' | xargs strace -tt -T -f -y -yy -s 1024 -o /tmp/unicorn.txt ``` Be aware that strace can have major impacts to system performance when it is running. #### Strace Resources - See the [strace zine](https://wizardzines.com/zines/strace/) for a quick walkthrough. - 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). - We have a [series of GitLab Unfiltered videos](https://www.youtube.com/playlist?list=PL05JrBw4t0KoC7cIkoAFcRhr4gsVesekg) on using strace to understand GitLab. ### The Strace Parser tool Our [strace-parser tool](https://gitlab.com/wchandler/strace-parser) can be used to provide a high level summary of the `strace` output. It is similar to `strace -C`, but provides much more detailed statistics. MacOS and Linux binaries [are available](https://gitlab.com/gitlab-com/support/toolbox/strace-parser/-/tags), or you can build it from source if you have the Rust compiler. #### How to use the tool First run the tool with no arguments other than the strace output filename to get a summary of the top processes sorted by time spent actively performing tasks. You can also sort based on total time, # of system calls made, PID #, and # of child processes using the `-S` or `--sort` flag. The number of results defaults to 25 processes, but can be changed using the `-c`/`--count` option. See `--help` for full details. ```shell $ ./strace-parser strace.txt Top 25 PIDs ----------- pid active (ms) wait (ms) total (ms) % active syscalls ---------- ---------- --------- --------- --------- --------- 8795 689.072 45773.832 46462.902 16.89% 23018 13408 679.432 55910.891 56590.320 16.65% 28593 6423 554.822 13175.485 13730.308 13.60% 13735 ... ``` Based on the summary, you can then view the details of system calls made by one or more processes using the `-p`/`--pid` for a specific process, or `-s`/`--stats` flags for a sorted list. `--stats` takes the same sorting and count options as summary. ```shell $ ./strace-parse strace.text -p 6423 PID 6423 13735 syscalls, active time: 554.822ms, total time: 13730.308ms syscall count total max avg min errors (ms) (ms) (ms) (ms) --------------- -------- ---------- ---------- ---------- ---------- -------- epoll_wait 628 13175.485 21.259 20.980 0.020 clock_gettime 7326 199.500 0.249 0.027 0.013 stat 2101 110.768 19.056 0.053 0.017 ENOENT: 2076 ... --------------- Parent PID: 495 Child PIDs: 8383, 8418, 8419, 8420, 8421 Slowest file access times for PID 6423: open (ms) timestamp error file name ----------- --------------- --------------- ---------- 29.818 10:53:11.528954 /srv/gitlab-data/builds/2018_08/6174/954448.log 12.309 10:53:46.708274 /srv/gitlab-data/builds/2018_08/5342/954186.log 0.039 10:53:49.222110 /opt/gitlab/embedded/service/gitlab-rails/app/views/events/event/_note.html.haml 0.035 10:53:49.125115 /opt/gitlab/embedded/service/gitlab-rails/app/views/events/event/_push.html.haml ... ``` In the example above, we can see that file opening times on `/srv/gitlab-data` are extremely slow, about 100X slower than `/opt/gitlab`. When nothing stands out in the results, a good way to get more context is to run `strace` on your own GitLab instance while performing the action performed by the customer, then compare summaries of both results and dive into the differences. #### Stats for the open syscall Rough numbers for calls to `open` and `openat` (used to access files) on various configurations. Slow storage can cause the dreaded `DeadlineExceeded` error in Gitaly. Also [see this entry](../operations/filesystem_benchmarking.md) in the handbook for quick tests customers can perform to check their file system performance. Keep in mind that timing information from `strace` is often somewhat inaccurate, so small differences should not be considered significant. |Setup | access times | |:--------------|:--------------| | EFS | 10 - 30ms | | Local Storage | 0.01 - 1ms | ## Networking ### Ports ```shell # Find the programs that are listening on ports netstat -plnt ss -plnt lsof -i -P | grep ``` ### Internet/DNS ```shell # Show domain IP address dig +short example.com nslookup example.com # Check DNS using specific nameserver # 8.8.8.8 = google, 1.1.1.1 = cloudflare, 208.67.222.222 = opendns dig @8.8.8.8 example.com nslookup example.com 1.1.1.1 # Find host provider whois | grep -i "orgname\|netname" # Curl headers with redirect curl --head --location "https://example.com" # Test if a host is reachable on the network. `ping6` works on IPv6 networks. ping example.com # Show the route taken to a host. `traceroute6` works on IPv6 networks. traceroute example.com mtr example.com # List details of network interfaces ip address # Check local DNS settings cat /etc/hosts cat /etc/resolv.conf systemd-resolve --status # Capture traffic to/from a host sudo tcpdump host www.example.com ``` ## Package Management ```shell # Debian/Ubuntu # List packages dpkg -l apt list --installed # Find an installed package dpkg -l | grep apt list --installed | grep # Install a package dpkg -i .deb apt-get install apt install # CentOS/RedHat # Install a package yum install dnf install # RHEL/CentOS 8+ rpm -ivh .rpm # Find an installed package rpm -qa | grep ``` ## Logs ```shell # Print last lines in log file where 'n' # is the number of lines to print tail -n /path/to/log/file ```