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 for users with experience with Linux. If you are currently having an issue with GitLab, you may want to check your support options first, before attempting to use this information.
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
# 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
shutdown -h now
reboot
Permissions
# change the user:group ownership of a file/dir
chown root:git <file_or_dir>
# make a file executable
chmod u+x <file>
Files and directories
# 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' <filename>
See all set environment variables
env
Searching
Filenames
# search for a file in a filesystem
find . -name 'filename.rb' -print
# locate a file
locate <filename>
# see command history
history
# search CLI history
<ctrl>-R
File contents
# -B/A = show 2 lines before/after search_term
grep -B 2 -A 2 search_term <filename>
# -<number> shows both before and after
grep -2 search_term <filename>
# Search on all files in directory (recursively)
grep -r search_term <directory>
# Grep namespace/project/name of a GitLab repository
grep 'fullpath' /var/opt/gitlab/git-data/repositories/@hashed/<repo hash>/.git/config
# search through *.gz files is the same except with zgrep
zgrep search_term <filename>
# Fast grep printing lines containing a string pattern
fgrep -R string_pattern <filename or directory>
CLI
# View command history
history
# Run last command that started with 'his' (3 letters min)
!his
# Search through command history
<ctrl>-R
# Execute last command with sudo
sudo !!
Managing resources
Memory, Disk, & CPU usage
# 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
# strace a process
strace -tt -T -f -y -yy -s 1024 -p <pid>
# -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 puma processes
ps auwx | grep puma | awk '{ print " -p " $2}' | xargs strace -tt -T -f -y -yy -s 1024 -o /tmp/puma.txt
Be aware that strace can have major impacts to system performance when it is running.
Strace Resources
- See the strace zine for a quick walkthrough.
- Brendan Gregg has a more detailed explanation of how to use strace.
- We have a series of GitLab Unfiltered videos on using strace to understand GitLab.
The Strace Parser tool
Our strace-parser tool 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, or you can build it from source if you have the Rust compiler.
How to use the tool
First run the tool with summary flag 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.
$ ./strace-parser sidekiq_trace.txt summary -c15 -s=pid
Top 15 PIDs by PID #
-----------
pid actv (ms) wait (ms) user (ms) total (ms) % of actv syscalls children
------- ---------- ---------- ---------- ---------- --------- --------- ---------
16706 0.000 0.000 0.000 0.000 0.00% 0 0
16708 0.000 0.000 0.000 0.000 0.00% 0 0
16716 0.000 0.000 0.000 0.000 0.00% 0 0
16717 0.000 0.000 0.000 0.000 0.00% 0 0
16718 0.000 0.000 0.000 0.000 0.00% 0 0
16719 0.000 0.000 0.000 0.000 0.00% 0 0
16720 0.389 9796.434 1.090 9797.912 0.02% 16 0
16721 0.000 0.000 0.000 0.000 0.00% 0 0
16722 0.000 0.000 0.000 0.000 0.00% 0 0
16723 0.000 0.000 0.000 0.000 0.00% 0 0
16804 0.218 11099.535 1.881 11101.634 0.01% 36 0
16813 0.000 0.000 0.000 0.000 0.00% 0 0
16814 1.740 11825.640 4.616 11831.996 0.10% 57 0
16815 2.364 12039.993 7.669 12050.026 0.14% 80 0
16816 0.000 0.000 0.000 0.000 0.00% 0 0
PIDs 93
real 0m12.287s
user 0m1.474s
sys 0m1.686s
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.
./strace-parser sidekiq_trace.txt p 16815
PID 16815
80 syscalls, active time: 2.364ms, user time: 7.669ms, total time: 12050.026ms
start time: 22:46:14.830267 end time: 22:46:26.880293
syscall count total (ms) max (ms) avg (ms) min (ms) errors
----------------- -------- ---------- ---------- ---------- ---------- --------
futex 5 10100.229 5400.106 2020.046 0.022 ETIMEDOUT: 2
restart_syscall 1 1939.764 1939.764 1939.764 1939.764 ETIMEDOUT: 1
getpid 33 1.020 0.046 0.031 0.018
clock_gettime 14 0.420 0.038 0.030 0.021
stat 6 0.277 0.072 0.046 0.031
read 6 0.170 0.036 0.028 0.020
openat 3 0.126 0.045 0.042 0.038
close 3 0.099 0.034 0.033 0.031
lseek 3 0.089 0.035 0.030 0.021
ioctl 3 0.082 0.033 0.027 0.023 ENOTTY: 3
fstat 3 0.081 0.034 0.027 0.022
---------------
Slowest file open times for PID 16815:
dur (ms) timestamp error filename
---------- --------------- --------------- ---------
0.045 22:46:16.771318 - /opt/gitlab/embedded/service/gitlab-rails/config/database.yml
0.043 22:46:26.877954 - /opt/gitlab/embedded/service/gitlab-rails/config/database.yml
0.038 22:46:22.174610 - /opt/gitlab/embedded/service/gitlab-rails/config/database.yml
In the example above, we can see which files took longer to open for PID 16815.
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 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 - 30 ms |
| Local Storage | 0.01 - 1 ms |
Networking
Ports
# Find the programs that are listening on ports
netstat -plnt
ss -plnt
lsof -i -P | grep <port>
Internet/DNS
# 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 <ip_address> | 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
# Debian/Ubuntu
# List packages
dpkg -l
apt list --installed
# Find an installed package
dpkg -l | grep <package>
apt list --installed | grep <package>
# Install a package
dpkg -i <package_name>.deb
apt-get install <package>
apt install <package>
# CentOS/RedHat
# Install a package
yum install <package>
dnf install <package> # RHEL/CentOS 8+
rpm -ivh <package_name>.rpm
# Find an installed package
rpm -qa | grep <package>
Logs
# Print last lines in log file where 'n'
# is the number of lines to print
tail -n /path/to/log/file