Linux Possible Issues, Solutions and Helper Commands

Provides common issues and solutions that may be encountered in Linux systems. Offers practical commands and solutions for topics such as system time settings, disk and memory controls, processor information, file operations, network configurations, and certificate management.

Other

To check system time with “date” command and fix if it’s in wrong timezone

sudo ln -sf /usr/share/zoneinfo/Europe/Istanbul /etc/localtime

To manually set date if it’s in correct timezone but still wrong

date -s '2014-12-25 12:34:56'

If Ubuntu no-key error is received

# W: GPG error: https://download.docker.com/linux/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 7EA0A9C3F273FCD8

To fix SSH working with dash instead of bash due to user definition

chsh -s /bin/bash $(whoami)

To see applications killed by OOM Killer

dmesg -T | egrep -i 'killed process'

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Disk Checks

To See Current Usage and Limits on All Disks

df -h

To See Current Usage and Limits at a Specific Address

du -sh <FILE_PATH>

Memory Checks

Memory Information

cat /proc/meminfo

To See Current Memory Amount and Limits

free -g

Processor Checks

CPU Information

less /proc/cpuinfo
lscpu

To See Only Processor Count

cat /proc/cpuinfo | grep processor | wc -l

To See Total Thread Count

ps -eo nlwp | tail -n +2 | awk '{ num_threads += $1 } END { print num_threads }'

cat /proc/sys/kernel/threads-max

To See Total Thread Count Belonging to Java Application

ps -ef | grep java
# Java's id is written below, example: 10057
ps huH p 10057 | wc -l

Taking dump

# jcmd -> when under JRE
sudo -u apinizer ./jcmd 26032 Thread.print > ./Thread.dump
sudo -u apinizer ./jcmd 26032 GC.heap_dump ./Heap.dump

Determining Process Count for Current User

ulimit -Su 30000

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Jar Operations

Extracting File from Jar

jar -xf apinizer-gateway-2.1.2.jar

Creating Jar from File

jar -cf apinizer-gateway-2.1.2.jar **

Creating Jar from File with manifest File

/opt/apinizerManager/jre8162linux/bin/jar -cfm apinizer-gateway-2.1.2.jar ./META-INF/MANIFEST.MF **

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File Operations

Transferring Folder to Another Server with SCP

scp -r /opt/apinizerManager/* <REMOTE_SERVER_USER>@<REMOTE_SERVER_ID>:<PATH_TO_COPY_TO>

To Convert tar.gz File to zip

tar xzf jre282linux.tar.gz && zip jre282linux.zip $(tar tf jre282linux.tar.gz)

To See Open Files

lsof | wc -l

To See Settings Related to Files

cat /etc/sysctl.conf
# system-wide maximum number of files
cat /proc/sys/fs/file-max
cat /proc/sys/fs/file-nr

To See Top Ten File Descriptors with Most Open Files

cd /proc
for pid in [0-9]*
do
    echo "PID = $pid with $(ls /proc/$pid/fd/ | wc -l) file descriptors"
done | sort -rn -k5 | head | while read -r _ _ pid _ fdcount _
do
  command=$(ps -o cmd -p "$pid" -hc)
  printf "pid = %5d with %4d fds: %s\n" "$pid" "$fdcount" "$command"
done

Processes with Most Open Files

lsof -Fnst | awk '
    { field = substr($0,1,1); sub(/^./,""); }
    field == "p" { pid = $0; }
    field == "t" { if ($0 == "REG") size = 0; else next; }
    field == "s" { size = $0; }
    field == "n" && size != 0 { print size, $0; }
' | sort -k1n -u | tail -n42 | sed 's/^[0-9]* //'

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Firewall Operations

sudo iptables -A PREROUTING -t nat -i ens160 -p tcp --dport 443 -j REDIRECT --to-port 9443
iptables-save > /opt/apinizerManager/iptables.conf

# The following is written into startManager
iptables-restore < /opt/apinizerManager/iptables.conf

max_allowed_packet=500M
lower_case_table_names=1
innodb_log_file_size=2G

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Wget Operations

Downloading File from Google Drive with Wget

wget --load-cookies /tmp/cookies.txt "https://docs.google.com/uc?export=download&confirm=$(wget --quiet --save-cookies /tmp/cookies.txt --keep-session-cookies --no-check-certificate 'https://docs.google.com/uc?export=download&id=Adres_Unique_Idsi_yZ8JlXtWwdOQg_5h' -O- | sed -rn 's/.*confirm=([0-9A-Za-z_]+).*/\1\n/p')&id=Adres_Unique_Idsi_yZ8JlXtWwdOQg_5h" -O apinizer.zip && rm -rf /tmp/cookies.txt

Sorting by File Size

sudo lsof | grep REG | grep -v "stat: No such file or directory" | grep -v DEL | awk '{if ($NF=="(deleted)") {x=3;y=1} else {x=2;y=0}; {print $(NF-x) "  " $(NF-y) } }' | sort -n -u | numfmt --field=1 --to=iec

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Network Operations

To See Process or Service Using a Specific Port

netstat -ltnp | grep -w ':80'

Running Ports

ss -antp

Example output:

State Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN 0 128 0.0.0.0:22 0.0.0.0:* users:(("sshd",pid=734,fd=4))
ESTAB 0 0 173.249.11.199:22 3.83.11.233:48610 users:(("sshd",pid=44179,fd=5),("sshd",pid=44178,fd=5))

Other frequently used parameters:

• -t: Show only TCP sockets on Linux
• -u: Display only UDP sockets on Linux
• -l: Show listening sockets. For example, TCP port 22 is opened by SSHD server.
• -p: List process name that opened sockets
• -n: Don’t resolve service names i.e. don’t use DNS

---

Certificate Operations

Getting server certificate from Linux server

openssl s_client -showcerts -connect <URL>:443 </dev/null 2>/dev/null|openssl x509 -outform PEM > mycertfile.pem

Certificate import to jar file application

/opt/apinizerManager/jre8282linux/jre/bin/keytool -import -trustcacerts -keystore /opt/apinizerManager/jre8282linux/jre/lib/security/cacerts -storepass changeit -alias <CERTIFICATE_ALIAS> -import -file <CERTIFICATE_FILE>

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Vi Editor Operations

To write all commands related to Vi and vim, first press the escape key to enter command mode and most should start with : (colon) character.

To exit without saving edits

:q!

To exit by saving edits

:wq

Replace All

:%s/old_word/new_word/g

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CURL Operations

Parameters:

• -k: To go to SSL addresses without verification
• -v: For log
• -m 10: For timeout
• -u <USERNAME>: To send username and prompt for password
• --output response.txt: To save output

Sending SOAP request and logging response

curl -D - --header "Content-Type:text/xml;charset=UTF-8" --header "SOAPAction:XXX" -d '<SOAP_BODY_GOES_HERE>' https://api.adres/apigateway/relativepath >> response.txt

Accessing Basic auth WSDL address (will ask for password) and saving it

curl -u username "http://api.address.com?wsdl" --output response.wsdl