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I regularly use the Linux shell (Bash), but I often forget some useful commands or shell tricks. Yes, I can remember some commands, but they definitely won’t be used only once for specific tasks, so I started writing down these little tricks for the Linux shell in a text file on my Dropbox account, and now I’ve decided to share them with you. I will update this list in the future. Remember, some of these tips require additional software to be installed on your Linux distribution.
Check if a remote port is open in bash:
echo >/dev/tcp/8.8.8.8/53 && echo “open”
Pause a process:
Ctrl + z
Bring a process to the foreground:
fg
(Note: A suspended process does not execute. If you want it to run in the background, you can use the bg command and specify the job number obtained from the jobs command.)
Generate a random hexadecimal number, n is the number of characters:
openssl rand -hex n
Execute commands from a file in the current shell (Note: this file is not a bash script, such as .bashrc, bash_profile, etc.):
source /home/user/file.name
Extract the first 5 characters of a string:
${variable:0:5}
Open SSH debug mode (Note: very useful when you encounter SSH connection issues):
ssh -vvv user@ip_address
SSH connection using a pem key:
ssh user@ip_address -i key.pem
Use wget to get a complete directory listing to a local directory:
wget -r –no-parent –reject “index.html*” http://hostname/ -P /home/user/dirs
Create multiple directories at once:
mkdir -p /home/user/{test,test1,test2}
List processes and subprocesses in a tree format:
ps axwef
Create a war file:
jar -cvf name.war file
Test disk write speed:
dd if=/dev/zero of=/tmp/output.img bs=8k count=256k conv=fdatasync; rm -rf /tmp/output.img
Test disk read speed:
hdparm -Tt /dev/sda
Get the md5 value of text:
echo -n “text” | md5sum
Check XML syntax:
xmllint –noout file.xml
Extract a tar.gz file to a specified directory:
tar zxvf package.tar.gz -C new_dir
Get HTTP headers using curl:
curl -I http://www.example.com
Modify the timestamp of some files or directories (format: YYMMDDhhmm):
touch -t 0712250000 file
Download from ftp using wget:
wget -m ftp://username:password@hostname
Generate a random password (16 characters long in this example):
LANG=c < /dev/urandom tr -dc _A-Z-a-z-0-9 | head -c${1:-16};echo;
Quickly create a backup of a file (extension is .bkp):
cp some_file_name{,.bkp}
Access Windows shares:
smbclient -U “DOMAINuser” //dc.domain.com/share/test/dir
Run a command from history (here it is the 100th command in history):
!100
Unzip to a directory:
unzip package_name.zip -d dir_name
Input multiple lines of text (press CTRL + d to exit):
cat > test.txt
Create an empty file or clear an existing file:
> test.txt
Update the date from the Ubuntu NTP server:
ntpdate ntp.ubuntu.com
netstat shows all listening TCP ports for IPv4:
netstat -lnt4 | awk ‘{print $4}’ | cut -f2 -d: | grep -o ‘[0-9]*’
Convert qcow2 image to raw format:
qemu–img convert –fqcow2 –Oraw precise–server–cloudimg–amd64–disk1.img
precise–server–cloudimg–amd64–disk1.raw
Repeat a command and display its output (default repeats every 2 seconds):
watch ps -ef
Display all users:
getent passwd
Remount the root filesystem in read-write mode:
mount -o remount,rw /
Mount a directory (suitable when symbolic links do not work):
mount –bind /source /destination
Send DNS dynamic updates to DNS:
nsupdate <
Recursive grep all directories:
grep -r “some_text” /path/to/dir
List the 10 largest open files in the system:
lsof / | awk ‘{ if($7 > 1048576) print $7/1048576 “MB “$9 }’ | sort -n -u | tail
Display free memory in MB:
free -m | grep cache | awk ‘/[0-9]/{ print $4″ MB” }’
Open vim and jump to the end of the file:
vim + some_file_name
Clone a specific branch in git (in this example, the master branch):
git clone [email protected]:name/app.git -b master
Switch to another branch in git (in this example, the develop branch):
git checkout develop
Delete a branch in git (in this example, myfeature):
git branch -d myfeature
Delete a remote branch in git:
git push origin :branchName
Push a new branch to remote in git:
git push -u origin mynewfeature
Print the last cat command in history:
!cat:p
Run the last cat command in history:
!cat
Find all empty subdirectories in /home/user:
find /home/user -maxdepth 1 -type d -empty
Get lines 50 to 60 of text from test.txt:
< test.txt sed -n ‘50,60p’
Re-run the last executed command with sudo privileges (if it was: mkdir /root/test, the next will run: sudo mkdir /root/test) (Note: When you forget to use sudo for a command, you can re-execute it this way without typing the full command again):
sudo !!
Create a temporary RAM filesystem – ramdisk (please create the /tmpram directory first):
mount -t tmpfs tmpfs /tmpram -o size=512m
Grep for complete words (Note: not parts of other words):
grep -w “name” test.txt
Append text to a file with elevated permissions:
echo “some text” | sudo tee -a /path/file
List all supported kill signals:
kill -l
Generate a random password (16 characters long in this example):
openssl rand -base64 16
Do not record the last session in bash history:
kill -9 $$
Scan the network to find open ports:
nmap -p 8081 172.20.0.0/16
Set git email:
git config –global user.email “[email protected]”
If you have uncommitted commits, sync with master:
git pull –rebase origin master
Move all files containing ‘txt’ in their names to /home/user:
find -iname “*txt*” -exec mv -v {} /home/user \;
Merge and display corresponding lines from two files line by line:
paste test.txt test1.txt
Progress bar in shell:
pv data.log
Send data to a server using netcat:
echo “hosts.sampleHost 10 `date +%s`” | nc 192.168.200.2 3000
Convert tabs to spaces:
expand test.txt > test1.txt
Skip bash history:
<<space>>cmd
Return to the previous working directory:
cd –
Split a large tar.gz file into several files (each 100MB), and restore:
split–b100m /path/to/large/archive /path/to/output/files
cat files* > archive
Get HTTP status value using curl:
curl -sL -w “%{http_code}\n” www.example.com -o /dev/null
When Ctrl + c does not work:
Ctrl + \
Get file owner:
stat -c %U file.txt
List block devices:
lsblk -f
Find files with trailing spaces:
find . -type f -exec egrep -l ” +$” “{}” \;
Find files indented with tabs:
find . -type f -exec egrep -l $’t’ “{}” \;
Print horizontal lines with “=”
printf ‘%100sn’ | tr ‘ ‘ =
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