I’m using Linux shell (Bash) on daily basis, but I often forgot some useful command or shell tip. Yes, I can remember commands, but I can’t say that if I used it just once for specific task. Then I started to write Linux shell tips in text file on my Dropbox account and now I decided to share that. This list will be updated over time. Also keep in mind that for some tips you will need to install additional software on your Linux distribution.
UPDATE: November 25, 2013
Check if remote port is open with bash:
echo >/dev/tcp/8.8.8.8/53 && echo "open"
Suspend process: 
Ctrl + z
Move process to foreground:
fg
Generate random hex number where n is number of characters:
openssl rand -hex n
Execute commands from a file in the current shell:
source /home/user/file.name
Substring for first 5 characters:
${variable:0:5}
SSH debug mode:
ssh -vvv user@ip_address
SSH with pem key:
ssh user@ip_address -i key.pem
Get complete directory listing to local directory with wget:
wget -r --no-parent --reject "index.html*" http://hostname/ -P /home/user/dirs
Create multiple directories:
mkdir -p /home/user/{test,test1,test2}
List processes tree with child processes:
ps axwef
Create war file:
jar -cvf name.war file
Test disk write speed:
dd if=/dev/zero of=/tmp/output.img bs=8k count=256k conv=fdatasync; rm -rf /tmp/output.img
Test disk read speed:
hdparm -Tt /dev/sda
Get md5 hash from text:
echo -n "text" | md5sum
Check xml syntax:
xmllint --noout file.xml
Extract tar.gz in new directory:
tar zxvf package.tar.gz -C new_dir
Get HTTP headers with curl:
curl -I http://www.example.com
Modify timestamp of some file or directory (YYMMDDhhmm):
touch -t 0712250000 file
Download from ftp using wget:
wget -m ftp://username:password@hostname
Generate random password (16 char long in this case):
LANG=c < /dev/urandom tr -dc _A-Z-a-z-0-9 | head -c${1:-16};echo;
Quickly create a backup of a file:
cp some_file_name{,.bkp}
Access Windows share:
smbclient -U "DOMAIN\user" //dc.domain.com/share/test/dir
Run command from history (here at line 100):
!100
Unzip to directory:
unzip package_name.zip -d dir_name
Multiline text (CTRL + d to exit):
cat > test.txt
Create empty file or empty existing one:
> test.txt
Update date from Ubuntu NTP server:
ntpdate ntp.ubuntu.com
netstat show all tcp4 listening ports:
netstat -lnt4 | awk '{print $4}' | cut -f2 -d: | grep -o '[0-9]*'
Convert image from qcow2 to raw:
qemu-img convert -f qcow2 -O raw precise-server-cloudimg-amd64-disk1.img \
                                 precise-server-cloudimg-amd64-disk1.raw
Run command repeatedly, displaying it's output (default every two seconds):
watch ps -ef
List all users:
getent passwd
Mount root in read/write mode:
mount -o remount,rw /
Mount a directory (for cases when symlinking will not work):
mount --bind /source /destination
Send dynamic update to DNS server:
nsupdate < <EOF
update add $HOST 86400 A $IP
send
EOF
Recursively grep all directories:
grep -r "some_text" /path/to/dir
List ten largest open files:
lsof / | awk '{ if($7 > 1048576) print $7/1048576 "MB "$9 }' | sort -n -u | tail
Show free RAM in MB:
free -m | grep cache | awk '/[0-9]/{ print $4" MB" }'
Open Vim and jump to end of file:
vim + some_file_name
Git clone specific branch (master):
git clone git@github.com:name/app.git -b master
Git switch to another branch (develop):
git checkout develop
Git delete branch (myfeature):
git branch -d myfeature
Git delete remote branch:
git push origin :branchName
Git push new branch to remote:
git push -u origin mynewfeature
Print out the last cat command from history:
!cat:p
Run your last cat command from history:
!cat
Find all empty subdirectories in /home/user:
find /home/user -maxdepth 1 -type d -empty
Get all from line 50 to 60 in test.txt:
< test.txt sed -n '50,60p'
Run last command (if it was: mkdir /root/test, below will run: sudo mkdir /root/test):
sudo !!
Create temporary RAM filesystem - ramdisk (first create /tmpram directory):
mount -t tmpfs tmpfs /tmpram -o size=512m
Grep whole words:
grep -w "name" test.txt
Append text to a file that requires raised privileges:
echo "some text" | sudo tee -a /path/file
List all supported kill signals:
kill -l
Generate random password (16 characters long in this case):
openssl rand -base64 16
Do not log last session in bash history:
kill -9 $$
Scan network to find open port:
nmap -p 8081 172.20.0.0/16
Set git email:
git config --global user.email "me@example.com"
To sync with master if you have unpublished commits:
git pull --rebase origin master
Move all files with "txt" in name to /home/user:
find -iname "*txt*" -exec mv -v {} /home/user \;
Put the file lines side by side:
paste test.txt test1.txt
Progress bar in shell:
pv data.log
Send the data to server with 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
Go to the previous working directory:
cd -
Split large tar.gz archive (100MB each) and put it back:
split –b 100m /path/to/large/archive /path/to/output/files
cat files* > archive
Get HTTP status code with curl:
curl -sL -w "%{http_code}\\n" www.example.com -o /dev/null
When Ctrl + c not works:
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 with tabs indentation:
find . -type f -exec egrep -l $'\t' "{}" \;
Print horizontal line with "=":
printf '%100s\n' | tr ' ' =