Linux/Unix Command Line Tools

cloud-image: set initial password

NOTE: img can be a qcow2:

$ sudo virt-customize -a ubuntu-16.04-server-cloudimg-amd64-disk1.img --root-password password:linux

Or also:

# guestfish --rw -a <qcow2 image file name>
><fs> run
><fs> list-filesystems
><fs> mount /dev/vda1 /
><fs> vi /etc/shadow

#NOTE: REMOVE the `!!` from the file

><fs> umount /
><fs> exit

Or also:

# modprobe nbd max_part=8 &&
# qemu-nbd -c /dev/nbd0 image.qcow2
# mount /dev/nbd0p1 /mnt
# chroot a sh -c "echo 'root:password' | chpasswd"
# umount /mnt
# qemu-nbd -d /dev/nbd0

nohup: keep a command running even when the terminal is closed

$ nohup <command> <command options> &

Avoid generating nohup.log:

$ nohup <command> <command options> >dev/null &

sed

Delete lines

With a given pattern (/pattern/d):

$ sed "/pattern/d" file.txt

In a given range (i,jd):

$ sed "2,14d" file.txt

Delete empty lines:

$ sed -e '/^$/d'

or, with awk:

$ awk ''

equivalent to:

$ awk '$0'

equivalent to:

$ awk '$0 != "" {print $0;}'

Print only a given pattern found in a given line

Make sed print only relevant pattern. E.g. only print the "..." part in the lines having the format "... bar". Do not print lines not having this pattern.

sed -n 's/ \+bar//p'

Join lines

sed -z 's/\n//g'

ss: socket statistics

$ ss --tcp
$ ss --udp
$ ss -plunt

start-stop-daemon: daemonize a process

On regular Linux:

$ start-stop-daemon --start --oknodo --startas /root/usb/coredns --  -conf /root/usb/Corefile -pidfile /var/run/coredns.pid

On OpenWRT:

$ start-stop-daemon -S -b -x /root/usb/coredns --  -conf /root/usb/Corefile -pidfile /var/run/coredns.pid

Enable DPMS

$ xset +dpms

Force DPMS on

$ xset dpms force on

rcconf

rcconf: configure startup services on Debian

find

Execute a command for each result of find:

Either use ';' or \;:

$ find /path -exec cmd1 '{}' ';'

Multiple commands are possible:

$ find /path -exec cmd1 '{}' ';' -exec cmd2 '{}' ';'

Create a RAM disk

mount -t tmpfs -o size=512m tmpfs /mnt/ramdisk

Specify a match group with grep

-P: Perl-style regex
-o: only print the match:
(regext) \K: reset the starting point of the reported match. Any previously consumed characters are no longer included in the final match.

E.g.: make grep print ONLY the next word after 'foo', and nothing else:

$ grep -P -o 'foo \K[^ ]+'

Or, used sed -n 's///p' ...

mtr - my tracerouter

mtr combines the functionality of the traceroute and ping programs in a single network diagnostic tool.

openssl cheats

See a certificate content:

$ openssl x509 -text -noout -in /PATH/TO/THE/CERT.crt

copy ALL files from a directory

Use the -T option.

This copies all files in /source-dir into /dest-dir.

$ cp -aT /source-dir /dest-dir

SSH

SSH port forwarding

Use ssh -L local-port:host-addr:host-port -N [-f].

-L local-port:host:host-port: Forward the local-port TCP connection to the given remote host:host-port. This makes ssh listen locally local-port
-N: do not execute a command (because of -L)
-f: go in background

SSH VNC tunnel

Start the VNC server remotely:

$ vncserver

Setup the SSH tunnel locally:

$ ssh -L 5901:127.0.0.1:5901 -N -f -p SSH-PORT -l pi example.com

Run the VNC client locally:

$ xtigervncviewer

Terminate stuck session without closing the terminal window

Use the escape sequence - input one char by one: Enter + ~ + .
If the SSH session is nested, add more ~. Eg. SSH in SSH: Enter + ~ + ~ + .

How to avoid storing key fingerprints of each host in `~/.ssh/known_hosts`

Host 10.* 192.168.* 172.*
    UserKnownHostsFile /dev/null
    StrictHostKeyChecking no
    LogLevel ERROR

NOTE: Host can be a single host, or a white space separated list. Use * to define patterns.

The line LogLevel ERROR suppresses the Warning: Permanently added ... warning message printed at every connection.

sudo

sudo does not forward some signals sent from the same process group. So, if you have a script does does this this this this:

sudo some-command &
pid=$!
sudo kill -TERM $pid

the command will never be SIGTERM-ed. A workaround is to:

setsid sudo kill -TERM $pid

Explanation is only found in the sudo source code:

/*
 * Do not forward signals sent by a process in the command's process
 * group, as we don't want the command to indirectly kill itself.
 * For example, this can happen with some versions of reboot that
 * call kill(-1, SIGTERM) to kill all other processes.
 */

tmux

Copy lines into buffer:

prefix + :, type capture-pane -S -<NUM OF LINES TO SAVE>

Save buffer to file:

prefix + :, type save-buffer filename.txt

Move pane to a new window

While on current pane: prefix :break-pane

update-alternatives

Basic usage

$ sudo update-alternatives --display vi
$ sudo update-alternatives --install /usr/bin/vi vi /usr/bin/nvim 99
$ sudo update-alternatives --install /usr/bin/vi vi /usr/bin/vim 100
$ sudo update-alternatives --config vi
$ sudo update-alternatives --remove-all vi

xset: configure display features

Query settings

$ xset q

xargs

Run a command per line using -I replstring. E.g.:

$ ls -1 | xargs -I file stat file

QEMU

Boot an img file:

qemu-system-x86_64 -enable-kvm -drive file=openwrt-23.05.3-x86-64-generic-ext4-combined-efi.img -m 2G -smp 4 -machine type=q35,accel=kvm -serial mon:stdio -nographic

Add networking via slirp:

-netdev user,id=qemu-net,net=192.168.1.0/24,dhcpstart=192.168.1.100 -device virtio-net-pci,netdev=qemu-net

NOTE - to make this work, remember that:

you need to setup the guest network i/f in DHCP mode

pinging from inside the guest wont work without this extra sysctl config done on the host:

Set what range of group ids is allowed to use ICMP sockets in /etc/sysctl.conf (make sure to reload with sysctl --load=/etc/sysctl.conf ...)

net.ipv4.ping_group_range = 0 1000

Add host ports forward:

Append to -netdev user,... the extra option hostfwd. Syntax is hostfwd=hostip:hostport-guestip:guestport.

E.g., to add SSH forward, use ,hostfwd=tcp::2222-:22. You can then SSH into the guest using ssh -p2222 root@localhost

inotifywatch, inotifywait

Detect when a given file is accessed on the system.

unshare

Create a new user namespace, map root user to current user:

unshare --user --map-auto --map-root-user

Linux/Unix Command Line Tools

cloud-image: set initial password

Meta

nohup: keep a command running even when the terminal is closed

sed

Delete lines

Print only a given pattern found in a given line

Join lines

ss: socket statistics

start-stop-daemon: daemonize a process

Enable DPMS

Force DPMS on

rcconf

find

Create a RAM disk

Specify a match group with grep

mtr - my tracerouter

openssl cheats

copy ALL files from a directory

SSH

SSH port forwarding

SSH VNC tunnel

Terminate stuck session without closing the terminal window

How to avoid storing key fingerprints of each host in `~/.ssh/known_hosts`

sudo

tmux

Copy lines into buffer:

Save buffer to file:

Move pane to a new window

update-alternatives

Basic usage

xset: configure display features

Query settings

xargs

QEMU

inotifywatch, inotifywait

unshare

Tech Notes

Table Of Contents

Related Topics

Linux/Unix Command Line Tools

cloud-image: set initial password

Meta

nohup: keep a command running even when the terminal is closed

sed

Delete lines

Print only a given pattern found in a given line

Join lines

ss: socket statistics

start-stop-daemon: daemonize a process

Enable DPMS

Force DPMS on

rcconf

find

Create a RAM disk

Specify a match group with grep

mtr - my tracerouter

openssl cheats

copy ALL files from a directory

SSH

SSH port forwarding

SSH VNC tunnel

Terminate stuck session without closing the terminal window

How to avoid storing key fingerprints of each host in ~/.ssh/known_hosts

sudo

tmux

Copy lines into buffer:

Save buffer to file:

Move pane to a new window

update-alternatives

Basic usage

xset: configure display features

Query settings

xargs

QEMU

inotifywatch, inotifywait

unshare

How to avoid storing key fingerprints of each host in `~/.ssh/known_hosts`