Linux CLI Cheatsheet
Below are some short snippets of code to help you perform day-to-day tasks in Linux. If you can't find what you are looking for, make sure to look at the BASH cheatsheet.
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- Nmap Cheatsheet
JSON
Allo the following examples use jq which can be installed with sudo apt install jq.
If you have some command or script that outputs in JSON, then it may not be in a readable format. To have it output to the terminal in a readable format, just pipe it to jq like so:
. someScript.py | jq
If you then want that output to go to a file, do the following:
echo '[1,2,3,4,5,6,7,8,9,10]' | jq . > my-output-file.json
For more examples of using jq, refer to the following:
Misc
Set File Timestamp
We all know that you can use touch $FILENAME to update a file to say it was edited right now. However, if you want to manually set a time that is in the past, you can use:
touch -d '1 June 2018 11:02' $FILENAME
Here is a little php snippet for programmatically setting time of files:
<?php
$filename = "test-php-file.txt"; // specify name for file to create/edit timestmap of
$timestamp = time() - (60*60); // set a unix timestamp here.
$dateString = date("d F Y H:i:s", $timestamp);
$cmd = "touch -d '{$dateString}' {$filename}";
shell_exec($cmd);
Download File And Set Custom Name
The following command will use wget to download a file, but set your own custom name for the file when it is downloaded.
wget --output-document="my-custom-filename.zip" http://my.domain.com/path-to-file
Get Time Taken For Command To Run
Sometimes it's useful to know how long a command takes. All you have to do is wrap it like so:
CMD="YOUR COMMAND GOES HERE"
utime="$( TIMEFORMAT='%lU';time ( $CMD ) 2>&1 1>/dev/null )"
echo "$utime"
CentOS - Check If Reboot Required
#!/bin/bash
LAST_KERNEL=$(rpm -q --last kernel | perl -pe 's/^kernel-(\S+).*/$1/' | head -1)
CURRENT_KERNEL=$(uname -r)
test $LAST_KERNEL = $CURRENT_KERNEL || echo REBOOT
Get Ubuntu Release Name
lsb_release -cs
This should output something like:
xenial
Generate Random Password
This is my favourite option from How-To-Geek: 10 Ways to Generate a Random Password from the Command Line
openssl rand -base64 32
If you need it to not contain special characters, you could use:
head /dev/urandom | tr -dc A-Za-z0-9 | head -c$NUM_CHARACTERS
... or if hexadecimal is ok (all lowercase):
openssl rand -hex $NUM_CHARACTERS
Filesystem
Copy File(s) But Don't Overwrite
Sometimes you want to copy a file to another location, but are worries about the possibility of overwriting something that already exists. If that's the case, use the -n or --no-clobber parameter.
cp -n $SOURCE $DEST
Move File(s) But Don't Overwrite
In the same situation as above, but for moving file(s).
mv -i
or
mv --interactive
cp -n, this will propt you if you wish to overwrite files, whereas cp -n will just immediately exit with an error.
Get Folder Sizes
If you've ever completely filled a system to the point where you can't even install ncdu to track where all your storage is gone, you can get the size of all the folders at your current level by executing:
du -sh *
However, if you can use ncdu instead, that is a much better tool.
Move all Files of a Type To Own Folder
If you have a jumble of files with no apparent structure, perhaps from a backup, and want to move a ceratain file type (in this example PDFs) into their own directory then you can execute the following:
mkdir pdf
find . -name "*.pdf" -type f -exec /bin/mv {} ./pdf \;
Obviously if you just wanted to copy the files instead, use:
mkdir pdf
find . -name "*.pdf" -type f -exec /bin/cp {} ./pdf \;
List All Files Within All Subdirectories
find . -type f
If you wish to also list the directories:
find .
List All Files With Specific File Extensions
find ./ -type f \( -iname \*.jpg -o -iname \*.png \)
List all Files Over A Certain Size
The following command will recursively find individual files larger than [size] in MB. This could be useful for finding really large files to consider compressing/deleting.
find . -type f -size +[size]M
File Manipulation
Replace Text In Files (Sed)
I use variations of the sed command to manipulate config files for automated scripts
SEARCH="search text here"
REPLACE="replace text here"
FILEPATH="/path/to/file.conf"
sed -i "s;$SEARCH;$REPLACE;" $FILEPATH
If you want to replace an entire line, then just use .* as shown below.
SEARCH="bind-address.*"
REPLACE="bind-address = $PRIVATE_IP
FILEPATH="/etc/mysql/my.cnf"
sudo sed -i "s;$SEARCH;$REPLACE;" $FILEPATH
You can use [[:space:]]\+ when you don't know how many whitespaces or tabs there are. E.g.
SEARCH="datadir[[:space:]]\+= /var/lib/mysql"
REPLACE="datadir = /media/data/mysql"
FILEPATH="/etc/mysql/my.cnf"
sudo sed -i "s;$SEARCH;$REPLACE;" $FILEPATH
To replace an entire line that contains text that you are looking for in the middle of the line, you can use the following:
SEARCH="^.*\bYour pattern or sentence goes here\b.*$"
REPLACE="This will replace the entire line"
FILEPATH="$DIR/irapcms.sql"
sed -i "s;$SEARCH;$REPLACE;" $FILEPATH
Write To File
echo "my content here" | tee $FILEPATH
echo "my content here" > $FILEPATH
cat << EOF > [filepath]
[script content]
EOF
Write To File With Sudo Privs
echo "my content here" | sudo tee $FILEPATH
cat << EOF | sudo tee $FILEPATH
[script content]
EOF
cat << EOF > [filepath]
[script content]
EOF
Append To File
echo "my content here" >> $FILEPATH
Append To File (With Sudo Privs)
echo "my content here" | sudo tee -a $FILEPATH
cat << EOF | sudo tee -a $FILEPATH
[script content]
EOF
Replace Text In Files (Sed)
I use variations of the sed command to manipulate config files for automated scripts
SEARCH="search text here"
REPLACE="replace text here"
FILEPATH="/path/to/file.conf"
sed -i "s;$SEARCH;$REPLACE;" $FILEPATH
If you want to replace an entire line, then just use .* as shown below.
SEARCH="bind-address.*"
REPLACE="bind-address = $PRIVATE_IP
FILEPATH="/etc/mysql/my.cnf"
sudo sed -i "s;$SEARCH;$REPLACE;" $FILEPATH
You can use [[:space:]]\+ when you don't know how many whitespaces or tabs there are. E.g.
SEARCH="datadir[[:space:]]\+= /var/lib/mysql"
REPLACE="datadir = /media/data/mysql"
FILEPATH="/etc/mysql/my.cnf"
sudo sed -i "s;$SEARCH;$REPLACE;" $FILEPATH
Disks
Get Utilization
df -h
Or you could install pydf for a simpler interface.
Erase Disk
The best way to erase a disk is to use secure erase. For an SSD, this will reach areas that the other commands won't, and return it to peak performance.
Alternatively, you can use shred
shred -n 1 -vz /dev/sdb
... or use scrub
scrub -p dod /dev/sdb
... or you could use dd.
Check Sata Connection of Drives
dmesg | grep -i ahci | grep -i --color Gbps
Check the SATA Connection Of Specific Drive
smartctl -a /dev/sda | grep "^SATA"
Get Block Size
sudo blockdev --getbsz /dev/sd[x]
The output will be in bytes, so typically this will be 4096 to represent a 4k block size.
User Management
Adding Users
The simplest way to add users is to use the command below which will ask you a series of questions and set up their home directory.
sudo adduser [username]
If you are wanting to add a user as part of a script, or just don't want to set up a home directory, you can use
sudo useradd [username]
In that format, it will create a user without a home directory or even a password, use the options -m to automatically create a home directory, and -p to specify the encrypted password.
For example, the commands below will create a user called test with a random password:
USER="test"
PASSWORD=`openssl rand -base64 16`
PASSWORD=${PASSWORD::-2}
ENCRYPTED_PASSWORD=$(openssl passwd -1 ${PASSWORD})
sudo useradd -m -p $ENCRYPTED_PASSWORD $USER
echo "Please log in with password: $PASSWORD"
Deleting Users
sudo deluser [user]
A full-blown command in which we specify the user ID, group ID, whether to create a home, and the shell to use would be:
sudo useradd \
--no-create-home \
--uid 1019 \
--gid 100 \
--shell /usr/sbin/nologin \
myUserName
Get A User's ID
id -u $USERNAME
Change User's ID
The following command would change the programster user to user ID 1026
sudo usermod -u 1026 programster
Change User's Group
The following command would change the programster user to group ID 1026.
sudo groupmod -g 1026 programster
You can also do the following to set a user's primary group:
usermod -g primary_group_name programster
Group Management
Create Group
sudo groupadd myGroupName
If you wish to specify the ID of the group, you would use:
sudo groupadd --gid 1003 myGroupName
List All Groups
The following command will list all the groups on the system.
cut -d: -f1 /etc/group | sort
Add User To Group
sudo adduser $USERNAME $GROUPNAME
Remove User From Group
sudo deluser $USERNAME $GORUPNAME
Networking
Output IPs
hostname -I
If you want each IP on a newline, use the command below:
hostname -I | xargs | tr [:space:] '\n'
Stop Processes That Are Using Port
The following command will kill all processes that are using the port 1234.
lsof -ti:1234 | xargs kill -9
Blink NIC
If you have multiple NICs in a computer and want to know which one is which, you can make it blink. The command below will make eth0 blink for 30 seconds.
ethtool -p eth0 30
Hashes
md5sum $FILEPATH | awk '{print $1}'
sha1sum $FILEPATH | awk '{print $1}'
sha256sum $FILEPATH | awk '{print $1}'
Copy CD To Disk
If you have a CD-Rom drive (remember those things?), and want to create an ISO version on your computer, you can do it like so:
dd if=/dev/cdrom of=filename.iso
References
- Nixcraft - Linux Command To Find SATA Link Speed
- Unix & Linux - How to measure time of program execution and store that inside a variable
- Nixcraft - How to Change a USER and GROUP ID on Linux For All Owned Files
- Unix And Linux - How to change primary group?
- Stack Overflow - Linux how to copy but not overwrite?
- Unix & Linux - How to use find command to search for multiple extensions
First published: 16th August 2018