Essential Linux Command Cheat Sheet: From File Management to SELinux

Command Connectors

Semicolon ; runs commands sequentially. Ampersand & runs the following command regardless of the previous command’s exit status. Double ampersand && runs the next command only if the previous command succeeded (exit status 0). Pipe | passes the standard output of the left command to the standard input of the right command. Double pipe || runs the right‑hand command only when the left command fails (non‑zero exit status).

System Resource Monitoring

df – Disk Free

Shows filesystem space usage, including space held by deleted files that are still open by processes.

du – Disk Usage

Example to list the size of each entry in the current directory and sort it descending:

du -sh * | sort -nr

ps – Process Status

List processes and filter, e.g.: ps -ef | grep sync Use grep -v grep to exclude the grep process itself.

Vim Editing Modes and Search‑Replace

Modes

Command mode : press ESC to enter; commands are not echoed.

Insert mode : i, a, o, c, r, s start text insertion.

Ex (last‑line) mode : press : to enter the command line, e.g. wq, q!.

Search and replace

General syntax:

:<range> s/<search_pattern>/<replace_string>/<flags>

range

– lines to operate on ( % for whole file, 200,250 for specific lines). search_pattern – regular expression to find. replace_string – replacement text. flags – g global, c confirm, [...c (or [...i) for case‑insensitive.

Examples:

:%s/python/Python3/g
:200,250 s/python/Python3/g
:%s/python\|py/Python3/g

File Search Commands

grep

# Search a specific file
grep 'string' filename

# Search all files in a directory
grep 'string' dirPath/*

# Recursive search
grep -r 'string' ./

find

# Find a file by name
find / -name install.log

# Files modified more than 10 days ago
find . -mtime +10

# Files larger than 25 KB
find . -size 25k

# Files between 20 KB and 50 KB and list details
find /etc -size +20k -a -size -50k -exec ls -lh {} \;

whereis & which

# Locate binaries and manuals
whereis command_name   # -b for binaries only, -m for manuals only
which command_name

locate

# Fast filename search using a pre‑built database
locate filename
# Update the database
updatedb

File Transfer

scp

# Download a single file
scp user@host:/remote/file.txt /local/path

# Download a directory recursively
scp -r user@host:/remote/dir /local/dir

# Upload a single file
scp /local/file.txt user@host:/remote/path

# Upload a directory recursively
scp -r /local/dir user@host:/remote/dir

rsync

# Pull a remote directory, excluding specific sub‑directories
rsync -azh -e ssh --exclude 'l_temp' --exclude 'l_tmp/claimImageList' user@host:/remote/dir /local/dir

# Sync with log and delete options
rsync -az /src/dir /dest/dir --log-file=./rsync.log --exclude='unwanted.jar' --delete

sftp

sftp user@host
put local_file remote_path
get remote_path local_path

Linux Text Utilities

sort

Common options include -n (numeric), -r (reverse), -k (key field), etc.

split

split -b 1024M large.tar.gz part_
cat part_* > large.tar.gz
md5sum large.tar.gz

Mount / Unmount and NFS

Mount utilities

# Show mounted devices
mount

# Mount all entries from /etc/fstab
mount -a

# Mount a specific filesystem
mount -t ext4 /dev/sdb1 /mnt/data

# Unmount
umount /dev/sdb1

NFS server (CentOS)

sudo yum install nfs-utils
sudo systemctl start rpcbind nfs-server nfs-lock nfs-idmap
sudo systemctl enable rpcbind nfs-server nfs-lock nfs-idmap

echo "/nfs/data *(rw,sync,no_root_squash,no_all_squash)" | sudo tee -a /etc/exports
sudo exportfs -rav

sudo firewall-cmd --permanent --zone=public --add-service=nfs
sudo firewall-cmd --permanent --zone=public --add-service=mountd
sudo firewall-cmd --permanent --zone=public --add-service=rpc-bind
sudo firewall-cmd --reload

NFS client (Ubuntu)

sudo apt update
sudo apt install nfs-common
sudo mount nfs-server:/srv/nfs/share /mnt

Automation with expect

#!/usr/bin/expect
spawn ssh [email protected]
expect "password:"
send "mypassword\r"
expect "$ "
send "ls -l\r"
expect eof

Key commands: spawn (start a process), expect (wait for output), send (send input), interact (hand control back to the user).

Other Useful Commands

openssl encryption / decryption

tar -zcf - host.conf | openssl des3 -salt -k ZYProduct@2022 | dd of=host.des
openssl des3 -d -k "ZYProduct@2022" -in host.des | tar zxf -

eval

Evaluates its arguments as a shell command: eval [argument].

xargs

# Find files containing "password"
find . -name "*.txt" | xargs grep "password"

# Pass each line as a separate argument
echo "hello world" | xargs -n 1

echo with colors

echo -e "\e[1;33m test \e[0m"

User and File Management

# Create group and user
groupadd mysql
useradd -u 544 -d /home/mysql -g mysql -m mysql

# Change ownership
chown zhuo:zhuogroup test.php
chown -R zhuo:zhuogroup testdir

SELinux Policy Management with semanage

List, add, modify, or delete port and file‑context definitions.

# List all ports
semanage port -l

# Add a TCP port 8081 for http_port_t
semanage port -a -t http_port_t -p tcp 8081

# Delete the entry
semanage port -d -t http_port_t -p tcp 8081

Check SELinux status with getenforce, edit /etc/sysconfig/selinux, and reboot if changes are made.