Master Essential Linux Commands: A Quick Reference Guide

Common Linux Commands

tree -L 1 /

– display a one‑level directory tree of the root. ls -l / – list files and directories with detailed information. ls -d / – show the directory entry for the root. ls -d /* – list all top‑level directories and files. cd - – return to the previous working directory. cd . – stay in the current directory. cd .. – move to the parent directory. cd ~ – change to the current user's home directory. pwd – print the absolute path of the current working directory. cp – copy files or directories (e.g., cp source dest). alias – define or display command aliases (e.g., alias ll='ls -l --color=auto'). unalias – remove a previously defined alias (e.g., unalias mv). cat -n /root/.bashrc – display file contents with line numbers. find / -type f -name "useradd" – search for regular files named useradd under the root directory. head /etc/services – show the first ten lines of a file. tail /etc/passwd – show the last ten lines of a file. tail -f /var/log/messages – continuously monitor a file for new content. rpm -ivh /mnt/Packages/elinks.el6_3.x86_64.rpm – install an RPM package. rpm -qa | grep mysql – list installed RPM packages containing "mysql". yum -y install ftp – install the FTP client using yum. yum grouplist – list available and installed package groups. yum -y groupinstall "PostgreSQL 数据库服务器" – install the PostgreSQL server group. yum info – display detailed information about a package or group. yum groupinfo – show information about a yum group. yum search all "keyword" – search package descriptions for a keyword. seq 5 – generate a sequence of numbers from 1 to 5. seq 3 5 – generate numbers from 3 to 5. seq 3 3 10 – generate numbers from 3 to 10 with a step of 3. seq -s "" 5 – output 12345 without separators. seq -w 10 – generate two‑digit numbers padded with leading zeros. hostname – display the system's host name. uname -r – show the kernel release version. uname -a – display all system information. uname -m – show the machine hardware name (e.g., x86_64). whoami – print the effective user ID (e.g., root). su - ping – switch to user ping with a login shell. logout – exit the current login shell. history – display the command history for the current user. history -d 122 – delete the entry at line 122 from the history. history -c – clear the entire command history. runlevel – show the previous and current runlevel (e.g., N 3). init 5 – change the system to runlevel 5. chkconfig sshd on – enable the SSH service to start at boot. chkconfig --list sshd – list the runlevel settings for the SSH service. which ps – locate the full path of the ps executable. whereis jobs – locate binary, source, and manual pages for jobs. whereis -b ping – locate only the binary for ping. ifdown eth0 && ifup eth0 – restart the eth0 network interface. /etc/init.d/network reload – reload network configuration. blkid – display UUIDs and other attributes of block devices.

Built‑in Bash commands include alias, bg, cd, echo, history, jobs, kill, pwd, type, ulimit, unalias, etc. cat /etc/sysconfig/network-scripts/ifcfg-eth0 – view network interface configuration (DEVICE, ONBOOT, NM_CONTROLLED, BOOTPROTO, IPADDR, NETMASK). cat /etc/resolv.conf – view DNS resolver configuration (nameserver entries).

Key points about DNS configuration:

Client DNS can be set in the network interface file (ifcfg‑eth0).

Client DNS can also be set in /etc/resolv.conf.

Settings in the interface file take precedence over /etc/resolv.conf. cat /etc/hosts – view static hostname‑to‑IP mappings used for local name resolution. cat /etc/sysconfig/network – view the system's hostname configuration file. cat /etc/fstab – view filesystem mount information for automatic mounting at boot. df -h – display disk space usage in human‑readable format. mount – list currently mounted filesystems. mount -t ext4 -o loop,noatime,noexec /dev/sdb1 /mnt – manually mount an ext4 filesystem with specific options. dd if=/dev/zero of=/dev/sdb1 bs=4096 count=10 – create a blank filesystem image. mkfs.ext4 /dev/sdb1 – format /dev/sdb1 with the ext4 filesystem.

Testing manual mount procedure:

Create a virtual block device: dd if=/dev/zero of=/dev/sdb1 bs=4906 count=100.

Format it: mkfs.ext4 /dev/sdb1.

Mount it: mount -t ext4 -o loop,noatime,noexec /dev/sda1 /mnt. fsck -A /dev/sda – check and repair all filesystems on /dev/sda.