Red Hat RHCSARHCE 7 Cert Guide（一）

zhangleivod

1.time ls: This executes the ls command where the bash internal time
        shows information about the time it took to complete this command.
2.which time:This shows the filename /bin/time that was found in the
        $PATH variable.
3./bin/time ls(time ls):there also is an internal commandtime , the time
        command from the path will not be executed unless you tell the
        shell specifically to do so.
4.STDIN(File Description Number:0)                STDOUT(1)                STDERROR(2)
        STDOUT:    >(same as 1>):overwrite    >>(same as 1>>):append
        STDERR:    2>      2>&1:Redirects STDERR to the same destination as STDOUT
        STDIN: <(same as 0<)
5.A device file on Linux is a file that is used to access specific hardware
6.If you want to discard a commands output, you can redirect to /dev/null.
7.ls ilweghi Documents 2>/dev/null  >output
8.ls -R |less (-R|--recursive)
9.Only the user root has permission to write to device files directly.
10. .bash_history(last 1000commands,in the home directory,are kept in memory)
11.history(CTL+R|!number|!sometext|history -c:clear the in-memory history|history -w)
12.vim(vi improved|yum install vim-enhanced):
        ^:Goes to the first position in the current line
        $:Goes to the last position in the current line.
        :%s/old/new/g :Replaces all occurrences of old with new
        !ls: Adds the output of ls (or any other command) in the current file.
13.evn(printenv):To get an overview of the current variables defined in your shell environment, type the env command.
14./etc/profile:This is the generic file that is processed by all users upon login
        /etc/bashrc:This file is processed when subshells are started.
        ~/.bash_profile:In this file, user-specific login shell variables can be defined.
        ~/.bashrc:In this user-specific file, subshell variables can be defined.
15./etc/motd,/etc/issue
        /etc/motd: Messages in /etc/motd display after a user has successfully logged in to a shell.
                (Notice that users in a graphical environment do not see its contents after a graphical
                login.)
        /etc/issue:The contents of this file display before the user logs in.
16.the .bashrc file is included in the login procedure, the variable is set after logging in.
17.--help|man command|help command|man -k command(partial command)|apropos(same as the command man -k)
18.Man pages:
        1:Executable programs or shell commands
        5:File formats and conventions
        8:System administration commands
19:man -f commands:displays a short description of the item as found in the man database(mandb)
        mandb:Updating mandb(Just run the mandb command as root without any arguments)
20.pinfo or info:type n to go to the next page, p to go to the previous page,
        or u to move up in the hierarchy.
21./usr/share/doc:You will not typically find much information about a
        command like ls , but some services do provide useful information in /usr/share/doc.
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1.Filesystem Hierarchy Standard (FHS)
        man 7 hier:this file system hierarchy is described in `man 7 hier`
        / :The root directory. This is where the file system tree starts.
        /bin:In here, you find executable programs that are needed to repair a system in
                a minimal troubleshooting mode. This directory is essential during boot.
        /boot:Contains all files and directories that are needed to boot the Linux kernel
        /dev:Device files that are used for accessing physical devices. This directory is
                essential during boot.
        /etc:Contains configuration files that are used by programs and services that
                are used on your server. This directory is essential during boot.
        /home:Used for local user home directories
        /lib,/lib64:Shared libraries that are used by programs in /boot, /bin and /sbin.
        /media,/mnt:Directories that are used for mounting devices in the file system tree.
        /opt:This directory is used for optional packages that may be installed on your
                server.
        /proc:This directory is used by the proc file system. This is a file system
                structure that gives access to kernel information.
        /root:The home directory of the root user.
        /run:Contains process and user specific information that has been created since
                the last boot.
        /sbin:Like /bin, but for system administration commands that are not necessarily
                needed by regular users.
        /srv:Directory that may be used for data that is used by services like NFS, FTP
                and HTTP .
        /sys:Used as an interface to different hardware devices that is managed by the
                Linux kernel and associated processes.
        /tmp:Contains temporary files that may be deleted without any warning during
                boot.
        /usr:Directory that contains subdirectories with program files, libraries for
                these program files and documentation about them. Typically, many
                subdirectories exist in this directory that mimic the contents of the /
                directory. The contents of /usr are not required during boot.
        /var:Directory that contains files which may change in size dynamically, such as
                log files, mail boxes, and spool files.
2./proc/mounts:The mount command gives an overview of all mounted devices. To get this
        information, the /proc/mounts file is read, where the kernel keeps information
        about all current mounts.
3.df -Th(-T option shows which file system type|-h option summarizes the output of the command in
        a human-readable way)
4.findmnt
        tmpfs:These are kernel devices that are used to create a temporary file system
                in RAM.
5.Wildcard Overview
        *:Refers to an unlimited number of all characters    ls *
        ?:Used to refer to one specific character that can be any character   ls c?t
        [auo]:Refers to one character that may be selected from the range that is specified
                between square brackets
6./tmp:This brings you to the /tmp directory, where all users have write permissions
7.mkdir /home/$USER/files:($USER , which is substituted with your current username)
8.rmdir:it works only if the directory is empty and does not contain any files.
9.ls -d:Shows the names of directories(ls -d zhanglei*)
10.cp -R(-r:--recursion):cp -R /etc /tmp
        cp -a(-dR) :ensures that permissions and all other file properties will be kept while copying.
11.Inode(except filename):Names are stored in the directory
        1.The data block where the file contents are stored
        2.The creation, access, and modification date
        3.Permissions
        4.File owners
12.Hard Link：
        1.Hard links must exist all on the same device.
        2.You cannot create hard links to directories.
        3.The number of aliases the original file has. When the last name is removed,
                the contents are also removed.