Commands and Keywords that every Linux user MUST know Part 3

Here is the list of keywords and phrases that every Linux newbie should learn or at least have on file Part 3, For more commands check Part1 and Part2

renice -- alter priority of running processes.
Renice alters the scheduling priority of one or more running processes. The following who parameters are interpreted as process ID's, process group ID's, or user names. Renice'ing a process group causes all processes in the process group to have their scheduling priority altered. Renice'ing a user causes all processes owned by the user to have their scheduling priority altered. By default, the processes to be affected are specified by their process ID's.

return -- return from a function.
The return utility shall cause the shell to stop executing the current function or dot script. If the shell is not currently executing a function or dot script, the results are unspecified.

rm -- remove files and directories.
rm removes each specified file. By default, it does not remove directories.

rmdir -- remove empty directories.
Remove directories if they are empty.

rsync -- a fast, versatile, remote (and local) file-copying tool.
Rsync is a fast and extraordinarily versatile file copying tool. It can copy locally, to/from another host over any remote shell, or to/from a remote rsync daemon. It offers a large number of options that control every aspect of its behavior and permit very flexible specification of the set of files to be copied. It is famous for its delta-transfer algorithm, which reduces the amount of data sent over the network by sending only the differences between the source files and the existing files in the destination. Rsync is widely used for backups and mirroring and as an improved copy command for everyday use. Rsync finds files that need to be transferred using a “quick check” algorithm (by default) that looks for files that have changed in size or in last-modiâ€ fied time. Any changes in the other preserved attributes (as requested by options) are made on the destination file directly when the quick check indiâ€ cates that the file's data does not need to be updated.

scp -- secure copy (remote file copy program).
scp copies files between hosts on a network. It uses ssh(1) for data transfer, and uses the same authentication and provides the same security as ssh(1). Unlike rcp(1), scp will ask for passwords or passphrases if they are needed for authentication. File names may contain a user and host specification to indicate that the file is to be copied to/from that host. Local file names can be made explicit using absolute or relative pathnames to avoid scp treating file names containing ‘:’ as host specifiers. Copies between two remote hosts are also permitted.

screen -- screen manager with VT100/ANSI terminal emulation.
Screen is a full-screen window manager that multiplexes a physical terminal between several processes (typically interactive shells). Each virtual termiâ€ nal provides the functions of a DEC VT100 terminal and, in addition, several control functions from the ISO 6429 (ECMA 48, ANSI X3.64) and ISO 2022 stanâ€ dards (e.g. insert/delete line and support for multiple character sets). There is a scrollback history buffer for each virtual terminal and a copy-and- paste mechanism that allows moving text regions between windows.

sdiff -- side-by-side merge of file differences.
In the simplest case, two file names FROM-FILE and TO-FILE are given, and `diff' compares the contents of FROM-FILE and TO-FILE. A file name of `-' stands for text read from the standard input. As a special case, `diff - -' compares a copy of standard input to itself.

sed -- stream editor for filtering and transforming text.
Sed is a stream editor. A stream editor is used to perform basic text transformations on an input stream (a file or input from a pipeline). While in some ways similar to an editor which permits scripted edits (such as ed), sed works by making only one pass over the input(s), and is consequently more effiâ€ cient. But it is sed's ability to filter text in a pipeline which particularly distinguishes it from other types of editors.

select -- synchronous I/O multiplexing.
select allow a program to monitor multiple file descriptors, waiting until one or more of the file descriptors become "ready" for some class of I/O operation (e.g., input possible). A file descriptor is considered ready if it is possible to perform the corresponding I/O operation (e.g., read(2)) without blocking.

seq -- print a sequence of numbers.
seq prints the numbers from FIRST to LAST by INCREMENT. By default, each number is printed on a separate line. When INCREMENT is not specified, it defaults to `1', even when FIRST is larger than LAST. FIRST also defaults to `1'. So `seq 1' prints `1', but `seq 0' and `seq 10 5' produce no output. Floating-point numbers may be specified (using a period before any fractional digits).

set -- set or unset options and positional parameters.

sftp -- secure file transfer program.
sftp is an interactive file transfer program, similar to ftp, which performs all operations over an encrypted ssh transport. It may also use many feaâ€ tures of ssh, such as public key authentication and compression. sftp connects and logs into the specified host, then enters an interactive command mode.

shift -- shift positional parameters.
The positional parameters shall be shifted. Positional parameter 1 shall be assigned the value of parameter (1+n), parameter 2 shall be assigned the value of parameter (2+n), and so on. The parameters represented by the numbers "$#" down to "$#-n+1" shall be unset, and the parameter '#' is updated to reflect the new number of positional parameters.

shopt -- shows option settings for the current shell.

shutdown -- shuts down or restarts system.
shutdown brings the system down in a secure way. All logged-in users are notified that the system is going down, and login is blocked. It is possible to shut the system down immediately or after a specified delay. All processes are first notified that the system is going down by the signal SIGTERM. This gives programs like vi the time to save the file being edited, mail and news processing programs a chance to exit cleanly, etc. shutdown does its job by signalling the init process, asking it to change the runlevel. Runlevel 0 is used to halt the system, runlevel 6 is used to reboot the system, and runlevel S is used to put to system into a state where administrative tasks can be performed; this is the default if neither the -h or -r flag is given to shutdown.

sleep -- delay for a specified amount of time.
sleep pauses for an amount of time specified by the sum of the values of the command line arguments.

sort -- sort lines of text files.
sort sorts, merges, or compares all the lines from the given files, or standard input if none are given or for a FILE of `-'. By default, `sort' writes the results to standard output.

source -- Evaluate a file or resource as a Tcl script.
This command takes the contents of the specified file or resource and passes it to the Tcl interpreter as a text script. The return value from source is the return value of the last command executed in the script. If an error occurs in evaluating the contents of the script then the source command will return that error. If a return command is invoked from within the script then the remainder of the file will be skipped and the source command will return normally with the result from the return command.

split -- split a file into pieces.
Output fixed-size pieces of INPUT to PREFIXaa, PREFIXab, ...; default size is 1000 lines, and default PREFIX is `x'. With no INPUT, or when INPUT is -, read standard input.

ssh -- OpenSSH SSH client (remote login program).
ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine. It is intended to replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. X11 connections and arbitrary TCP ports can also be forâ€ warded over the secure channel.

su -- change user ID or become superuser.
The su command is used to become another user during a login session. Invoked without a username, su defaults to becoming the superuser. The optional argument - may be used to provide an environment similar to what the user would expect had the user logged in directly. Additional arguments may be provided after the username, in which case they are supplied to the user´s login shell. In particular, an argument of -c will cause the next argument to be treated as a command by most command interpreters. The command will be executed by the shell specified in /etc/passwd for the target user.

stat -- display file or file system status.
With no option, stat reports all information about the given files. But it also can be used to report the information of the file systems the given files are located on. If the files are links, `stat' can also give information about the files the links point to.

sum -- checksum and count the blocks in a file.
sum prints the checksum for each FILE followed by the number of blocks in the file (rounded up). If more than one FILE is given, file names are also printed (by default). (With the `--sysv' option, corresponding file names are printed when there is at least one file argument.)

suspend -- suspend current shell.

sync -- flush file system buffers.
sync writes any data buffered in memory out to disk. This can include (but is not limited to) modified superblocks, modified inodes, and delayed reads and writes. This must be implemented by the kernel; The `sync' program does nothing but exercise the `sync' system call.

tac -- concatenate and print files in reverse.
tac copies each FILE (`-' means standard input), or standard input if none are given, to standard output, reversing the records (lines by default) in each separately.

tail -- output the last part of files.
tail prints the last part (10 lines by default) of each FILE; it reads from standard input if no files are given or when given a FILE of `-'.

tar -- The GNU version of the tar archiving utility.
A tarfile may be made on a tape drive, however, it is also common to write a tarfile to a normal file. The first argument to tar must be one of the options Acdrtux, followed by any optional functions. The final arguments to tar are the names of the files or directories which should be archived. The use of a directory name always implies that the subdirectories below should be included in the archive.

tee -- read from standard input and write to standard output and files.
The tee command copies standard input to standard output and also to any files given as arguments. This is useful when you want not only to send some data down a pipe, but also to save a copy.

test -- check file types and compare values.
test returns a status of 0 (true) or 1 (false) depending on the evaluation of the conditional expression EXPR. Each part of the expression must be a separate argument.

time -- time a simple command or give resource usage.
The time command runs the specified program command with the given arguments. When command finishes, time writes a message to standard error giving timing statistics about this program run. These statistics consist of the elapsed real time between invocation and termination, the user CPU time, the sum of the tms_utime and tms_cutime values in a struct tms as returned by times, and the system CPU time (the sum of the tms_stime and tms_cstime values in a struct tms as returned by times).

touch -- change file timestamps.
Update the access and modification times of each FILE to the current time. A FILE argument that does not exist is created empty. A FILE argument string of - is handled specially and causes touch to change the times of the file associated with standard output. Mandatory arguments to long options are mandatory for short options too.

top -- display Linux tasks.
That program provides a dynamic real-time view of a running system. It can display system summary information as well as a list of tasks currently being manâ€ aged by the Linux kernel. The types of system summary information shown and the types, order and size of information displayed for tasks are all user conâ€ figurable and that configuration can be made persistent across restarts.

traceroute* -- print the route packets trace to network host.
*it needs to be executed as root.
traceroute tracks the route packets taken from an IP network on their way to a given host. It utilizes the IP protocol's time to live (TTL) field and attempts to elicit an ICMP TIME_EXCEEDED response from each gateway along the path to the host. The only required parameter is the name or IP address of the destination host.

trap -- trap signals (executes certain command when the shell recieve a particular signal)
If action is '-', the shell shall reset each condition to the default value. If action is null ( "" ), the shell shall ignore each specified condition if it arises. Otherwise, the argument action shall be read and executed by the shell when one of the corresponding conditions arises. The action of trap shall override a previous action (either default action or one explicitly set). The value of "$?" after the trap action completes shall be the value it had before trap was invoked.

tr -- translate or delete characters.
Translate, squeeze, and/or delete characters from standard input, writing to standard output.

true -- do nothing, successfully.
true does nothing except return an exit status of 0, meaning "success". It can be used as a place holder in shell scripts where a successful command is needed, although the shell built-in command `:' (colon) may do the same thing faster.

tsort -- perform topological sort.
Write totally ordered list consistent with the partial ordering in FILE. With no FILE, or when FILE is -, read standard input.

tty -- print the file name of the terminal connected to standard input.
tty prints the file name of the terminal connected to its standard input. It prints `not a tty' if standard input is not a terminal.

type -- write a description of command type.
The type utility shall indicate how each argument would be interpreted if used as a command name.

ulimit -- set or report file size limit.
The ulimit utility shall set or report the file-size writing limit imposed on files written by the shell and its child processes (files of any size may be read). Only a process with appropriate privileges can increase the limit.

umask -- get or set the file mode creation mask.
The umask utility shall set the file mode creation mask of the current shell execution environment to the value speciâ€ fied by the mask operand. This mask shall affect the initial value of the file permission bits of subsequently created files.

umount -- unmount file systems.
The umount command detaches the file system(s) mentioned from the file hierarchy. A file system is specified by giving the directory where it has been mounted. Giving the special device on which the file system lives may also work, but is obsolete, mainly because it will fail in case this device was mounted on more than one directory.

unalias -- remove alias definitions.
The unalias utility shall remove the definition for each alias name specified. The aliases shall be removed from the current shell execution environment.

uname -- print system information.
Print certain system information. With no OPTION, same as -s.

uniq -- report or omit repeated lines.
Filter adjacent matching lines from INPUT (or standard input), writing to OUTPUT (or standard output). With no options, matching lines are merged to the first occurrence. Mandatory arguments to long options are mandatory for short options too.

useradd* -- create a new user or update default new user information.
*it needs to be executed as root.
When invoked without the -D option, the useradd command creates a new user account using the values specified on the command line plus the default values from the system. Depending on command line options, the useradd command will update system files and may also create the new user´s home directory and copy initial files.

userdel* -- delete a user account and related files.
*it needs to be executed as root.
The userdel command modifies the system account files, deleting all entries that refer to the user name LOGIN. The named user must exist.

usermod* -- modify a user account.
*it needs to be executed as SuperUser a.k.a root
usermod will not allow you to change the name of a user who is logged in. You must make certain that the named user is not executing any processes when this command is being executed if the user´s numerical user ID is being changed. You must change the owner of any crontab files manually. You must change the owner of any at jobs manually. You must make any changes involving NIS on the NIS server.

users -- print the user names of users currently logged in to the current host.
users prints on a single line a blank-separated list of user names of users currently logged in to the current host. Each user name corresponds to a login session, so if a user has more than one login session, that user's name will appear the same number of times in the output.

vdir -- list directory contents.
List information about the FILEs (the current directory by default). Sort entries alphabetically if none of -cftuvSUX nor --sort. Mandatory arguments to long options are mandatory for short options too.

w -- Show who is logged on and what they are doing.
w displays information about the users currently on the machine, and their processes. The header shows, in this order, the current time, how long the system has been running, how many users are currently logged on, and the system load averages for the past 1, 5, and 15 minutes. The following entries are displayed for each user: login name, the tty name, the remote host, login time, idle time, JCPU, PCPU, and the command line of their current process. The JCPU time is the time used by all processes attached to the tty. It does not include past background jobs, but does include currently running backâ€ ground jobs.

watch -- execute a program periodically, showing output fullscreen.
watch runs command repeatedly, displaying its output (the first screenfull). This allows you to watch the program output change over time. By default, the program is run every 2 seconds; use -n or --interval to specify a different interval.

wc -- print newline, word, and byte counts for each file.
Print newline, word, and byte counts for each FILE, and a total line if more than one FILE is specified. With no FILE, or when FILE is -, read standard input.

wget -- network downloader.
Wget is non-interactive, meaning that it can work in the background, while the user is not logged on. This allows you to start a retrieval and disconnect from the system, letting Wget finish the work. By contrast, most of the Web browsers require constant user's presence, which can be a great hindrance when transferring a lot of data.

whereis -- locate the binary, source, and manual page files for a command.
whereis locates source/binary and manuals sections for specified files. The supplied names are first stripped of leading pathname components and any (sinâ€ gle) trailing extension of the form .ext, for example, .c. Prefixes of s. resulting from use of source code control are also dealt with. whereis then attempts to locate the desired program in a list of standard Linux places.

who -- show who is logged on.

whoami -- show current user

write -- send massage to another user logged in the system.

zcat -- compress or expand files.

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