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UNIX Commands

Command Basics

• UNIX commands are mainly written in C, but also supports programs written in any language.

• UNIX is case-sensitive to the filenames. (Most commands are in lowercase.)

• UNIX does NOT require an extension for command names, but it can be used either for convenience or for conforming to an application requirement.

  • e.g., .c and .java extensions are required for C and Java programs, repectively, to be compiled.

  • e.g., shell and Perl scripts don't need the .sh or pl extensions, but we often provide them for easier identification.

• Unlike other commands, shell is invoked the moment you log in and runs until you log out.

• External Commands

  • Commands that exist on disk as separate files.

• Internal Commands

  • Commands that are built into the shell executable.

    • e.g., cd command (There's no file in the system named cd.)

PATH Variable

• PATH is a variable that shell maintains in its own environment (thus a.k.a. an environment variable).

• PATH is set to a list of colon-delimited directories; making it a string as a whole.

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  $ echo $PATH
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  /bin:/usr/bin:usr/ucb:/usr/spg4/bin:.
  

  There are five directories in this list separated by a delimiter, the colon. (The last one, ., represents the current directory.)

  When a command is entered, the shell looks in each of these directories to locate the file with that name.

  When the netscape command is not available in any of these directories:

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  $ netscape
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  netscape: command not found
  

  In case netscape exists somewhere else on the system, try:

  • adding that directory to PATH

  • using a pathname (like /usr/local/bin/netscape if the command is located in /usr/local/bin).

  • switching to the directory where the executable is located and executing it from there.

• To execute the program in the current directory, invoke it with ./ at front.

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  1
  $ ./myprogram
  

Commands to Locate a Command

• which

  • which command tells you the directory in PATH that contains the command:

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    $ which grep
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    /usr/bin/grep
    

    After searching the directories of PATH in sequence, which abandons its search the moment it locates a file named grep.

    grep (global regular expression printer) searches for patterns in a file.

  • which is not the required by POSIX.

• whereis

  • Unlike which, whereis searches a larger list than PATH.

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    whereis grep
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    grep: /usr/bin/grep /usr/share/man/man1/grep.1.gz /usr/share/info/grep.info.gz
    

  • whereis command is usable in BSD-based UNIX systems.

  • whereis is not the required by POSIX.

• type

  • type doesn't display files but only points to the version that will be executed.

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    $ type echo
    2
    echo is a shell builtin
    

    Remember! Not all UNIX commands are files; some are built into the shell. which and whereis do not consider this possibility.

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    $ which echo
    2
    /usr/bin/echo
    

    This file is not the echo that will be executed but it is the built-in echo command that will be executed in reality. type gives a more authentic information whereas which gives only the half true information.

Command Structure

• UNIX command line: command + (space delmited) areguments

  • Even if a lot of whitespace (space, tab, new line, form feed) are provided in the command line, the shell ensures that arguments are separated by a single space before the command is run.

• An option begins with a hyphen '-', and changes a command's default behavior.

  • e.g., ls shows only the filenames, the -l and -t options show their attributes as well.

  • Never create a filename that begins with a hyphen as a command might treat it as one of its options.

  • Linux offers --[word] type options as well as all of the UNIX-type options.

    • e.g., ls --classify is equivalent to ls -F.

• The filename will generally be a command's last argument, after all options. However, this is not always true; some options use filenames as their own arguments.

Flexibility of Command Usage

• In UNIX, you can specify multiple commands in one line by using ;(a metacharacter) as the delimiter of commands.

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  1
  ls ; cat file1
  

• On the other hand, a command line can be split into multiple physical lines.

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  1
  $ echo "This is
  2
  > a three-line
  3
  > text message"
  4
  This is
  5
  a three-line
  6
  text message
  

  The appearance of > (or ?) indictes that the command line is not complete (e.g., the absence of a matching quote or parenthesis). If you can't fix it, just interrupt the command line with [Ctrl-c] or [Ctrl-u].

  In C shell command line will look a little different:

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  1
  % echo "This is\      (Primary prompt in C shell is '%')
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  ? a three-line\       (Secondary prompt is often '?')
  3
  ? text message"
  4
  This is
  5
  a three-line
  6
  text message
  

• Command output need not always be seen on the terminal.

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  who > userlist.txt    (Output saved in 'userlist.txt')
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  who | wc -l           ('who' piped to 'wc -l')
  

man: On-Line Help

• man, short for manual, is called the man documentation which remains the most complete and authoritative guide to the UNIX system.

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  $ man ls              (To view the manual page of 'ls')
  

  The output is sent to a pager program, which displays the contents of a file one page (screen) at a time.

• man is always preconfigured to be used with a specific pager.

  • more, Berkeley's pager, a superior alternative to the original AT&T pg command (now obsolete), which is now available universally.

  • less, the standard pager used on Linux systems, but also available for UNIX platforms.

• On the man page, the key you press can be considered to be one of man's (rather, the pager's) internal commands which doesn't show up on the screen.

  • e.g., To quit the pager, and ultimately man, press q.

• man page also includes important system files and system calls used by the commands.

Navigation and Search

• The navigation commands may vary across the UNIX implementations.

  • e.g., f or spacebar advances by one scree, b moves back one screen.

• Search

  • e.g., /filename[Enter] calls up the page containing the word filename. (Press n to find next.)

Further Help with man -k and man -f

• POSIX requires man to support only one option -k.

  • Most UNIX systems also offer the appropos command that emulates man -k.

• man -k searches the NAME section of all man pages that contain the keyword.

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  $ man -k cron
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  anacrontab (5)       - configuration file for anacron
  3
  anacron (8)          - runs commands periodically
  4
  cron (8)             - daemon to execute scheduled commands (Vixie Cron)
  5
  crontab (1)          - maintain crontab files for individual users (Vixie Cron)
  6
  crontab (5)          - tables for driving cron
  

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  $ man -k chmod
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  chmod (1)            - change file mode bits
  3
  chmod (2)            - change permissions of a file
  4
  fchmod (2)           - change permissions of a file
  5
  fchmodat (2)         - change permissions of a file
  

The man Documentation

• Generally, man documentation includes following eight sections.

• Examples

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  man 2 read            (Look up the 'read' system call)
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  man -s2 read          (Solaris uses the '-s' option)
  

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  man passwd            (Look up Section 1 only)
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  man 4 passwd          (Look up Section 4)
  3
  man -s4 passwd        (Solaris uses the '-s' option)
  

Understanding a man Page

• A man page is divided into a number of compulsory and optional sections.

  • Although not every command has all sections, the first three (NAME, SYNOPSIS and DESCRIPTION) are generally seen in all man pages.

• NAME provides one-line introduction to the command.

• SYNOPSIS shows the syntax used by the command.

  • e.g., mycmd [ -a | -b | -A ] [ -kj ] [ file ... ]

    • Bracket ([]) means optional; otherwise, required. So mycmd can be used by itself without arguments.

    • Ellipsis (...) implies that there can be more. So mycmd can be used with more than one filename as argument.

    • Pipe (|) delimited options mean that only one on the either side of the pipe can be used. So, only one of the options -a, -b and -A, can be used.

• DESCRIPTION (often the largest section) presents a detailed description.

• OPTIONS lists all options used by the command.

• EXIT STATUS lists possible error conditions and their numeric representation.

  • This is important when writing shell scripts and C programs in order to determine the actual cause of termination of a program.

Using man to Understand man

• man itself is a UNIX command so it can be viewed in it's own documentation.

  • The variable PAGER can be chosen.

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    1
    $ PAGER=less ; export PAGER     (Set this shell variable and export it before you run 'man')
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    $ man ls
    

info and --help in Linux

• Linux offers two additional help facilities; info command and --help option.

• info is GNU's info reader for browsing Texinfo documentation.

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  $ info grep           (See the documentation organized in 'nodes')
  

  Tips!

  • [Tab] to naviate through the nodes.

  • n to visit the next node.

  • p to visit previous node

  • u to return to the previous level where [Enter] was pressed.

  • l to move to the previously visied note.

  • Within a page, [PageUp] and [PageDown] (or spacebar) also work.

• --help option displays a compact listing of all options.

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  $ grep --help         (Shows grep's options in a compact manner)
  

  This is useful when a command has too many options and you need to view them quickly.

echo: Displaying Messages

• echo command is often used to,

  • evalute shell variables.

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    $ echo $SHELL
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    /bin/bash
    

  • (when used in shell scripts) to display messages on your terminal.

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    $ echo "Filename not entered"     (Shell version of echo used here)
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    Filename not entered
    

  • (when used in shell scripts) to issue prompts for taking user input. e.g., In AT&T version (System V), echo command can be used with escape sequences, but BSD doesn't recognize them. (-n option is an alternative to the \c sequence.)

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    $ echo "Enter filename: \c"   (System V)
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    $ echo -n "Enter filename: "  (BSD)
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    Enter filename: $ _           (Prompt and cursor in the same line)
    

    An escape sequence begins with a \ and is followed by a single character or a zero-prefixed number. Here, \c is an escape sequene.

    echo interprets a number as octal when it is preceded by \0.

• Different shells will respond differently to escape sequences.

  • For this reason, PSOXI recommands to use printf instead of echo.

• In Linux, where Bash is the standard shell, e option is required to interpret the escape sequences properly.

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  $ echo -e "Enter filename: \c"
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  Enter filename: $ _
  

printf: Alternative to echo

• printf esixts as an external command, but it's only Bash that has printf built in.

• If you can, choose printf over echo.

• printf recognizes escape sequences.

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  $ printf "No filename entered\n"      (Unlike echo, '\n` is required in printf to explicitly specify a new line)
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  No filename entered
  

• printf also uses format specifiers to format the output. (Like in C.)

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    $ printf "My current shell is%s\n" $SHELL
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    My current shell is/bin/bash
  

  Unlike in C though, the parentheses and comma between the format specifier and its matching arguments are absent.

script: Recording Your Session

• script is used to record your login session in a file.

  • It is not included in POSIX but is very useful.

• script should be followed by the name of the file to place the log in, and the exit command should be used to stop logging.

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  $ script mylog
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  Script started, file is mylog
  3
  $ echo "This is a test log."          (Another shell; recording begins here) 
  4
  This is a test log.
  5
  $ exit                                (Or use [Ctrl-d])
  6
  exit      
  7
  Script done, file is mylog
  8
  $ _                                   (Back to login shell)
  

  At the termination of above session recording, mylog will be containing:

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  Script started on 2022-02-11 15:18:56-06:00 [TERM="xterm-256color" TTY="/dev/pts/2" COLUMNS="102" LINES="57"]
  2
  $ echo "This is a test log."
  3
  This is a test log.
  4
  $ exit
  5
  exit
  6
  Script done on 2022-02-11 15:19:42-06:00 [COMMAND_EXIT_CODE="0"]
  

  If the filename is not provided, the session record will get recorded in the file named typescript by default.

  script overwrites any previous typescript that my exist. (Use -a option to append.)

Using Email with mailx

• mailx is the only mail utility that POXIS requires all UNIX systems to support.

• An email message is identified by a sender and a recipient, both of which appear as headers in the message.

• Recieved mail is deposited in a mailbox, a text file that may contain binary attachments in encoded form.

  • A viewed message is moved from the mailbox to the mbox. (a.k.a. folders)

Sending Mail

• Interactive Sending Mode

  • mailx command works in the interactive sending mode when invoked with the emil address of the recipient as argument.

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    klee@t480s:~$ mailx klee          ('klee' is on the same host)
    2
    Cc: klee
    3
    Subject: Interactive Sending Mode Test 1
    4
    This is an interactive sending mode test 1.                       
    5
    klee@t480s:~$ _                   ([Ctrl-d] or dot sends the mail)
    

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    1
    klee@t480s:~$ mailx klee          ('klee' is on the same host)
    2
    Cc: klee
    3
    Subject: Interactive Sending Mode Test 2
    4
    This is an interactive sending mode test 2, without Cc.
    5
    klee@t480s:~$ _                   ([Ctrl-d] or dot sends the mail)
    

    After sending the above two mails, the file /var/mail/klee contains:

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    1
    From klee@t480s  Fri Feb 11 16:30:39 2022
    2
    Return-Path: <klee@t480s>
    3
    X-Original-To: klee@t480s
    4
    Delivered-To: klee@t480s
    5
    Received: by t480s (Postfix, from userid 1000)
    6
        id 9451B240156; Fri, 11 Feb 2022 16:30:39 -0600 (CST)
    7
    To: <klee@t480s>
    8
    Cc: <klee@t480s>
    9
    Subject: Interactive Sending Mode Test 2
    10
    X-Mailer: mail (GNU Mailutils 3.7)
    11
    Message-Id: <20220211223039.9451B240156@t480s>
    12
    Date: Fri, 11 Feb 2022 16:30:39 -0600 (CST)
    13
    From: klee <klee@t480s>
    14
    
    15
    This is an interactive sending mode test 1.                       
    16
    
    17
    From klee@t480s  Fri Feb 11 16:37:49 2022
    18
    Return-Path: <klee@t480s>
    19
    X-Original-To: klee@t480s
    20
    Delivered-To: klee@t480s
    21
    Received: by t480s (Postfix, from userid 1000)
    22
        id CBD8C240157; Fri, 11 Feb 2022 16:37:49 -0600 (CST)
    23
    To: <klee@t480s>
    24
    Subject: Interactive Sending Mode Test 2
    25
    X-Mailer: mail (GNU Mailutils 3.7)
    26
    Message-Id: <20220211223749.CBD8C240157@t480s>
    27
    Date: Fri, 11 Feb 2022 16:37:49 -0600 (CST)
    28
    From: klee <klee@t480s>
    29
    
    30
    This is an interactive sending mode test 2, without Cc.
    

• Noninteractive Sending Mode

  • UNIX specific way to send mails.

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    $ mailx -s "Noninteractive Sending Mode Test" klee < message.txt
    

    Where message.txt contains:

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    1
    This is a noninteractive sending mode test.
    

    After sending the above mail, the file /var/mail/klee contains:

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    From klee@t480s  Fri Feb 11 16:53:08 2022
    2
    Return-Path: <klee@t480s>
    3
    X-Original-To: klee@t480s
    4
    Delivered-To: klee@t480s
    5
    Received: by t480s (Postfix, from userid 1000)
    6
        id B52A3240157; Fri, 11 Feb 2022 16:53:08 -0600 (CST)
    7
    Subject: Noninteractive Sending Mode Test
    8
    To: <klee@t480s>
    9
    X-Mailer: mail (GNU Mailutils 3.7)
    10
    Message-Id: <20220211225308.B52A3240157@t480s>
    11
    Date: Fri, 11 Feb 2022 16:53:08 -0600 (CST)
    12
    From: klee <klee@t480s>
    13
    
    14
    This is non-interactive sending mode test.
    

  • Although it is not required by POSIX, most mailx versions support the -c (carbon copy) and -b (blind carbon copy) options.

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    mailx -s "Greetings" -c "sunny,yena" -b jaesoo klee < message.txt
    

    Recipient list of this mail will be:

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    1
    To: klee@t480s
    2
    Cc: sunny@t480s, yena@t480s
    3
    Bcc: jaesoo@t480s
    

  • What's remarkable about the Noninteractive Sending Mode?

    • Subject and recipients can be obtained from shell variables.

    • Message body could come from the output of another program.

    • Designing automated mailing lists is possible by using these features.

Receiving Mail

• Incoming mail is appended to the mailbox, a text file named after the user-id of the recipient.

  • mailbox location: /var/mail (/var/spool/mail in Linux.)

    e.g., /var/mail/klee

• When you are prompted with the following message:

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  You have new mail in /var/mail/klee
  

  run the mailx command in the receiving mode (without using an argument) to check the mailbox and the system will display the headers and some credentials of all incoming mail still saved in the mailbox:

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  1
  $ mailx
  2
  "/var/mail/klee": 3 messages 3 new
  3
  >N   1 klee               Fri Feb 11 16:30  14/393   Interactive Sending Mode Test 1
  4
   N   2 klee               Fri Feb 11 16:37  13/378   Interactive Sending Mode Test 2
  5
   N   3 klee               Fri Feb 11 16:53  13/432   Noninteractive Sending Mode Test
  6
  ? _
  

  Press the number or [Enter] to view the message that the number or the > is positioned on, respectively.

  Press q to quit.

• Already seen mails will be moved from the mailbox to the mbox located in the user's home directory.

mailx Internal Commands

• Replying to mail

  • r (reply) command switches mailx to the sending mode and retrieves the sender's detail.

• Saving messages

  • w command saves one or more messages in separate files.

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    $ w myfile              (Appends current message to 'myfile')
    2
    $ w 1 2 3 myfile        (Appends first three messages to 'myfile')
    

    To view these messages with their headers:

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    $ mailx -f myfile
    

• Deleting Mail

  • d (delete) command marks a mail for deletion; the mail gets deleted when quitting mailx.

passwd: Changing Your Password

• passwd command changes the user's password.

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  $ passwd
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  Changing password for klee.
  3
  Current password:                         (Asks for old password)
  4
  New password:                             (New password to be entered twice)
  5
  Retype new password:
  6
  passwd: password updated successfully
  

  The new password is encrypted (scrambled) by the system and stored in the file /etc/shadow (/etc/passwd on older systems).

  The system will authenticate with this file everytime the user logs in.

• Nonprivileged user can change only their own password.

• Privileged user, the system administrator, can change any user's password using the same command.

uname: Your Machine's Name and Operating System

• The uname command displays certain features of the operating system running on your machine.

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  $ uname
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  Linux                 (Output depends on the system)
  

  By default, it simply displays the name of the operating system.

• Current release

  • -r option displays the version of your operating system.

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    $ uname -r
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    5.13.0-28-generic     (In Linux, this means 'Kernel version 5.13')
    

• Machine name

  • -n option displays your hostname.

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    $ uname -r
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    t480s
    

    Every machine has a name (the hostname), and if your network is connected to the Internet, this hostname is a component of your machine's domain name.

who: Know the Users

• The who command shows the information of users logged in.

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  $ who
  2
  root      console     Feb 12 08:51    (:0)
  3
  hkim      pts/10      Feb 12 08:59    (pc123.sunnykim.com)
  

  1^st column: user-ids currently working on the system. (2 users)

  2^nd column: filenames of the devices associated with the respective terminals. (klee's terminal has the name pts/10 (a file named 10 in the pts directory).

  3^rd, 4^th, 5^th comumns: date and time of logging in.

  Last column: hostname from where the user logged in. (skim have logged in remotely from the different hostl.)

• To specify who invoked the who command, use the argument am and i.

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  $ who am i
  2
  hkim      pts/10      Feb 12 08:59    (pc123.sunnykim.com)
  

  This didn't work on Ubuntu 20.04 (5.13.0-28-generic).

date: Displaying the System Date

• The UNIX internal clock stores the number of seconds elapsed since the Epoch: January 1, 1970.

• The date command shows the date and time to the nearest second:

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  $ date
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  Fri 11 Feb 2022 10:07:22 PM CST
  

• +-prefixed format specifiers as arguments can be used:

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  $ date +%m            (Print 'month' only)
  2
  02
  

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  $ date +%h            (Print 'month' name only)
  2
  Feb
  

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  $ date +"%h %m"       (Print them combined)
  2
  Feb 02
  

  When using multiple format specifiers, enclose them within quotes (single or double) and use + symbol as a prefix.

  d: The day of the mont (1 to 31)

  y: The last two digits of the year

  H, M and S: The hour, minute, and second, repectively

  D: The date in the format mm/dd/yy

  T: The time in the format hh:mm:ss

stty: When Things Go Wrong

• The stty command changes terminal settings. (Displays them when used with the -a option.

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  $ stty -a
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  speed 38400 baud; rows 57; columns 102; line = 0;
  3
  intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>;
  4
  start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 0;
  5
  -parenb -parodd -cmspar cs8 -hupcl -cstopb cread -clocal -crtscts
  6
  -ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon -ixoff -iuclc -ixany -imaxbel
  7
  iutf8
  8
  opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
  9
  isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke -flusho
  10
  -extproc
  

• Keyword forms:

  • keyword = value

  • Keyword or -keyword (The - prefix means that the option is turned off.)

• Changing the settings

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  $ stty intr DEL           (Define [Delete] key as the 'interrupt' key)
  2
  $ stty intr /^c           (Define it back to [Ctrl-c])
  

  All keys shown above can be mapped differently the same way.

• Resetting the settings

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  $ stty sane               (Restore sanity to the terminal)
  2
  $ reset                   (Restore sanity to the terminal)
  

• When things go wrong:

The X Window System

• X Window System is the default GUI for UNIX systems. It is a lightweight windowing system that allows for graphical windows to be displayed on a UNIX system.

  • Displays every application in a separate window.

  • Uses mouse to run programs, display menus, select options, cut-copy-paste operations.

  • Because X (i.e., X Windows) uses a bit-mapped display (where every pixel on the screen is handled individually), web browsers like Mozilla and Konqueror MUST run under X.

• Unlike Microsoft Windows, the look-and-feel of the X Window system varies across UNIX and Linux systems.

  • e.g., Common Desktop Environment (CDE), GNOME, KDE, etc.

Terminal Emulator

• X supports terminal emulator program that runs a shell in a window.

  • You can use every UNIX command on this window.

  • You can invoke any graphical X program on this window.

File Manager

• Every X implementation offers a file management program that can perform copying, moving and deleting, etc.