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--- os_cp:redirects_pipes [2023/03/22 01:27]
jkonczak utworzono
+++ os_cp:redirects_pipes [2026/03/24 14:33] (current)
jkonczak [more, less]
@@ Line 1: / Line 1: @@
-===== Standard streams =====
-|  K&R C[[https://archive.org/details/TheCProgrammingLanguageFirstEdition|[1]]] [[https://en.wikipedia.org/wiki/C_(programming_language)#K&R_C|[2]]]<code c>printf("Please type your name:\n")
-scanf("%s", name);</code>  |  Python<code python>print("Please type your name:")
-name = input()</code>  |
-Did you ever wonder how does a program know where to read input from and where
-the print-like functions should output?
-**In UNIX world a program expects to have three files already open upon
-start – standard input, standard output and standard error.
-These are called the [[https://en.wikipedia.org/wiki/Standard_streams|standard streams]].**
-\\
-<small>
-The standard input/output library of the C programming language – ''stdio.h'' –
-bases on this concept. C was created by one of the authors of UNIX.
-</small>
-Basic I/O functions in most programming languages by default read from the
-standard input, and output data to standard output. \\
-The rationale of standard error stream is to convey information on what went
-wrong while executing a program. Programming languages usually offer dedicated
-functions to output data to standard error.
-In Unix-like, as well as POSIX-compatible systems, the operating system is
-responsible for abstracting files away – the user should not worry about
-details of accessing a file.
-\\
-When the user wants to open a file, the user provides the file name and gets an
-identifier – a **file descriptor** in return. \\
-(A file descriptor is in fact an index in an array of files maintained for the
-process by the OS.) \\
-To do standard operations such as reading or writing data, the user just tells
-which operation shall be executed, on which file descriptor, and the user shall
-provide the details of the operations (such as where to put data read from
-the file and how many bytes shall be read).
-A child process inherits all file descriptors from its parent.
-**The three standard streams are the files represented by first three file
-descriptors – 0 is always used for standard input, 1 for stand output and 2 for
-standard error.**
-The files do not need to be ordinary files – Unix-like systems abstract almost
-everything with a [[https://en.wikipedia.org/wiki/Everything_is_a_file|file]].
-\\
-For instance, a terminal device is a file (even if it were a real teletype).
-By default, a shell opens the terminal as file 0, 1 and 2.
 ===== Outputting file / text / sequences =====
@@ Line 58: / Line 7: @@
 Name comes from //con__cat__enate//. \\
 ''cat'' numbers lines with the ''-n'' switch and outputs non-printable characters
-as ''^//x//'', ''M-//x//'', … with the ''-v'' switch.
+as ''^//x//'', ''M-//x//'', … with the ''-v'' switch((This notation corresponds to keys one would have to press to input the byte. To see all bytes (''\t'' and ''\n'' is replaced by ''X''), try: \\ ''perl -e'for(0..15){printf"\t%x_",$_};print"\n";for$l(0..15){printf"_%x",$l;for$h(0..15){$c=$h<<4|$l;$c=88 if $c==9||$c==10;printf("\t%c",$c)}print"\n"}'|cat -v'')).
 ''**paste** //file_1// [//file2//]...'' reads round-robin one line from each input
@@ Line 65: / Line 14: @@
 <small>
+''**join** [-t //separator//] [-1 //fieldNumberInFile1//] [-2 //fieldNumberInFile2//] //file1// //file2//''
+parses two sorted files and prints their lines joined on specified fields.
 ''**fold** [-w //width//] [//file//]...'' outputs input files (or standard input)
 forcing a line break whenever a line would exceed //width// (that defaults to 80).
 \\
 With the ''-s'' switch ''fold'' breaks lines on spaces (or at //width// if there are no spaces).
+''**column** [-x] [//file//]...'' works just like ''cat'' if the longest line in the //file//s (or standard input) would not fit twice within the terminal width. \\ Else, it prints the input in as many columns as fit the terminal, filling column first (or, with ''-x'', rows first).
+\\
+''**column -t** [//file//]...'' does a completely different thing: it detects columns in input (by a separator that defaults to whitespace) and outputs the input as a table.
 ''**od** [-t x1]'', ''**hexdump** [-C]'', and ''**xxd**'' show binary files.
 </small>
+++++ Examples | {{page>so:redirects:cat&inline}} ++++
 ~~Exercise.#~~ Print any file with ''cat''. Print two files at once with ''cat''. \\
@@ Line 88: / Line 46: @@
 ==== Printing text ====
-''**echo** //text//'' outputs //text// followed by a newline (unless ''-n'' is specified). \\
+''**echo** //text//'' outputs //text// followed by a newline. \\
-The ''-e'' switch turns backslash escapes into corresponding characters, e.g., ''\t''
+<small>
-becomes a tab and ''\n'' a newline (cf. manual).
+''echo'' is present in almost any shell of any operating system,
+but there is no consensus on how to interpret switches and how to treat
+backslashes in the text.
+\\
+In Bash, the  ''-n'' switch supresses the newline, and ''-e'' turns backslash
+escapes into corresponding characters, e.g., ''\t'' becomes a tab and ''\n''
+a newline (cf. manual).
+</small>
 ''**printf** //format// [//arguments//]...'' works roughly the same as the ''printf'' function in C.
 <small>
-''**figlet** [//text//]'' outputs //text// or the standard input by using ascii-art font.
+Here and there, the course materials make use of the following toy commands:
+\\
+''**figlet** [//text//]'' outputs //text// or the standard input by using an ascii-art font.
+\\
 ''**cowsay** [//text//]'' makes a cow say the //text// (or the standard input).
 </small>
-~~Exercise.#~~ Try ''echo -e 'foo\n\nbaz' '' \\ and ''echo -e '\n\n one \033[A \033[A two \033[B \033[B \n \033[1;31m red \033[0m' ''
+++++ Examples | {{page>so:redirects:echo&inline}} ++++
+~~Exercise.#~~ Try the commands ''echo -e 'foo\n\nbaz' '' \\ and ''echo -e '\n\n one \033[A \033[A two \033[B \033[B \n \033[1;31m red \033[0m' '' \\ <small>[[https://en.wikipedia.org/wiki/ANSI_escape_code|ANSI escape codes]] are well summarized [[https://gist.github.com/fnky/458719343aabd01cfb17a3a4f7296797|here]]</small>
 ~~Exercise.#~~ Try ''printf "|%4.2f|%3s|%-20s|\n|%4.2f|%3s|%-20s|\n" 3.1428 pi circumference/radius 9.8 g gravity''
@@ Line 116: / Line 85: @@
 \\
 With the ''-w'' switch, ''seq'' makes all number of equal width (e.g., ''seq -w 8 11'' outputs 08, 09, 10 and 11).
+++++ Examples | {{page>so:redirects:seq&inline}} ++++
 ~~Exercise.#~~ Generate a sequence of numbers from 1 to 15.
 ~~Exercise.#~~ Generate a sequence of numbers from 64 to 1024 with step of 64.
+===== Standard streams =====
+|  K&R C[[https://archive.org/details/TheCProgrammingLanguageFirstEdition|[1]]] [[https://en.wikipedia.org/wiki/C_(programming_language)#K&R_C|[2]]]<code c>printf("Please type your name:\n")
+scanf("%s", name);</code>  |  Python<code python>print("Please type your name:")
+name = input()</code>  |
+Did you ever wonder how does a program know where to read input from and where
+the print-like functions should output?
+**In UNIX world a program expects to have three files already open upon
+start – standard input, standard output and standard error.
+These are called the [[https://en.wikipedia.org/wiki/Standard_streams|standard streams]].**
+\\
+<small>
+The standard input/output library of the C programming language – ''stdio.h'' –
+bases on this concept. C was created by one of the authors of UNIX.
+</small>
+Basic I/O functions in most programming languages by default read from the
+standard input, and output data to standard output. \\
+The rationale of standard error stream is to convey information on what went
+wrong while executing a program. Programming languages usually offer dedicated
+functions to output data to standard error.
+In Unix-like, as well as POSIX-compatible systems, the operating system is
+responsible for abstracting files away – the user should not worry about
+details of accessing a file.
+\\
+When the user wants to open a file, the user provides the file name and gets an
+identifier – a **file descriptor** in return. \\
+(A file descriptor is in fact an index in an array of files maintained for the
+process by the OS.) \\
+To do standard operations such as reading or writing data, the user just tells
+which operation shall be executed, on which file descriptor, and the user shall
+provide the details of the operations (such as where to put data read from
+the file and how many bytes shall be read).
+A child process inherits all file descriptors from its parent.
+**The three standard streams are the files represented by first three file
+descriptors – 0 is always used for standard input, 1 for stand output and 2 for
+standard error.**
+The files do not need to be ordinary files – Unix-like systems abstract almost
+everything with a [[https://en.wikipedia.org/wiki/Everything_is_a_file|file]].
+\\
+For instance, a terminal device is a file (even if it were a real teletype).
+By default, a shell opens the terminal as file 0, 1 and 2.
 ===== Redirections =====
@@ Line 161: / Line 182: @@
 ~~Exercise.#~~ Redirect standard output and the standard error of the ''find /var/spool/'' command to the same file.
+++++ Examples | {{page>so:redirects:out-en&inline}} ++++
 === Input redirections ===
@@ Line 189: / Line 212: @@
 ~~Exercise.#~~ Use ''bc'' to calculate ''sqrt(2.0000)'' in non-interactive mode and redirect its output to a file.
+++++ Examples | {{page>so:redirects:in&inline}} ++++
 <small>
@@ Line 211: / Line 236: @@
 | ''>>'' | opens //word// for appending and replaces //file_number// with the file |
 | ''<>'' | opens //word// for reading and writing and replaces //file_number// with the file |
-| ''<<'' | 1) creates a temporary file \\ 2) reads an input line \\ 3) if the line is //word//, go to step 6 \\ 4) if there are no quotes (a pair of ''"'' or <html><code>'</code></html>) in //word//, performs the expansion((E.g., ''$VAR'' is substituted with its value, ''`date`'' is replaced by output of the date command etc.)) on the line \\ 5) writes the line to the temporary file \\ 6) opens the temporary file for reading  \\ 7) replace //file_number// with the file \\ The command is run once this is done |
+| ''<<'' | 1) creates a temporary file \\ 2) reads an input line \\ 3) if the line is //word//, goes to step 7 \\ 4) if there are no quotes (a pair of ''"'' or <html><code>'</code></html>) enclosing the //word//, performs the expansion((E.g., ''$VAR'' is substituted with its value, ''`date`'' is replaced by output of the date command etc.)) on the line \\ 5) appends the line to the temporary file \\ 6) goes to step 2 \\ 7) opens the temporary file for reading  \\ 8) replace //file_number// with the file \\ The command is run once this is done |
 | ''<<-''| same as ''<<'', but after step 2 adds a step: \\ 2a) erase all leading tab characters (''\t'') \\ warning: spaces are not erased |
 | ''<<<''| warning: this is a Bash extension \\ 1) creates a temporary file \\ 2) writes //word// to it \\ 3) writes a newline to it \\ 4) opens the temporary file for reading  \\ 5) replace //file_number// with the file \\ The command is run once this is done |
-| ''<&'' | if //word// is a number: duplicates a readable descriptor number //word// to the number //file_number// \\  if //word/// is ''-'': closes a descriptor number //file_number// |
+| ''<&'' | if //word// is a number: duplicates a readable descriptor number //word// to the number //file_number// \\  if //word// is ''-'': closes a descriptor number //file_number// |
-| ''>&'' | if //word// is a number: duplicates a writeable descriptor number //word// to the number //file_number//  \\  if //word/// is ''-'': closes a descriptor number //file_number// |
+| ''>&'' | if //word// is a number: duplicates a writeable descriptor number //word// to the number //file_number//  \\  if //word// is ''-'': closes a descriptor number //file_number// |
 | ''&>'' | warning: this is a Bash extension \\ warning: this does not allow providing //file_number// (in fact, ''&'' is the file number) \\ opens //word// for writing, truncates it and replaces streams 1 and 2 with the file |
 | ''&>>''| warning: this is a Bash extension \\ warning: this does not allow providing //file_number// (in fact, ''&'' is the file number) \\ opens //word// for appending and replaces streams 1 and 2 with the file |
@@ Line 231: / Line 256: @@
 standard output (unless you dare to face the consequences).
-~~Exercise.#~~ Swap standard input with standard output of the
+~~Exercise.#~~ Swap standard error with standard output of the
 ''cat /etc/motd /etc/shadow'' command. Test if you did this correctly by adding
 ''|rev'' at the end (that will put standard output backwards).
@@ Line 305: / Line 330: @@
 All that is written to a pipe is stored in the main memory (so it never occupies
 disk space, regardless if it is a named pipe) until some program reads it.
+~~Exercise.#~~ Run ''echo '2+2*2' ''. Then, pipe it through ''bc''.
 ~~Exercise.#~~ ''echo'' some text. Then, echo the text and pipe it through ''xxd''.
@@ Line 319: / Line 346: @@
 ~~Exercise.#~~ Create a named pipe //p//. Redirect input of ''fold'' from //p//
 in one terminal, and redirect output of ''ps -eF'' to //p// in another terminal.
-\\ Then repeat the commands, running the''ps'' before the ''fold''.
+\\ Then repeat the commands, running the ''ps'' before the ''fold''.
 </small>
+++++ Examples | {{page>so:redirects:pipes1-en&inline}} ++++
 ===== Filters =====
@@ Line 349: / Line 378: @@
 ~~Exercise.#~~ Run ''seq 25 > //file//''. Then run ''tail -f //file//'' in one
 terminal and append (with output redirection) some data to //file//.
+++++ Examples | {{page>so:pipes_filters:head_tail&inline}} ++++
 ==== grep ====
@@ Line 360: / Line 391: @@
 <small>
 The ''grep'' program accepts several regular expression grammars.
-POSIX [[https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html|specifies]]
+POSIX [[https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap09.html|specifies]]
 basic (default for ''grep'') and extended regular expressions (selectable with
 ''egrep'' or ''grep -E'' ). See manual for your implementation of ''grep'' for
@@ Line 395: / Line 426: @@
 \\
 List all files containing ''ecdsa'' in ''~/.ssh''.
+++++ Examples | {{page>so:pipes_filters:grep&inline}} ++++
 ==== cut ====
@@ Line 401: / Line 434: @@
 bytes (''-b //spec//'') / fields (''-f //spec//'') in each line.
 \\
-A filed is any number of characters separated by a single-character
+A field is any number of characters separated by a single-character
 delimiter (''-d //delim//'', defaults to tab).
@@ Line 416: / Line 449: @@
 ''egrep '^[Ee]{2}' /usr/share/myspell/en_US.dic'' a slash and all that follows
 it.
+++++ Examples | {{page>so:pipes_filters:cut-en&inline}} ++++
 ==== sort ====
@@ Line 460: / Line 495: @@
 ~~Exercise.#~~ Sort the file by the second column (alphabetically) and by the third column (numerically).
 </small>
+++++ Examples | {{page>so:pipes_filters:sort&inline}} ++++
 ==== wc, uniq, nl ====
@@ Line 492: / Line 529: @@
 \\
 Then pipe it through ''sort'' and ''uniq'' so that you see how many times each result was hit.
+++++ Examples | {{page>so:pipes_filters:wc_uniq&inline}} ++++
 <small>
@@ Line 526: / Line 565: @@
   * to squeeze all spaces
   * remove all the letters ''rwx''
+++++ Examples | {{page>so:pipes_filters:tr-en&inline}} ++++
 <small>
@@ Line 568: / Line 609: @@
 not human-readable file format.
 \\
-''[[https://github.com/wofr06/lesspipe|lesspipe]]'' is the leading implementation
+''[[https://lesspipe.org|lesspipe]]'' is the leading implementation
 of this feature.
 </small>
@@ Line 594: / Line 635: @@
 <small>
 ~~Exercise.#~~ Open a PDF file (e.g., ''/usr/share/doc/packages/apparmor-docs/techdoc.pdf'') with less, with and without -L option. \\
-Open a ''tar'' archive with less (e.g., ''/usr/share/doc/packages/automake/amhello-1.0.tar.gz'').
+Open a ''tar'' archive with less (e.g., ''/usr/share/doc/packages/automake/amhello-1.0.tar.gz'').\\
+View a directory (e.g., ''/usr/include'') with less.
 </small>

Jan Kończak

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