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Created page with "* Simple filter commands: head, tail, cut, sort, tr, wc * grep utility * stdin, stdout, stderr * Redirection and piping * /dev/null file = head and tail commands = These com..."
* Simple filter commands: head, tail, cut, sort, tr, wc
* grep utility
* stdin, stdout, stderr
* Redirection and piping
* /dev/null file
= head and tail commands =
These commands display the beginning or the end of a file respectively. By default, 10 lines are displayed. The entire file will be displayed if it is less than 10 lines in length
<source lang="bash"># head [-line_count] file
$ head -3 users.log
</source>
= cut =
* Selects fields or columns from files or standard input
* Range can be specified in multiple ways:
** 1-10 - first 10
** 3-8 - 3rd to 8th
** -10 - up to 10th
** 2- - from 2nd until the end of line
** 1-3,4,10- - combination of above
* Important options:
** -c - cut characters
** -f - cut fields
* Default field delimiter is the tab
* Other field delimiter can be specified using the -d option
* Field delimiter must be a single character, only one delimiter is supported
* If special characters are used for delimiters they must be quoted
<source lang="bash"># will cut first 2 characters
$ cut -c 1-2
# will cut 2nd and 5th field
$ cut -f 2,5
# will cut first 2 fields delimited with a comma
$ cut -d, -f1-2
# space is the field delimiter
$ cut -d” “ -f1
</source>
= sort command =
* Sorts single files or standard input
* Merges and sorts multiple files
* Is able to sort by fields
* Popular options:
** -f - ignore case in comparisons
** -n - numeric sort
** -u - display unique entries
** -r - reverse sort
= wc =
* Counts the number of lines, words and/or characters in a file
* Usage: wc option [filename]
* Options:
** -l - count lines
** -w - count words (delimited by whitespace)
** -m - count characters
* If no option is specified all 3 counts are displayed
= grep utility =
* Searches for literal text and text patterns
* Pattern-based searches will be covered in detail next week
* Example usage: <code>grep student *</code>
* Works with files and/or standard input
* Acts like a filter - outputs only lines which are successfully matched to a given regular expression
* A successful match can be entire line or any part of it
* The entire line that has the match inside will be displayed
* Useful grep options
** -i - ignores case
** -n - numbers lines in the output
** -v - reverse match
** -c - displays the count of matched lines
= Standard Input and Standard Output =
* Standard input (stdin) is a general term which describes how or where a command receives information from
* When no information is coming from standard input a command usually has defaults or expects an argument (parameter). Typically such parameter would be a file name
* Standard output (stdout) describes the place where or how the commands sends its output
* For most commands the standard input and output are your terminal's keyboard and screen
* Standard input can be redirected from a file or piped from another command
* Standard output can be redirected to a file or piped to another command
= Standard Input Redirection =
<code>command < filename</code>
* Example: <code>tr ‘a-z’ ‘A-Z’ < ls.txt=</code>
* Used for commands which do not accept a file as argument
= Standard Output Redirection =
<code>command > filename</code>
* Redirtects a command's standard output to a file
* Stdout redirection is represented by the <code>></code> symbol.
* Example: <code>ls > ls.txt</code> will redirect output from the <code>ls</code> command into a file called <code>ls.txt</code>. In other words the output of <code>ls</code> will be saved to <code>ls.txt</code>
* If the file exists already its content will be replaced
* To append to a file, the <code>>></code> symbol can be used
= Standard Error =
* In addition to standard input and standard output UNIX commands have standard error
* Standard error is the place where error messages are sent to
* By default error messages are sent to the terminal
* Standard error can be redirected by using the <code>2></code> or <code>2>></code> redirection operators
* Sometimes you might want to redirect the standard error to the same place as standard output
* Use the <code>2>&1</code> redirection for that
= Inter-process communication =
* Commands can send their standard output directly to standard input of other commands
* A few simple commands can form a more powerful one
* No temporary files are necessary
* This is achieved by using pipes and tees
= Pipes =
* Pipes are represented by <code>|</code>
* Many commands can be “piped” together, but filter commands use them especially often
* Each filter processes the initial input based on it's design
* Filters must be chained in specific order
* Example piping use: <code>ls | less</code>
= Tee =
* UNIX pipe with the tee utility can be used to split the flow of information
* Example: <code>ls | tee unsorted.txt | sort</code>
= /dev/null file =
* The /dev/null file (sometimes called the bit bucket or black hole) is a special system file that discards all data written into in
* Useful to discard unwanted command output,
* Example: <code>find / -name “homer” 2> /dev/null</code>
* Also, /dev/null can provide null data (EOF only) to processes reading from it
* Useful to purge (empty) files etc, for example: <code>cat /dev/null > ~/.bash_history</code>
= “Here” documents =
* The << symbol indicates a “here” document
<source lang="bash">sort << EOF
word
name
car
EOF
</source>
* Anything between EOF…EOF is sent to the standard input of a utility
* You can use some other string/symbol instead of “EOF”
* grep utility
* stdin, stdout, stderr
* Redirection and piping
* /dev/null file
= head and tail commands =
These commands display the beginning or the end of a file respectively. By default, 10 lines are displayed. The entire file will be displayed if it is less than 10 lines in length
<source lang="bash"># head [-line_count] file
$ head -3 users.log
</source>
= cut =
* Selects fields or columns from files or standard input
* Range can be specified in multiple ways:
** 1-10 - first 10
** 3-8 - 3rd to 8th
** -10 - up to 10th
** 2- - from 2nd until the end of line
** 1-3,4,10- - combination of above
* Important options:
** -c - cut characters
** -f - cut fields
* Default field delimiter is the tab
* Other field delimiter can be specified using the -d option
* Field delimiter must be a single character, only one delimiter is supported
* If special characters are used for delimiters they must be quoted
<source lang="bash"># will cut first 2 characters
$ cut -c 1-2
# will cut 2nd and 5th field
$ cut -f 2,5
# will cut first 2 fields delimited with a comma
$ cut -d, -f1-2
# space is the field delimiter
$ cut -d” “ -f1
</source>
= sort command =
* Sorts single files or standard input
* Merges and sorts multiple files
* Is able to sort by fields
* Popular options:
** -f - ignore case in comparisons
** -n - numeric sort
** -u - display unique entries
** -r - reverse sort
= wc =
* Counts the number of lines, words and/or characters in a file
* Usage: wc option [filename]
* Options:
** -l - count lines
** -w - count words (delimited by whitespace)
** -m - count characters
* If no option is specified all 3 counts are displayed
= grep utility =
* Searches for literal text and text patterns
* Pattern-based searches will be covered in detail next week
* Example usage: <code>grep student *</code>
* Works with files and/or standard input
* Acts like a filter - outputs only lines which are successfully matched to a given regular expression
* A successful match can be entire line or any part of it
* The entire line that has the match inside will be displayed
* Useful grep options
** -i - ignores case
** -n - numbers lines in the output
** -v - reverse match
** -c - displays the count of matched lines
= Standard Input and Standard Output =
* Standard input (stdin) is a general term which describes how or where a command receives information from
* When no information is coming from standard input a command usually has defaults or expects an argument (parameter). Typically such parameter would be a file name
* Standard output (stdout) describes the place where or how the commands sends its output
* For most commands the standard input and output are your terminal's keyboard and screen
* Standard input can be redirected from a file or piped from another command
* Standard output can be redirected to a file or piped to another command
= Standard Input Redirection =
<code>command < filename</code>
* Example: <code>tr ‘a-z’ ‘A-Z’ < ls.txt=</code>
* Used for commands which do not accept a file as argument
= Standard Output Redirection =
<code>command > filename</code>
* Redirtects a command's standard output to a file
* Stdout redirection is represented by the <code>></code> symbol.
* Example: <code>ls > ls.txt</code> will redirect output from the <code>ls</code> command into a file called <code>ls.txt</code>. In other words the output of <code>ls</code> will be saved to <code>ls.txt</code>
* If the file exists already its content will be replaced
* To append to a file, the <code>>></code> symbol can be used
= Standard Error =
* In addition to standard input and standard output UNIX commands have standard error
* Standard error is the place where error messages are sent to
* By default error messages are sent to the terminal
* Standard error can be redirected by using the <code>2></code> or <code>2>></code> redirection operators
* Sometimes you might want to redirect the standard error to the same place as standard output
* Use the <code>2>&1</code> redirection for that
= Inter-process communication =
* Commands can send their standard output directly to standard input of other commands
* A few simple commands can form a more powerful one
* No temporary files are necessary
* This is achieved by using pipes and tees
= Pipes =
* Pipes are represented by <code>|</code>
* Many commands can be “piped” together, but filter commands use them especially often
* Each filter processes the initial input based on it's design
* Filters must be chained in specific order
* Example piping use: <code>ls | less</code>
= Tee =
* UNIX pipe with the tee utility can be used to split the flow of information
* Example: <code>ls | tee unsorted.txt | sort</code>
= /dev/null file =
* The /dev/null file (sometimes called the bit bucket or black hole) is a special system file that discards all data written into in
* Useful to discard unwanted command output,
* Example: <code>find / -name “homer” 2> /dev/null</code>
* Also, /dev/null can provide null data (EOF only) to processes reading from it
* Useful to purge (empty) files etc, for example: <code>cat /dev/null > ~/.bash_history</code>
= “Here” documents =
* The << symbol indicates a “here” document
<source lang="bash">sort << EOF
word
name
car
EOF
</source>
* Anything between EOF…EOF is sent to the standard input of a utility
* You can use some other string/symbol instead of “EOF”
