329 lines
11 KiB
Markdown
329 lines
11 KiB
Markdown
# AWK
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AWK (awk) is a domain-specific language designed for text processing and typically used as a data extraction and reporting tool. Similar to the **[Sed](sed)** and **[Grep](grep)** commands, it is a filter, and is a standard feature of most Unix-like operating systems, like **[Linux](linux)**.
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## Usage
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### Unix/Linux
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```bash
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awk '/pattern/ {print "$1"}' # standard Unix shells
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```
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### DOS/Win
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```bash
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awk '/pattern/ {print "$1"}' # compiled with DJGPP, Cygwin
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awk "/pattern/ {print \"$1\"}" # GnuWin32, UnxUtils, Mingw
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```
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Note that the DJGPP compilation (for DOS or Windows-32) permits an awk
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script to follow Unix quoting syntax `'/like/ {"this"}'`. HOWEVER, if the
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command interpreter is `CMD.EXE` or `COMMAND.COM`, single quotes will not
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protect the redirection arrows `(<, >)` nor do they protect pipes `(|)`.
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These are special symbols which require "double quotes" to protect them
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from interpretation as operating system directives. If the command
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interpreter is bash, ksh, zsh or another Unix shell, then single and double
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quotes will follow the standard Unix usage.
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Users of MS-DOS or Microsoft Windows must remember that the percent
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sign `(%)` is used to indicate environment variables, so this symbol must
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be doubled `(%%)` to yield a single percent sign visible to awk.
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To conserve space, use `'1'` instead of `'{print}'` to print each line.
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Either one will work.
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## Handy one-line Awk scripts
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### File Spacing
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```bash
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# double space a file
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awk '1;{print ""}'
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awk 'BEGIN{ORS="\n\n"};1'
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# double space a file which already has blank lines in it. Output file
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# should contain no more than one blank line between lines of text.
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# NOTE: On Unix systems, DOS lines which have only CRLF (\r\n) are
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# often treated as non-blank, and thus 'NF' alone will return TRUE.
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awk 'NF{print $0 "\n"}'
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# triple space a file
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awk '1;{print "\n"}'
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```
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### Numbering and Calculations
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```bash
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# precede each line by its line number FOR THAT FILE (left alignment).
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# Using a tab (\t) instead of space will preserve margins.
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awk '{print FNR "\t" $0}' files*
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# precede each line by its line number FOR ALL FILES TOGETHER, with tab.
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awk '{print NR "\t" $0}' files*
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# number each line of a file (number on left, right-aligned)
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# Double the percent signs if typing from the DOS command prompt.
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awk '{printf("%5d : %s\n", NR,$0)}'
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# number each line of file, but only print numbers if line is not blank
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# Remember caveats about Unix treatment of \r (mentioned above)
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awk 'NF{$0=++a " :" $0};1'
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awk '{print (NF? ++a " :" :"") $0}'
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# count lines (emulates "wc -l")
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awk 'END{print NR}'
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# print the sums of the fields of every line
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awk '{s=0; for (i=1; i<=NF; i++) s=s+$i; print s}'
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# add all fields in all lines and print the sum
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awk '{for (i=1; i<=NF; i++) s=s+$i}; END{print s}'
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# print every line after replacing each field with its absolute value
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awk '{for (i=1; i<=NF; i++) if ($i < 0) $i = -$i; print }'
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awk '{for (i=1; i<=NF; i++) $i = ($i < 0) ? -$i : $i; print }'
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# print the total number of fields ("words") in all lines
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awk '{ total = total + NF }; END {print total}' file
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# print the total number of lines that contain "Beth"
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awk '/Beth/{n++}; END {print n+0}' file
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# print the largest first field and the line that contains it
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# Intended for finding the longest string in field #1
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awk '$1 > max {max=$1; maxline=$0}; END{ print max, maxline}'
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# print the number of fields in each line, followed by the line
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awk '{ print NF ":" $0 } '
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# print the last field of each line
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awk '{ print $NF }'
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# print the last field of the last line
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awk '{ field = $NF }; END{ print field }'
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# print every line with more than 4 fields
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awk 'NF > 4'
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# print every line where the value of the last field is > 4
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awk '$NF > 4'
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```
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### String Creation
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```bash
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# create a string of a specific length (e.g., generate 513 spaces)
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awk 'BEGIN{while (a++<513) s=s " "; print s}'
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# insert a string of specific length at a certain character position
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# Example: insert 49 spaces after column #6 of each input line.
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gawk --re-interval 'BEGIN{while(a++<49)s=s " "};{sub(/^.{6}/,"&" s)};1'
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```
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### Array Creation
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```bash
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# These next 2 entries are not one-line scripts, but the technique
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# is so handy that it merits inclusion here.
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# create an array named "month", indexed by numbers, so that month[1]
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# is 'Jan', month[2] is 'Feb', month[3] is 'Mar' and so on.
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split("Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec", month, " ")
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# create an array named "mdigit", indexed by strings, so that
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# mdigit["Jan"] is 1, mdigit["Feb"] is 2, etc. Requires "month" array
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for (i=1; i<=12; i++) mdigit[month[i]] = i
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```
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### Text Conversion and Substitution
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```bash
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# IN UNIX ENVIRONMENT: convert DOS newlines (CR/LF) to Unix format
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awk '{sub(/\r$/,"")};1' # assumes EACH line ends with Ctrl-M
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# IN UNIX ENVIRONMENT: convert Unix newlines (LF) to DOS format
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awk '{sub(/$/,"\r")};1'
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# IN DOS ENVIRONMENT: convert Unix newlines (LF) to DOS format
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awk 1
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# IN DOS ENVIRONMENT: convert DOS newlines (CR/LF) to Unix format
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# Cannot be done with DOS versions of awk, other than gawk:
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gawk -v BINMODE="w" '1' infile >outfile
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# Use "tr" instead.
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tr -d \r <infile >outfile # GNU tr version 1.22 or higher
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# delete leading whitespace (spaces, tabs) from front of each line
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# aligns all text flush left
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awk '{sub(/^[ \t]+/, "")};1'
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# delete trailing whitespace (spaces, tabs) from end of each line
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awk '{sub(/[ \t]+$/, "")};1'
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# delete BOTH leading and trailing whitespace from each line
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awk '{gsub(/^[ \t]+|[ \t]+$/,"")};1'
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awk '{$1=$1};1' # also removes extra space between fields
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# insert 5 blank spaces at beginning of each line (make page offset)
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awk '{sub(/^/, " ")};1'
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# align all text flush right on a 79-column width
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awk '{printf "%79s\n", $0}' file*
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# center all text on a 79-character width
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awk '{l=length();s=int((79-l)/2); printf "%"(s+l)"s\n",$0}' file*
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# substitute (find and replace) "foo" with "bar" on each line
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awk '{sub(/foo/,"bar")}; 1' # replace only 1st instance
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gawk '{$0=gensub(/foo/,"bar",4)}; 1' # replace only 4th instance
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awk '{gsub(/foo/,"bar")}; 1' # replace ALL instances in a line
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# substitute "foo" with "bar" ONLY for lines which contain "baz"
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awk '/baz/{gsub(/foo/, "bar")}; 1'
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# substitute "foo" with "bar" EXCEPT for lines which contain "baz"
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awk '!/baz/{gsub(/foo/, "bar")}; 1'
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# change "scarlet" or "ruby" or "puce" to "red"
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awk '{gsub(/scarlet|ruby|puce/, "red")}; 1'
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# reverse order of lines (emulates "tac")
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awk '{a[i++]=$0} END {for (j=i-1; j>=0;) print a[j--] }' file*
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# if a line ends with a backslash, append the next line to it (fails if
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# there are multiple lines ending with backslash...)
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awk '/\\$/ {sub(/\\$/,""); getline t; print $0 t; next}; 1' file*
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# print and sort the login names of all users
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awk -F ":" '{print $1 | "sort" }' /etc/passwd
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# print the first 2 fields, in opposite order, of every line
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awk '{print $2, $1}' file
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# switch the first 2 fields of every line
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awk '{temp = $1; $1 = $2; $2 = temp}' file
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# print every line, deleting the second field of that line
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awk '{ $2 = ""; print }'
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# print in reverse order the fields of every line
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awk '{for (i=NF; i>0; i--) printf("%s ",$i);print ""}' file
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# concatenate every 5 lines of input, using a comma separator
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# between fields
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awk 'ORS=NR%5?",":"\n"' file
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```
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### Selective Printing of Certain Lines
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```bash
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# print first 10 lines of file (emulates behavior of "head")
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awk 'NR < 11'
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# print first line of file (emulates "head -1")
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awk 'NR>1{exit};1'
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# print the last 2 lines of a file (emulates "tail -2")
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awk '{y=x "\n" $0; x=$0};END{print y}'
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# print the last line of a file (emulates "tail -1")
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awk 'END{print}'
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# print only lines which match regular expression (emulates "grep")
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awk '/regex/'
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# print only lines which do NOT match regex (emulates "grep -v")
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awk '!/regex/'
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# print any line where field #5 is equal to "abc123"
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awk '$5 == "abc123"'
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# print only those lines where field #5 is NOT equal to "abc123"
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# This will also print lines which have less than 5 fields.
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awk '$5 != "abc123"'
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awk '!($5 == "abc123")'
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# matching a field against a regular expression
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awk '$7 ~ /^[a-f]/' # print line if field #7 matches regex
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awk '$7 !~ /^[a-f]/' # print line if field #7 does NOT match regex
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# print the line immediately before a regex, but not the line
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# containing the regex
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awk '/regex/{print x};{x=$0}'
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awk '/regex/{print (NR==1 ? "match on line 1" : x)};{x=$0}'
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# print the line immediately after a regex, but not the line
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# containing the regex
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awk '/regex/{getline;print}'
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# grep for AAA and BBB and CCC (in any order on the same line)
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awk '/AAA/ && /BBB/ && /CCC/'
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# grep for AAA and BBB and CCC (in that order)
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awk '/AAA.*BBB.*CCC/'
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# print only lines of 65 characters or longer
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awk 'length > 64'
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# print only lines of less than 65 characters
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awk 'length < 64'
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# print section of file from regular expression to end of file
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awk '/regex/,0'
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awk '/regex/,EOF'
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# print section of file based on line numbers (lines 8-12, inclusive)
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awk 'NR==8,NR==12'
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# print line number 52
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awk 'NR==52'
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awk 'NR==52 {print;exit}' # more efficient on large files
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# print section of file between two regular expressions (inclusive)
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awk '/Iowa/,/Montana/' # case sensitive
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```
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### Selective Deletion of Certain Lines
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```bash
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# delete ALL blank lines from a file (same as "grep '.' ")
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awk NF
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awk '/./'
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# remove duplicate, consecutive lines (emulates "uniq")
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awk 'a !~ $0; {a=$0}'
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# remove duplicate, nonconsecutive lines
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awk '!a[$0]++' # most concise script
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awk '!($0 in a){a[$0];print}' # most efficient script
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```
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## References
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For additional syntax instructions, including the way to apply editing
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commands from a disk file instead of the command line, consult:
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"sed & awk, 2nd Edition," by Dale Dougherty and Arnold Robbins
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(O'Reilly, 1997)
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"UNIX Text Processing," by Dale Dougherty and Tim O'Reilly (Hayden
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Books, 1987)
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"GAWK: Effective awk Programming," 3d edition, by Arnold D. Robbins
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(O'Reilly, 2003) or at http://www.gnu.org/software/gawk/manual/
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To fully exploit the power of awk, one must understand "regular
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expressions." For detailed discussion of regular expressions, see
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"Mastering Regular Expressions, 3d edition" by Jeffrey Friedl (O'Reilly,
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2006).
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The info and manual ("man") pages on Unix systems may be helpful (try
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"man awk", "man nawk", "man gawk", "man regexp", or the section on
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regular expressions in "man ed").
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USE OF '\t' IN awk SCRIPTS: For clarity in documentation, I have used
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'\t' to indicate a tab character (0x09) in the scripts. All versions of
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awk should recognize this abbreviation.
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