grep, awk and sed are three of the most useful command-line tools1 in *nix world. And this article will give you a brief introduction and basic usages of these three different commands.

grep

grep (Global Regular Expression Print) is used to search for specific terms in a file.

Different from awk and sed, grep can’t add/modify/remove the text in a specific file. But it’s useful when we just want to search and filter out matches.

典型用法

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# print every line that contains the word 'test'
grep 'test' file.txt

  • -E / -P: Use extended / Perl compatible regular expression syntax.

  • -n: Show line number before each line.

  • -o: Only show the matching segment of the line.

  • -v: Print all of the lines that DO NOT match the search pattern.

  • -c: Show the number of the lines that contains the search pattern.

  • -i: Ignore case.

awk

awk is a text pattern scanning and processing language, which is created by Aho, Weinberger & Kernighan. awk is mostly used for data extraction and reporting (dealing with .csv files).

Each awk procedure can be divided into three sections:

BEGIN { … initialization awk commands …}

{ … awk commands for each line of the file …}

END { … finalization awk commands …}

  • Control flow

if (condition) statement [ else statement ]

while (condition) statement

do statement while (condition)

for (expr1; expr2; expr3) statement

for (var in array) statement

break

continue

exit [ expression ]

  • 内置变量
Variable Meaning
$0 Current line
$1 - $n The nth field
FS Input field separator, default value is “ “
NF The number of fields/columns
NR The number of records/rows
FNR The number of records relative to the current input file
OFS The output field separator, default value is “ “
ORS The output record separator, default value is “\n”
  • 典型用法
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# print 1st and 4th column
awk '{print $1, $4}' file.txt

# same with 'cat file.txt'
awk '{print $0}' file.txt
  • -F: Set input field sparator
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# print 1st and 4th column separated by ':'
awk -F: '{print $1, $4}' file.txt
# or
awk 'BEGIN {FS = ":"} {print $1, $4}' file.txt
  • Pattern matching
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# print 2nd column when 1st field is precisely 'test'
awk '$1 == "test" { print $2 }' file.txt

# print 2nd column when 1st field contains 'test'
awk '$1 ~ /test/ { print $2 }' file.txt

# print 2nd column when 1st field DOES NOT contain 'test'
awk '$1 !~ /test/ { print $2 }' file.txt

# print 2nd column when this record contains 'test'
awk '/test/ { print $2 }' file.txt

# print 2nd column when this record DOES NOT contain 'test'
awk '! /test/ { print $2 }' file.txt
  • In-place editing: -i (GNU awk 4.1.0 or later…)

sed