awk

awk Dr. Tran, Van Hoai Faculty of Computer Science and Engineering HCMC Uni. of Technology hoai@cse.hcmut.edu.vn

What is "awk"? • awk = programming language • text/string manipulation within shell script • useful for input data in records/fields

Typical uses • Perform arithmetic and string operations • Use loops and conditionals • Produce formatted report • Process UNIX command arguments • Execute UNIX commands from script • …

Examples (1) • How to print the first three fields of each record on separate lines • Can we do by shell script and "cut" ? • We can do by "awk" in one command awk -F: '{ print $1; print $2; print $3 }' /etc/passwd

Syntax • Command forms awk [options] 'script' var=value file(s) awk [options] -f scriptfile var=value file(s) • Variable assignment on command-line • "value" can be • string, numeric constant • shell variable ($name) • command substitution (`cmd`)

Options • Standard options • Advanced options (see man pages)

Patterns and procedures • Script is pattern { action } • Both are optional • pattern missing, apply to all lines • action missing, matched line is printed

Patterns

Procedures • Separated by newline, semicolons • Contained in "{}" • Command groups

Example (2) • Print first field of each line { print $1 } • Print all lines containing pattern /pattern/ • Print first field of lines containing pattern /pattern/ { print $1 } • Select record having > 2 fields NF > 2

Example (3) • Print only lines which first field matches URGENT $1 ~ /URGENT/ { print $3, $2 } • Print number of lines matching pattern /pattern/ {x++} END {print x} • Sum up column 2 and print the total {total += $2} END { print total}

Example (4) • Print lines containing < 20 characters length($0) < 20 • Print lines of 7 fields and beginning with "Name:" NF==7 && /^Name:/

Example (5) • Print fields in reverse order one per line { for (i=NF; i>=1; i--) print $i; }

Built-in variables

Operators

awk functions and commands • Control flow • break, continue, do/while, exit, for, if/else, return, while • Arithmetic • rand, sin, cos,… • String • IO • print, printf, getline,…

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