How do use shell variables in an awk script?

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may be done in several ways. Some are better than others. This should cover most of them. If you have a comment, please leave below.

Using -v (The best way, most portable)

Use the -v option: (P.S. use a space after -v or it will be less portable. E.g., awk -v var= not awk -vvar=)

variable=”line onenline two” awk -v var=”$variable” \’BEGIN {print var}\’ line one line two

This should be compatible with most awk, and the variable is available in the BEGIN block as well:

If you have multiple variables:

awk -v a=”$var1″ -v b=”$var2″ \’BEGIN {print a,b}\’

Warning. As Ed Morton writes, escape sequences will be interpreted so t becomes a real tab and not t if that is what you search for. Can be solved by using ENVIRON[] or access it via ARGV[]

PS If you like three vertical bar as separator |||, it can\’t be escaped, so use -F”[|][|][|]”

Example on getting data from a program/function inn to awk (here date is used)

awk -v time=”$(date “%F %H:%M” -d \’-1 minute\’)” \’BEGIN {print time}\’

Variable after code block

Here we get the variable after the awk code. This will work fine as long as you do not need the variable in the BEGIN block:

variable=”line onenline two” echo “input data” | awk \'{print var}\’ var=”${variable}” or awk \'{print var}\’ var=”${variable}” file

  • Adding multiple variables:

awk \'{print a,b,$0}\’ a=”$var1″ b=”$var2″ file

  • In this way we can also set different Field Separator FS for each file.

awk \’some code\’ FS=\’,\’ file1.txt FS=\’;\’ file2.ext

  • Variable after the code block will not work for the BEGIN block:

echo “input data” | awk \’BEGIN {print var}\’ var=”${variable}”

Here-string

Variable can also be added to awk using a here-string from shells that support them (including Bash):

awk \'{print $0}\’ <<< “$variable” test

This is the same as:

printf \’%s\’ “$variable” | awk \'{print $0}\’

P.S. this treats the variable as a file input.

ENVIRON input

As TrueY writes, you can use the ENVIRON to print Environment Variables. Setting a variable before running AWK, you can print it out like this:

X=MyVar awk \’BEGIN{print ENVIRON[“X”],ENVIRON[“SHELL”]}\’ MyVar /bin/bash

ARGV input

As Steven Penny writes, you can use ARGV to get the data into awk:

v=”my data” awk \’BEGIN {print ARGV[1]}\’ “$v” my data

To get the data into the code itself, not just the BEGIN:

v=”my data” echo “test” | awk \’BEGIN{var=ARGV[1];ARGV[1]=””} {print var, $0}\’ “$v” my data test

Variable within the code: USE WITH CAUTION

You can use a variable within the awk code, but it\’s messy and hard to read, and as Charles Duffy points out, this version may also be a victim of code injection. If someone adds bad stuff to the variable, it will be executed as part of the awk code.

This works by extracting the variable within the code, so it becomes a part of it.

If you want to make an awk that changes dynamically with use of variables, you can do it this way, but DO NOT use it for normal variables.

variable=”line onenline two” awk \’BEGIN {print “\'”$variable”\'”}\’ line one line two

Here is an example of code injection:

variable=\’line onenline two” ; for (i=1;i<=1000; i) print i”\’ awk \’BEGIN {print “\'”$variable”\'”}\’ line one line two 1 2 3 . . 1000

You can add lots of commands to awk this way. Even make it crash with non valid commands.

Extra info:

Use of double quote

It\’s always good to double quote variable “$variable” If not, multiple lines will be added as a long single line.

Example:

var=”Line one This is line two” echo $var Line one This is line two echo “$var” Line one This is line two

Other errors you can get without double quote:

variable=”line onenline two” awk -v var=$variable \’BEGIN {print var}\’ awk: cmd. line:1: onenline awk: cmd. line:1: ^ backslash not last character on line awk: cmd. line:1: onenline awk: cmd. line:1: ^ syntax error

And with single quote, it does not expand the value of the variable:

awk -v var=\’$variable\’ \’BEGIN {print var}\’ $variable

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