How can I use ":" as an AWK field separator?

"-F" is a command line argument, not AWK syntax. Try:

 echo "1: " | awk -F  ":" '/1/ {print $1}'

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If you want to do it programatically, you can use the FS variable:

echo "1: " | awk 'BEGIN { FS=":" } /1/ { print $1 }'

Note that if you change it in the main loop rather than the BEGIN loop, it takes affect for the next line read in, since the current line has already been split.

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You have multiple ways to set : as the separator:

awk -F: '{print $1}'

awk -v FS=: '{print $1}'

awk '{print $1}' FS=:

awk 'BEGIN{FS=":"} {print $1}'

All of them are equivalent and will return 1 given a sample input "1:2:3":

$ awk -F: '{print $1}' <<< "1:2:3"
1
$ awk -v FS=: '{print $1}' <<< "1:2:3"
1
$ awk '{print $1}' FS=: <<< "1:2:3"
1
$ awk 'BEGIN{FS=":"} {print $1}' <<< "1:2:3"
1

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-F is an argument to awk itself:

$echo "1: " | awk -F":" '/1/ {print $1}'
1

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You can also use a regular expression as a field separator. The following will print "bar" by using a regular expression to set the number "10" as a separator.

echo "foo 10 bar" | awk -F'[0-9][0-9]' '{print $2}'

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Or you can use:

echo "1: " | awk  '/1/{print $1-":"}' 

This is a really funny equation.

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There isn't any need to write this much. Just put your desired field separator with the -F option in the AWK command and the column number you want to print segregated as per your mentioned field separator.

echo "1: " | awk -F: '{print $1}'
1

echo "1#2" | awk -F# '{print $1}'
1

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AWK works as a text interpreter that goes linewise for the whole document and that goes fieldwise for each line. Thus $1, $2...$n are references to the fields of each line ($1 is the first field, $2 is the second field, and so on...).

You can define a field separator by using the "-F" switch under the command line or within two brackets with "FS=...".

Now consider the answer of Jürgen:

echo "1: " | awk -F  ":" '/1/ {print $1}'

Above the field, boundaries are set by ":" so we have two fields $1 which is "1" and $2 which is the empty space. After comes the regular expression "/1/" that instructs the filter to output the first field only when the interpreter stumbles upon a line containing such an expression (I mean 1).

The output of the "echo" command is one line that contains "1", so the filter will work...

When dealing with the following example:

echo "1: " | awk '/1/ -F ":" {print $1}'

The syntax is messy and the interpreter chose to ignore the part F ":" and switches to the default field splitter which is the empty space, thus outputting "1:" as the first field and there will be not a second field!

The answer of Jürgen contains the good syntax...

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echo "1: " | "456:abc:515:xyz "

awk -F: NF=/1/

      1    |  456

UPDATE : realizing how verbose my previous answer was

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