# awk(1)
```markdown
awk [opt] program [input]
-F <sepstr> field separator string (can be regex)
program awk program
input file or stdin if not file given
```
## Input processing
Input is processed in two stages:
1. Splitting input into a sequence of `records`.
By default split at `newline` character, but can be changed via the
builtin `RS` variable.
2. Splitting a `record` into `fields`. By default strings without `whitespace`,
but can be changed via the builtin variable `FS` or command line option
`-F`.
Fields are accessed as follows:
- `$0` whole `record`
- `$1` field one
- `$2` field two
- ...
## Program
An `awk` program is composed of pairs of the form:
```markdown
pattern { action }
```
The program is run against each `record` in the input stream. If a `pattern`
matches a `record` the corresponding `action` is executed and can access the
`fields`.
```markdown
INPUT
|
v
record ----> ∀ pattern matched
| |
v v
fields ----> run associated action
```
Any valid awk `expr` can be a `pattern`.
An example is the regex pattern `/abc/ { print $1 }` which prints the first
field if the record matches the regex `/abc/`. This form is actually a short
version for `$0 ~ /abc/ { print $1 }`, see the regex comparison operator
below.
### Special pattern
awk provides two special patterns, `BEGIN` and `END`, which can be used
multiple times. Actions with those patterns are **executed exactly once**.
- `BEGIN` actions are run before processing the first record
- `END` actions are run after processing the last record
### Special variables
- `RS` _record separator_: first char is the record separator, by default
<newline>
- `FS` _field separator_: regex to split records into fields, by default
<space>
- `NR` _number record_: number of current record
- `NF` _number fields_: number of fields in the current record
### Special statements & functions
- `printf "fmt", args...`
Print format string, args are comma separated.
- `%s` string
- `%d` decimal
- `%x` hex
- `%f` float
Width can be specified as `%Ns`, this reserves `N` chars for a string.
For floats one can use `%N.Mf`, `N` is the total number including `.` and
`M`.
- `sprintf("fmt", expr, ...)`
Format the expressions according to the format string. Similar as `printf`,
but this is a function and return value can be assigned to a variable.
- `strftime("fmt")`
Print time stamp formatted by `fmt`.
- `%Y` full year (eg 2020)
- `%m` month (01-12)
- `%d` day (01-31)
- `%F` alias for `%Y-%m-%d`
- `%H` hour (00-23)
- `%M` minute (00-59)
- `%S` second (00-59)
- `%T` alias for `%H:%M:%S`
- `S ~ R`, `S !~ R`
The regex comparison operator, where the former returns true if the string
`S` matches the regex `R`, and the latter is the negated form.
The regex can be either a
[constant](https://www.gnu.org/software/gawk/manual/html_node/Regexp-Usage.html)
or [dynamic](
https://www.gnu.org/software/gawk/manual/html_node/Computed-Regexps.html)
regex.
## Examples
### Filter records
```bash
awk 'NR%2 == 0 { print $0 }' <file>
```
The pattern `NR%2 == 0` matches every second record and the action `{ print $0 }`
prints the whole record.
### Negative patterns
```bash
awk '!/^#/ { print $1 }' <file>
```
Matches records not starting with `#`.
### Range patterns
```bash
echo -e "a\nFOO\nb\nc\nBAR\nd" | \
awk '/FOO/,/BAR/ { print }'
```
`/FOO/,/BAR/` define a range pattern of `begin_pattern, end_pattern`. When
`begin_pattern` is matched the range is **turned on** and when the
`end_pattern` is matched the range is **turned off**. This matches every record
in the range _inclusive_.
An _exclusive_ range must be handled explicitly, for example as follows.
```bash
echo -e "a\nFOO\nb\nc\nBAR\nd" | \
awk '/FOO/,/BAR/ { if (!($1 ~ "FOO") && !($1 ~ "BAR")) { print } }'
```
### Access last fields in records
```bash
echo 'a b c d e f' | awk '{ print $NF $(NF-1) }'
```
Access last fields with arithmetic on the `NF` number of fields variable.
### Capture in variables
```bash
# /proc/<pid>/status
# Name: cat
# ...
# VmRSS: 516 kB
# ...
for f in /proc/*/status; do
cat $f | awk '
/^VmRSS/ { rss = $2/1024 }
/^Name/ { name = $2 }
END { printf "%16s %6d MB\n", name, rss }';
done | sort -k2 -n
```
We capture values from `VmRSS` and `Name` into variables and print them at the
`END` once processing all records is done.
### Capture in array
```bash
echo 'a 10
b 2
b 4
a 1' | awk '{
vals[$1] += $2
cnts[$1] += 1
}
END {
for (v in vals)
printf "%s %d\n", v, vals[v] / cnts [v]
}'
```
Capture keys and values from different columns and some up the values.
At the `END` we compute the average of each key.
### Run shell command and capture output
```bash
cat /proc/1/status | awk '
/^Pid/ {
"ps --no-header -o user " $2 | getline user;
print user
}'
```
We build a `ps` command line and capture the first line of the processes output
in the `user` variable and then print it.
