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| -rw-r--r-- | src/awk.md | 84 |
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@@ -17,7 +17,7 @@ Input is processed in two stages: but can be changed via the builtin variable `FS` or command line option `-F`. -Field are accessed as follows: +Fields are accessed as follows: - `$0` whole `record` - `$1` field one - `$2` field two @@ -42,3 +42,85 @@ record ----> ∀ pattern matched v v fields ----> run associated action ``` + +Any valid awk `expr` can be a `pattern`. + +### 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 + +### 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`. + +- `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` + + +## 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. + +### 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. + +### 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. |