awk(1)

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:

Splitting input into a sequence of records. +By default split at newline character, but can be changed via the +builtin RS variable.
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:

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.

INPUT
+  |
+  v
+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 +
FS field separator: regex to split records into fields, by default +
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

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

# /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

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.

+ +

Notes