multiple-files

docs/multiple_files.md

@@ -0,0 +1,50 @@
+# Multiple files ...
+> ... or how to apply different rules for different input files.
+
+## Common practice - `NR` and `FNR`
+
+Perhaps one of the most common approach is utilizing the internally [predefined variables](https://www.gnu.org/software/gawk/manual/html_node/Built_002din-Variables) [`NR`](https://www.gnu.org/software/gawk/manual/html_node/Auto_002dset.html#index-NR-variable-1) and [`FNR`](https://www.gnu.org/software/gawk/manual/html_node/Auto_002dset.html#index-FNR-variable-1).
+
+To illustrate this, we can create two files with some distinct data. You can run `seq -f "File_1 - Line_%g" 1 3` and `seq -f "File_2 - Line_%g" 1 3` and redirect the output to two separate files.
+
+```bash
+$ seq -f "File_1 - Line_%g" 1 3 
+File_1 - Line_1
+File_1 - Line_2
+File_1 - Line_3
+
+$ seq -f "File_2 - Line_%g" 1 3 
+File_2 - Line_1
+File_2 - Line_2
+File_2 - Line_3
+```
+
+Or use anonymous pipeline to run directly this example:
+```bash
+awk '{print "FNR: "FNR" NR: "NR"  => "$0}' <(seq -f "File_1 - Line_%g" 1 3) <(seq -f "File_2 - Line_%g" 1 3)
+FNR: 1 NR: 1  => File_1 - Line_1
+FNR: 2 NR: 2  => File_1 - Line_2
+FNR: 3 NR: 3  => File_1 - Line_3
+FNR: 1 NR: 4  => File_2 - Line_1
+FNR: 2 NR: 5  => File_2 - Line_2
+FNR: 3 NR: 6  => File_2 - Line_3
+```
+
+Note that `FNR` keeps the line number in the file, while `NR` keeps incrementing - i.e. referring to the line number irrelevant of the file.
+
+It is rather common (in the awk community) to treat differently the first file which usually is realized by adding check for `NR==FNR` that is **_true_** only for the first file.
+
+```awk
+...
+NR==FNR { commands } # runs only on the first file
+NR!=FNR { commands } # runs on any but the first file
+        { commands } # runs on all files
+...
+```
+
+## `FILENAME`, `ARGIND`
+
+- [`FILENAME`](https://www.gnu.org/software/gawk/manual/html_node/Auto_002dset.html#index-dark-corner-FILENAME-variable-1) - The name of the current input file. When no data files are listed on the command line, awk reads from the standard input and `FILENAME` is set to "-". `FILENAME` changes each time a new file is read.
+- [`ARGIND`](https://www.gnu.org/software/gawk/manual/html_node/Auto_002dset.html#index-ARGIND-variable) - Every time gawk opens a new data file for processing, it sets `ARGIND` to the index in ARGV of the file name. When gawk is processing the input files, `‘FILENAME == ARGV[ARGIND]`’ is always true.
+!!! warning
+    `ARGIND` is not working under OS X - you need gawk for this.

mkdocs.yml

@@ -6,7 +6,8 @@ nav:
   - 2.Teasing with grep: 2.Teasing_with_grep.md
   - 3.Shell we awk: 3.Shell_we_awk.md
   - 4.Brief commands: 4.Brief_commands.md
-  - 4.a Awk arrays: arrays.md
+  - "  a) Multiple files": multiple_files.md
+  - "  b) Awk arrays": arrays.md
   - 5.String manipulation: 5.String_manipulation.md
   - 6.One line programs: 6.One_line_programs.md
   - Python vs. awk: Python_vs_awk.md