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Merge pull request #492 from mkwardakov/remove-cats

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Fix #320: excessive cat usage
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Joshua Levy 2017-05-20 10:46:06 -07:00 committed by GitHub
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@ -347,9 +347,9 @@ A few examples of piecing together commands:
- It is remarkably helpful sometimes that you can do set intersection, union, and difference of text files via `sort`/`uniq`. Suppose `a` and `b` are text files that are already uniqued. This is fast, and works on files of arbitrary size, up to many gigabytes. (Sort is not limited by memory, though you may need to use the `-T` option if `/tmp` is on a small root partition.) See also the note about `LC_ALL` above and `sort`'s `-u` option (left out for clarity below).
```sh
cat a b | sort | uniq > c # c is a union b
cat a b | sort | uniq -d > c # c is a intersect b
cat a b b | sort | uniq -u > c # c is set difference a - b
sort a b | uniq > c # c is a union b
sort a b | uniq -d > c # c is a intersect b
sort a b b | uniq -u > c # c is set difference a - b
```
- Use `grep . *` to quickly examine the contents of all files in a directory (so each line is paired with the filename), or `head -100 *` (so each file has a heading). This can be useful for directories filled with config settings like those in `/sys`, `/proc`, `/etc`.
@ -367,7 +367,7 @@ A few examples of piecing together commands:
- Say you have a text file, like a web server log, and a certain value that appears on some lines, such as an `acct_id` parameter that is present in the URL. If you want a tally of how many requests for each `acct_id`:
```sh
cat access.log | egrep -o 'acct_id=[0-9]+' | cut -d= -f2 | sort | uniq -c | sort -rn
egrep -o 'acct_id=[0-9]+' access.log | cut -d= -f2 | sort | uniq -c | sort -rn
```
- To continuously monitor changes, use `watch`, e.g. check changes to files in a directory with `watch -d -n 2 'ls -rtlh | tail'` or to network settings while troubleshooting your wifi settings with `watch -d -n 2 ifconfig`.