Reading and filtering text

cat, less, head, tail, wc, grep, cut, sort, uniq and tr applied to a file: picking the right filter, the option that changes everything, and predicting the output on real data.

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grep -v pattern: what is it?
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A filter that keeps only the lines that do not contain the pattern.
grep -v '^#' config drops the comments. Not to be mixed up with -i (ignore case), -n (number the lines) and -c (count the lines without printing them).
What does cut -d: -f1,7 give on this /etc/passwd line?

```text
lea:x:1000:1000:Lea:/home/lea:/bin/bash
`
- lea:/bin/bash
- lea:/home/lea
- lea:x
- x:/bin/bash
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lea:/bin/bash
The fields are lea, x, 1000, 1000, Lea, /home/lea and /bin/bash: the first is the name, the seventh the shell. cut puts the delimiter back between the kept fields.
wc -l counts the … of a file; head -n 3 prints the … and tail -n 3 the ….
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wc -l counts the lines of a file; head -n 3 prints the first three and tail -n 3 the last three.
With no option, wc prints lines, words and bytes. With no -n, head and tail print ten lines.
cut -d: -f1,7: what is it?
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Splits each line at the colons and keeps fields 1 and 7.
-d sets the delimiter, -f picks the fields, numbered from 1. On /etc/passwd, those are the login name and the shell.
grep error journal.log does not find the line “ERROR disque plein” even though it is in the file. Which fix?
- Reverse the filter: grep -v error journal.log
- Search in depth: grep -r error journal.log
- Count the lines: grep -c error journal.log
- Ignore case: grep -i error journal.log
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Ignore case: grep -i error journal.log
grep is case-sensitive: error does not match ERROR. -v would keep the lines without the pattern, -r walks directories and -c would only print a number.

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