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0xmohit 2018-01-04 18:39:11 +00:00 committed by GitHub
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@ -140,9 +140,9 @@ Notes:
- In Bash, note there are lots of kinds of variable expansion. Checking a variable exists: `${name:?error message}`. For example, if a Bash script requires a single argument, just write `input_file=${1:?usage: $0 input_file}`. Using a default value if a variable is empty: `${name:-default}`. If you want to have an additional (optional) parameter added to the previous example, you can use something like `output_file=${2:-logfile}`. If `$2` is omitted and thus empty, `output_file` will be set to `logfile`. Arithmetic expansion: `i=$(( (i + 1) % 5 ))`. Sequences: `{1..10}`. Trimming of strings: `${var%suffix}` and `${var#prefix}`. For example if `var=foo.pdf`, then `echo ${var%.pdf}.txt` prints `foo.txt`.
- Brace expansion using `{`...`}` can reduce having to re-type similar text and automate combinations of items. This is helpful in examples like `mv foo.{txt,pdf} some-dir` (which moves both files), `cp somefile{,.bak}` (which expands to `cp somefile somefile.bak`) or `mkdir -p test-{a,b,c}/subtest-{1,2,3}` (which expands all possible combinations and creates a directory tree).
- Brace expansion using `{`...`}` can reduce having to re-type similar text and automate combinations of items. This is helpful in examples like `mv foo.{txt,pdf} some-dir` (which moves both files), `cp somefile{,.bak}` (which expands to `cp somefile somefile.bak`) or `mkdir -p test-{a,b,c}/subtest-{1,2,3}` (which expands all possible combinations and creates a directory tree). Brace expansion is performed before any other expansion.
- The output of a command can be treated like a file via `<(some command)`. For example, compare local `/etc/hosts` with a remote one:
- The output of a command can be treated like a file via `<(some command)` (known as Process Substitution). For example, compare local `/etc/hosts` with a remote one:
```sh
diff /etc/hosts <(ssh somehost cat /etc/hosts)
```
@ -352,7 +352,7 @@ A few examples of piecing together commands:
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`.
- Use `grep . *` to quickly examine the contents of all files in a directory (so each line is paired with the filename), or `more *` (so each file has a heading). This can be useful for directories filled with config settings like those in `/sys`, `/proc`, `/etc`.
- Summing all numbers in the third column of a text file (this is probably 3X faster and 3X less code than equivalent Python):