117 lines
2.4 KiB
Markdown
117 lines
2.4 KiB
Markdown
---
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title: "bash tips"
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tags: [ "Documentation", "Shell", "POSIX" ]
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---
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## Track Live Changes
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See changes in a file as it changes:
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`tail -f *somefile*`
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See changes in a directory, as it changes:
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`watch -d ls *directory*`
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Or use the `-g` flag to exit once the output changes.
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This command will look at whether you're connected to the internet, and turn into a rainbow once the connection hits.
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> watch -g ip address && clear && ip address | lolcat
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## Automatic Renaming
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There are a bunch of files:
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* Column CV.aux
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* Column CV.log
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* Column CV.out
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* Column CV.pdf
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* Column CV.tex
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* tccv.cls
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Goal: swap the word "Column" for "Alice" in all files.
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```
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IFS=$'\n'
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for f in $(find . -name "Col*"); do
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mv "$f" $(echo "$f" | sed s/Column/Alice/)
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done
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```
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IFS is the field separator. This is required to denote the different files as marked by a new line, and not the spaces.
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(Alternatively, just install `renameutils` and do `rename Column Alice *`)
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## Arguments and Input
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The `rm' program takes arguments, but not `stdin' from a keyboard, and therefore programs cannot pipe results into rm.
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To fix this, use `xargs` to turn the stdin into an argument.
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For example, if we have a list of files called `list.txt' then we could use cat as so:
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```bash
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cat list.txt | xargs rm
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```
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Of course if spaces are included in the file, you would have to account for that.
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## Numbers
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Add number to variables with:
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* `let "var=var+1"`
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* `let "var+=1"`
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* `let "var++"`
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* `((++var))`
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* `((var=var+1))`
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* `((var+=1))`
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* `var=$(expr $var + 1)`
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`((n--))` works identically.
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### POSIX WARNING
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The number commands above work in `bash`, but not in bare-ass POSIX shells, such as `dash`.
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Instead, you might do:
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```sh
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x=2
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x=$(( x +1 ))
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x=$(( x*x ))
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```
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## Finding Duplicate Files
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```bash
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find . -type f -exec md5sum '{}' ';' | sort | uniq --all-repeated=separate -w 15 > all-files.txt
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```
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## Output random characters
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```bash
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cat /dev/urandom | tr -cd [:alnum:] | dd bs=1 count=200 status=none && echo
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```
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## Temporary Working Directory
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Try something out in a random directory in `/tmp` so the files will be deleted when you next shut down.
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```bash
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mktemp -d
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```
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That gives you a random directory to mess about in.
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```bash
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dir=$(mktemp -d)
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for x in {A..Z}; do
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fortune > "$dir"/chimpan-$x
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done
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cd $dir
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```
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### POSIX WARNING
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These smart-brackets are a bash feature.
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If you try to use `{A..Z}` in dash, it will think of this as a single item.
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