lk/system/bash_tricks.md

title	tags
bash tips	Documentation Shell POSIX

Track Live Changes

See changes in a file as it changes:

tail -f *somefile*

See changes in a directory, as it changes:

watch -d ls *directory*

Or use the -g flag to exit once the output changes. This command will look at whether you're connected to the internet, and turn into a rainbow once the connection hits.

watch -g ip address && clear && ip address | lolcat

Automatic Renaming

There are a bunch of files:

• Column CV.aux
• Column CV.log
• Column CV.out
• Column CV.pdf
• Column CV.tex
• tccv.cls

Goal: swap the word "Column" for "Alice" in all files.

IFS=$'\n'
for f in $(find . -name "Col*"); do
mv "$f" $(echo "$f" | sed s/Column/Alice/)
done

IFS is the field separator. This is required to denote the different files as marked by a new line, and not the spaces.

(Alternatively, just install renameutils and do rename Column Alice *)

Arguments and Input

The rm' program takes arguments, but not stdin' from a keyboard, and therefore programs cannot pipe results into rm. To fix this, use xargs to turn the stdin into an argument. For example, if we have a list of files called `list.txt' then we could use cat as so:

cat list.txt | xargs rm

Of course if spaces are included in the file, you would have to account for that.

Numbers

Add number to variables with:

• let "var=var+1"
• let "var+=1"
• let "var++"
• ((++var))
• ((var=var+1))
• ((var+=1))
• var=$(expr $var + 1)

((n--)) works identically.

POSIX WARNING

The number commands above work in bash, but not in bare-ass POSIX shells, such as dash.

Instead, you might do:

x=2
x=$(( x +1 ))
x=$(( x*x ))

Finding Duplicate Files

find . -type f -exec md5sum '{}' ';' | sort | uniq --all-repeated=separate -w 15 > all-files.txt

Output random characters

cat /dev/urandom | tr -cd [:alnum:] | dd bs=1 count=200 status=none && echo

Temporary Working Directory

Try something out in a random directory in /tmp so the files will be deleted when you next shut down.

mktemp -d

That gives you a random directory to mess about in.

   dir=$(mktemp -d)
   for x in {A..Z}; do
      fortune > "$dir"/chimpan-$x
   done
   cd $dir

POSIX WARNING

These smart-brackets are a bash feature. If you try to use {A..Z} in dash, it will think of this as a single item.