Bash

Random bash tips and tricks to (try to) use Bash properly and efficiently...

Loops

To iterate over a sequence of numbers:

1. With a know fixed upper limit

   for i in {1..5}; do
       echo $i;
   done

2. With a possibly unknown upper limit

   END=5
   for i in $(seq 1 $END); do
       echo $i;
   done

3. If seq can't be used

   END=5
   for ((i=1;i<=END;i++)); do
       echo $i
   done

[ref]

Tar archives manipulation

Listing content

To list the content of an archive without extracting, use the --list/-t option:

tar --list --file <my-archive.tar>
tar -tf <my-archive.tar>

Creating archives

To create an archive, use the --create/-c option:

tar --create --file <my-archive.tar> <file1> <dir1>
tar -cf <my-archive.tar> <file1> <dir1>

Extracting the content

To extract the files of an archive, use the --extract/-x option:

tar --extract --file <my-archive.tar>
tar -xf <my-archive.tar>

This will extract all the content of the archive in the current folder

To extract in a different folder:

tar -xf <my-archive.tar> -C </my/folder>

Note: the folder needs to exists first.

To extract only specific files:

tar -xf <my-archive.tar> <file1> <dir1>

Note: The name of the files and/or dirs to extract must be the same as listed using the --list option.

Other

To manipulate .tar.gz archives, add the -z option to all commands.

By default, all commands will read from stdin or write to stdout, allowing incoming and outgoing piping. To read to or from an existing archive, use the --file/-f option.

Here documents

Here documents (or heredocs) are a way to provide multiline input directly to a command or script. The syntax is the following:

COMMAND <<DELIMITOR
...
...
DELIMITOR

DELIMITOR is used to control when to start and stop the input. It can be any string without spaces, and should then be choosed wiselly so that it won't apear in the input:

$ cat <<ThisIsADelimitor
Hello
There
ThisIsADelimitor
Hello
There
$ cat <<1
Hello
There
1
Hello
There

Heredocs allows for parameter expansion as well:

$ export FIRSTNAME='Obi-Wan'
$ export LASTNAME='Kenobi'
$ cat <<EOF
Hello $FIRSTNAME $LASTNAME
EOF
Hello Obi-Wan Kenobi

Default behavior of the heredocs is to preserve the leading tabs:

$ cat <<EOF
        hello
there
EOF
    hello
there

You can strip them by prefixing the delimitor with a dash:

$ cat <<-EOF
        hello
there
EOF
hello
there

This will strip all leading tabs, allowing the input to be more readable visually.

Note though that this strips only leading tabs, but not spaces.

Showing disk usage

To show disk usage, you can use df and du:

1. To show free space over all mounted drives:

   df -h

2. To check the size of a directory:

   du -sh /path/to/directory

3. To list sizes of all subdirectories:

   du -h --max-depth=1 /path/to/directory