Some Linux Commands

:-

Take ${val1:-val2} for example, if val1 is unset or null, return val2, otherwise return val1.

Example:

#!/bin/bash

default="default"
preset="preset"
value="This is ${preset:-"$default"} value"
echo $value  # This is preset value

#!/bin/bash

default="default"
value="This is ${preset:-"$default"} value"
echo $value  # This is default value

set -a

Definition from the Bash Manual

-a

Each variable or function that is created or modified is given the export attribute and marked for export to the environment of subsequent commands.

Honestly I haven’t fully comprehended the definition, but we can set up an example to see what it does.

1. Create foo.sh

   #!/usr/bin/env bash
   
   set -a
   . "./bar.sh"
   set +a
   echo "a=$a"
   echo "b=$b"
   echo "c=$c"

2. Create bar.sh

   #!/usr/bin/env bash
   
   a=1
   b=2
   c=3

3. Set executable permission

   chmod +x foo.sh
   chmod +x bar.sh

4. Source foo.sh

   . ./foo.sh

5. Result

   a=1
   b=2
   c=3

   As you can see, we can access all the variables defined in bar.sh in foo.sh, just as if they are marked as export. If we didn’t use set -a, the result would be

   a=
   b=
   c=

6. We can access it directly in the terminal too, they are exported all the way to the top bash environment.

   ➜  /tmp echo $a
   1
   ➜  /tmp echo $b
   2
   ➜  /tmp echo $c
   3

Bash Regular Expressions

We can use regular expressions with the help of =~, here is an example.

#!/usr/bin/env bash

foo=1
if [[ $foo =~ [[:digit:]]+$ ]]; then
    echo number
fi

Output

number

Notice that you cannot use \d or \\d to replace [[:digit:]], because \d is PCRE, while it uses POSIX regex here, which doesn’t recognize \d. If you think [[:digit:]] is too long, you can use [0-9] to replace it, which has the same effect.

#!/usr/bin/env bash

foo=1
if [[ $foo =~ [0-9]+$ ]]; then
    echo number
fi

Output

number