Shell scripting

Shell scripting

Shell scripts are good for automating repetitive shell tasks. Bash and other shells include the “usual” constructs found in programming languages, such as for loops, tests, if and case statements, but there is only one type of variable: strings.

Shells

Unix has variety of Shells. Bourne shell (sh), Bourne again shell (bash), C shell (csh), Korn shell (ksh), Tenex C shell (tcsh). Use the which or whereis unix commands to find out where a specific shell is located:

# which bash
/bin/bash

or:

# whereis bash
bash: /bin/bash /etc/bash.bashrc /usr/share/man/man1/bash.1.gz

Backup and restore using brace expansion

The brace expansion is present in two basic forms, string lists and ranges. It can be switched on and off under runtime by using the set builtin and the option -B and +B or the long option braceexpand. If brace expansion is enabled, the stringlist in SHELLOPTIONS contains braceexpand.

When doing:

$ echo {a,b}$PATH

the brace expansion does not expand the variable - this is done in a later step. Brace expansion just makes it being:

echo a$PATH b$PATH

Another common pitfall is to assume that a range like {1..200} can be expressed with variables using {$a..$b}. It simply is not possible, because it's the very first step in doing expansions. A possible way to achieve this, is using the eval command, which basically evaluates a commandline twice:

eval echo {$a..$b}

For instance, when embedded inside a for loop in a shell script:

for i in $(eval echo {$a..$b})

Backup script using brace expansion:

Restore script using brace expansion: