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shell scripting with bash

basic commands

echo prints out what comes after it on a line

tests for equality, existence, etc. to use in control structures

= is the string comparison operator. Note the two spaces, one on either side. Miss these out and bash will complain. Indeed, a="thing" (without the spaces) is the assignment rather than the comparison operator

File related tests: stolen from this shell scripting tutorial

• -b file = True if the file exists and is block special file.
• -c file = True if the file exists and is character special file.
• -d file = True if the file exists and is a directory.
• -e file = True if the file exists.
• -f file = True if the file exists and is a regular file
• -g file = True if the file exists and the set-group-id bit is set.
• -k file = True if the files' "sticky" bit is set.
• -L file = True if the file exists and is a symbolic link.
• -p file = True if the file exists and is a named pipe.
• -r file = True if the file exists and is readable.
• -s file = True if the file exists and its size is greater than zero.
• -s file = True if the file exists and is a socket.
• -t fd = True if the file descriptor is opened on a terminal.
• -u file = True if the file exists and its set-user-id bit is set.
• -w file = True if the file exists and is writable.
• -x file = True if the file exists and is executable.
• -O file = True if the file exists and is owned by the effective user id.
• -G file = True if the file exists and is owned by the effective group id.
• file1 -nt file2 = True if file1 is newer, by modification date, than file2.
• file1 ot file2 = True if file1 is older than file2.
• file1 ef file2 = True if file1 and file2 have the same device and inode numbers.
• -z string = True if the length of the string is 0.
• -n string = True if the length of the string is non-zero.
• string1 = string2 = True if the strings are equal.
• string1 != string2 = True if the strings are not equal.
• !expr = True if the expr evaluates to false.
• expr1 -a expr2 = True if both expr1 and expr2 are true.
• expr1 -o expr2 = True is either expr1 or expr2 is true.

Usage: if [ -d directory ] ; then echo "directory exists"; fi

control structures

The if statement takes the following form:

if [ condition ];
   echo "do something";
fi

To do if ... else, you need to add a "then"

if [ condition ];
then
   echo "do something";
else
   echo "do something else";
fi

doing mathematics in bash

To compare two integers use

• -eq for equals
• -ne for not-equals
• -gt (or > within double brackets)
• -ge (or >= within double brackets)
• -lt (or < within double brackets)
• -le (or <= within double brackets)

So if [ $a -gt $b ] is true if the value of $a is bigger than the value of $b

To add subtract etc... use subshells. So

command line arguments

Using command line arguments in a shell script - the equivalent of argv and argc etc. in c:

• $@ gives you all the arguments
• $# gives you the number of arguments
• $0 gives you the current file name (the name of the script)
• $1 gives you the first argument, $2 the second, and so on.

consider this script, called cl.sh

#!/bin/bash
echo there are $# arguments to $0: $*
echo the first argument: $1
echo the second argument: $2
echo the third argument: $3
echo here are all the arguments again: $@

which is not very interesting, but shows what's what. Run it with the arguments a b c d and this is the output:

~/bin $ ./cl.sh a b c d
there are 4 arguments to ./cl.sh: a b c d
the first argument: a
the second argument: b
the third argument: c
here are all the arguments again: a b c d
~/bin $