Bash Input/Output: Difference between revisions
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=Quoted String Expansion= | =Quoted String Expansion= | ||
{{Internal|Bash_Built-In_Variables#Quoted_String_Expansion_.24.27....27|Quoted String Expansion}} | $'\012' # Octal value | ||
$'\x0a' # Hexadecimal value | |||
More details: {{Internal|Bash_Built-In_Variables#Quoted_String_Expansion_.24.27....27|Quoted String Expansion}} | |||
=Reading from stdin= | =Reading from stdin= | ||
{{Internal|Bash_read#Confirmation_to_Proceed|Interactively Read Input from stdin}} | {{Internal|Bash_read#Confirmation_to_Proceed|Interactively Read Input from stdin}} |
Revision as of 00:52, 4 February 2020
Internal
Here-Docs
Echo Multiple Lines to stdout
cat <<EOF blah ${some_var} $(some-command) blah EOF
- It will perform variable substitution. If you don't want that, escape \${some_var}.
- `...` sequences should be escaped as shown here: \`...\` otherwise they will executed before the output is sent to stdout.
- \ will join lines. If you want "\" in the output, then you should escape it:
... blah blah \\ ...
Echo Multiple Lines into a File
cat > /tmp/test.txt << EOF
blah
EOF
or
(cat << EOF
blah
EOF
) > /tmp/test.txt
To append:
cat >> /tmp/test.txt << EOF
blah
EOF
Iterating over Lines from a File in the Same bash Process
With for
Note that the following approach works better for small files. If the file is large, the content will be first cached in memory, and it may look like the command is irresponsive:
With sed
This approach has the advantage that lines are read one by one and not buffered:
filename=... line_count=$(wc -l ${filename}) || exit 1 line_count=${line_count% *} line_count=$(echo ${line_count} | sed -e 's/ *//') line_number=1 while [ ${line_number} -le ${line_count} ]; do local line line=$(sed -n ${line_number}p ${filename}) # process line ... ((line_number++)) done
Read First Line from a File
head -i <file>
Read the First Line of a Command's Output
bash --version | head -n 1
Extracting a Line Specified by Its Number from a File
With head and tail
Extract line 15 (line numbers are 1-based):
cat ./test.txt | head -15 | tail -1
With sed
echo Formatting
Bold:
echo -e "\033[1msomething bold \033[0m"
Red:
echo -e "\033[31msomething red\033[0m"
Quoted String Expansion
$'\012' # Octal value $'\x0a' # Hexadecimal value
More details: